ScholarBank@NUShttps://scholarbank.nus.edu.sgThe DSpace digital repository system captures, stores, indexes, preserves, and distributes digital research material.Sun, 25 Aug 2019 23:04:16 GMT2019-08-25T23:04:16Z50951- Unsteady flow calculations with a parallel multiblock moving mesh algorithmhttps://scholarbank.nus.edu.sg/handle/10635/111506Title: Unsteady flow calculations with a parallel multiblock moving mesh algorithm
Authors: Tsai, H.M.; Wong, A.S.F.; Cai, J.; Zhu, Y.; Liu, F.
Abstract: A novel parallel dynamic moving mesh algorithm designed for multiblock parallel unsteady flow calculations using body-fitted grids is presented. The moving grid algorithm within each block uses a method of arc-length-based transfinite interpolation, which is performed independently on local processors where the blocks reside. A spring network approach is used to determine the motion of the corner points of the blocks, which may be connected in an unstructured fashion in a general multiblock method. A smoothing operator is applied to the points of the block face boundaries and edges to maintain grid smoothness and grid angles. A multiblock parallel Euler/Navier-Stokes solver using multigrid and dual-time stepping is developed along with the moving mesh method. Computational results are presented for the unsteady flow calculations of airfoils and wings with deforming shapes as found in flutter simulations.
Fri, 01 Jun 2001 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1115062001-06-01T00:00:00Z
- An embedded Cartesian grid Euler solver with radial basis function for boundary condition implementationhttps://scholarbank.nus.edu.sg/handle/10635/111535Title: An embedded Cartesian grid Euler solver with radial basis function for boundary condition implementation
Authors: Carolina, L.; Tsai, H.M.; Liu, F.
Abstract: A Cartesian grid approach for the solution of the Euler equations within the framework of a patched, embedded Cartesian field mesh is described. As Cartesian grids are not necessarily body-aligned, an accurate representation for the surface boundary is important. In this paper a gridless boundary treatment using a cloud of nodes in the vicinity of the body combined with the multiquadric radial basis function (RBF) for the conserved flux variables for boundary implementation is proposed. In the present work, the RBF is applied only at the boundary interface, while a standard structured Cartesian grid approach is used everywhere else. Flow variables for solid cell centers for boundary condition implementation are determined via the use of reflected node involving a local RBF fit for a cloud of grid points. RBF is well suited to approximate multidimensional scattered data without any mesh accurately. Compared to the least-square method, RBF offers greater flexibility in regions where point selection may be very limited since the resulting matrix will be non-singular regardless of the sampling point's location. This is particularly important in the context of computations involving complex geometries where eligible points selected may be very close to one another. It is also shown that it provides similar accuracy with less cloud of points. The use of a Cartesian field mesh for the non boundary regions allows for effective implementation of multigrid methods, and issues associated with global conservation are greatly mitigated. Several two and three-dimensional problems are presented to show the efficiency and robustness of the method. Copyright © 2008 by the authors.
Tue, 01 Jan 2008 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1115352008-01-01T00:00:00Z
- Computation of transonic diffuser flows by a lagged κ-ω turbulence modelhttps://scholarbank.nus.edu.sg/handle/10635/115654Title: Computation of transonic diffuser flows by a lagged κ-ω turbulence model
Authors: Xiao, Q.; Tsai, H.M.; Liu, F.
Abstract: The lag model proposed by Olsen and Coakley is applied in combination with the baseline κ-ω two-equation turbulence model to simulate the steady and unsteady transonic flows in a diffuser. A fully implicit time-accurate multigrid algorithm is used to solve the unsteady Navier-Stokes equations and the coupled κ-ω turbulence model equations. Two test cases are investigated, one with a weak shock in the channel corresponding to an exit-static-to-inlet-total pressure ratio Rp=0.82 and the other with a strong shock corresponding to Rp=0.72. Unsteady flows are induced by imposing fluctuating backpressure. Computational results are compared with experimental data and demonstrate notable improvement by the lag model for flows with strong shock-boundary-layer interactions.
Thu, 01 May 2003 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1156542003-05-01T00:00:00Z
- Euler solution using cartesian grid with a gridless least-squares boundary treatmenthttps://scholarbank.nus.edu.sg/handle/10635/115709Title: Euler solution using cartesian grid with a gridless least-squares boundary treatment
Authors: Koh, E.P.C.; Tsai, H.M.; Liu, F.
Abstract: An approach that uses gridless or meshless methods to address the problem of boundary implementation associated with the use of Cartesian grid is discussed. This method applies the gridless concept only at the interface, whereas a standard structured grid method is used everywhere else. The Cartesian grid is used to specify and distribute the computational points on the boundary surface but not to define the geometrical properties. Euler fluxes for the neighbors of cut cells are computed using the gridless method involving a local least-squares curve fit of a "cloud" of grid points. The boundary conditions implemented on the surface points are automatically satisfied in the process of evaluating the surface values in a similar least-squares fashion. The present method does not require the use of halo points. Subsonic, transonic, and supersonic flows are computed for the NACA 0012 and RAE 2822 airfoils, and the results compare well with solutions obtained by a standard Euler solver on body-fitted grids. The method is also used to calculate the flow over a three-element airfoil configuration, and the result is compared with the exact solution for this configuration obtained by conformal mapping.
Tue, 01 Feb 2005 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1157092005-02-01T00:00:00Z
- Side force suppression by dimples on ogive-cylinder bodyhttps://scholarbank.nus.edu.sg/handle/10635/117156Title: Side force suppression by dimples on ogive-cylinder body
Authors: Cui, Y.D.; Tsai, H.M.
Abstract: The effectiveness of dimples in suppressing the large side forces over an ogival-cylinder body at high angles of attack was examined. The experiments were made in the low speed wind-tunnel and water tunnel and the measurements were made at a freestream velocity with a freestream turbulent intensity less than 0.4%. A smooth ogive-cylinder model with a nose length of 122.5 mm and a total body length of 560 mm was used as the baseline. The side force is found to significantly reduced at an angle of attack of 40 deg, while at 50 deg, the side force show values comparable to those without dimples and with a higher level of fluctuations. The existence of dimples on the front portion of the ogive-cylinder model modifies the flowfield leading to reduced side force at high angles of attack. The flow structures for different roll angles are almost the same at 40 deg, and the vortex asymmetry is found at 50 deg.
Wed, 01 Apr 2009 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1171562009-04-01T00:00:00Z
- A parallel viscous flow solver on multi-block overset gridshttps://scholarbank.nus.edu.sg/handle/10635/115565Title: A parallel viscous flow solver on multi-block overset grids
Authors: Cai, J.; Tsai, H.M.; Liu, F.
Abstract: A multi-block overset grid method is presented to accurately simulate viscous flows around complex configurations. A combination of multi-block and overlapping grids is used to discretize the flow domain. A hierarchical grid system with layers of grids of varying resolution is developed to ensure inter-grid connectivity within a framework suitable for multi-grid and parallel computations. At each stage of the numerical computation, information is exchanged between neighboring blocks across either or both matched block boundaries and overlapping boundaries. Coarse-grain parallel processing is facilitated by the multi-block system. Numerical results of flows over multi-element airfoils and three-dimensional turbulent flows around wing-body aerodynamic configurations demonstrate the utility and efficiency of the method. © 2005 Elsevier Ltd. All rights reserved.
Fri, 01 Dec 2006 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1155652006-12-01T00:00:00Z
- Three-dimensional numerical simulation for detonation waves using WENO schemeshttps://scholarbank.nus.edu.sg/handle/10635/86097Title: Three-dimensional numerical simulation for detonation waves using WENO schemes
Authors: Dou, H.-S.; Tsai, H.M.; Khoo, B.C.; Qiu, J.
Abstract: The numerical method and the simulation results of propagation of three-dimensional detonation waves in a rectangular duct are reported. The systems of conservative laws of inviscid fluid combined with the one-step chemical reaction model are discretized in a Cartesian coordinates using the fifth-order WENO (Weighted Essentially NonOscillatory) scheme, and the final discretized variables are solved with a 3rd order TVD Rouge-Kutta method. Then, the process of the formation of the detonation pattern from the premixed gaseous state in 3D space is observed from the simulation results. The simulation result reveals that there are significant differences between the three-dimensional and two-dimensional detonations. As expected, the detonation structure and the reaction process are more complex in three-dimensional case. The simulation shows that under an initial disturbance, the detonation front finally develops to an unsteady three-dimensional distorted pattern and which translates between the walls. For a narrow duct, the flow front displays an unmistakenly spinning motion with a period. For the wide duct, the flow front shows a quasi-steady "rectangular mode" periodically. It is shown that the detonation mechanism depends on the coupling between the pressure and the velocity. The reaction process is dominated by the hot "spot" (zone with high pressure and high velocity) sweeping along the transverse direction for 2D and 3D detonations.
Mon, 01 Jan 2007 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/860972007-01-01T00:00:00Z
- A Comparative Study of Evolutionary Algorithm and Swarm Algorithm for Airfoil Shape Optimization Problemshttps://scholarbank.nus.edu.sg/handle/10635/117226Title: A Comparative Study of Evolutionary Algorithm and Swarm Algorithm for Airfoil Shape Optimization Problems
Authors: Tan, C.M.; Ray, T.; Tsai, H.M.
Abstract: Shape optimization of airfoils involves highly expensive, nonlinear objective(s) and constraint functions. Zero order, stochastic methods are often used to handle such problems. In this paper, we report the performance of two such methods; Evolutionary Algorithm (EA) and the Swarm Algorithm (SWARM) on five airfoil shape optimization problems. Both the EA and the SWARM algorithm used here are variants of their original form suited to handle multiple objectives and multiple constraints without aggregation. Their original form were meant to solve unconstrained single objective problems. The present studies indicate that the EA marginally outperforms SWARM for single objective problems in terms of the computational efficiency while for the multiple objective problems, the SWARM exhibits a better overall performance in locating the Pareto front. Both methods exhibit fast convergence capabilities and provide the designer the flexibility of cost function choices that is necessary to solve various forms of the shape optimization problems. © 2003 by T. Ray and H.M. Tsai.
Wed, 01 Jan 2003 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1172262003-01-01T00:00:00Z
- Aerodynamic design of cascades by using an adjoint equation methodhttps://scholarbank.nus.edu.sg/handle/10635/117227Title: Aerodynamic design of cascades by using an adjoint equation method
Authors: Yang, S.; Wu, H.-Y.; Liu, F.; Tsai, H.-M.
Abstract: A continuous adjoint equation method is developed for the aerodynamic design of cascade blades in a two-dimensional, inviscid, and compressible flow. A cost function based on a prescribed target pressure distribution is defined and the purpose of design is to minimize the value of the cost function. The adjoint equations and the corresponding boundary conditions are derived based on the Euler equations, the flow boundary conditions, and the definition of the cost function. Gradient information is obtained by solving the adjoint equations. A one-dimensional search algorithm is used to perform the optimization in the calculated gradient direction. Multigrid method is applied to accelerate the computation for both the Euler and the adjoint equations. Three transonic cascade blade design cases are tested. The results show that the method is effective and efficient for turbomachinery blade design. The effect of shape functions on the performance of the design method is discussed. © 2003 by the authors. Published by the American Institute of Aeronautics and Astronautics, Inc.
Wed, 01 Jan 2003 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1172272003-01-01T00:00:00Z
- Solution acceleration for steady flow using the Conservation Element and Solution Element (CE/SE) methodhttps://scholarbank.nus.edu.sg/handle/10635/117274Title: Solution acceleration for steady flow using the Conservation Element and Solution Element (CE/SE) method
Authors: DeLisse, A.; Tsai, H.M.
Abstract: The method of Space-Time Conservation Element and Solution Element (CE/SE) is a high-resolution and multidimensional numerical method for solving conservation laws. The CE/SE's unique feature is that both space and time are unified and treated as a single entity, and both local and global flux conservation in space and time is enforced. The method avoids the limitations of the traditional approaches to capture the physics more efficiently and realistically. The scheme being an explicit time stepping method is intended for time dependent problems and thus makes considerable demands on the computing requirements for steady state flow solutions. To overcome this difficulty, traditional techniques used to accelerate the convergence of flows to a steady state are considered in this paper. The methods used for explicit flow solvers, namely multigrid, enthalpy damping, and local time stepping, are applied in the context of the CE/SE method. The accuracy and efficiency of the techniques are verified by Euler calculations for a range of test problems in two dimensions. The capabilities of the methods are assessed individually and collectively. Results show that the computational efficiency for the CE/SE can be realized. © 2003 by by the authors.
Wed, 01 Jan 2003 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1172742003-01-01T00:00:00Z
- Single and multiobjective wing planform and airfoil shape optimization using a swarm algorithmhttps://scholarbank.nus.edu.sg/handle/10635/117273Title: Single and multiobjective wing planform and airfoil shape optimization using a swarm algorithm
Authors: Ng, K.Y.; Tan, C.M.; Ray, T.; Tsai, H.M.
Abstract: Shape optimization of wing planform and airfoils can manifest in different single and multiobjective, unconstrained and constrained forms. In this paper, we introduce a swarm algorithm embedded with schemes for parameter free constraint and multiobjective handling that provides the designer adequate flexibility and ease of modeling, all at a computational cost that is comparable with existing stochastic methods. Moreover, existing stochastic methods use fixed bound for the design variables and the final solution is always within this space. Such a-priori knowledge of the variable bounds is not always easy to provide as highlighted in the first two planform design examples where the final solution is close to the bounds and exploration beyond the initially defined bounds is necessary. In order to deal with such problems, we introduce an Adaptive Search Space Operator (ASO) that is capable of moving the bounds of the search space towards more promising regions through the use of shrinking, expanding and shifting. The benefits of such an operator are illustrated using two examples; a mathematical function and an inverse airfoil design problem. © 2003 by T. Ray and H.M. Tsai. Published by the American Institute of Aeronautics and Astronautics, Inc.
Wed, 01 Jan 2003 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1172732003-01-01T00:00:00Z
- Time-domain aeroelastic simulation by a coupled euler and integral boundary-layer methodhttps://scholarbank.nus.edu.sg/handle/10635/117281Title: Time-domain aeroelastic simulation by a coupled euler and integral boundary-layer method
Authors: Yang, S.; Zhang, Z.; Liu, F.; Luo, S.; Tsai, H.-M.; Schuster, D.M.
Abstract: An interactive boundary-layer method that solves the unsteady Euler equations coupled with Green's lag entrainment integral boundary-layer equations is presented for time domain aeroelastic computation. The three-dimensional unsteady Euler equations are solved on stationary body-fitted curvilinear grids. Unsteady boundary conditions on moving surfaces in an aeroelastic problem are accounted for by using approximate small-perturbation method without moving the computational grid. A semi-inverse method is used to couple the Euler and the boundary-layer solutions in order to compute flows with strong inviscid and viscous interactions. The method is tested on standard steady transonic flow computations for the NACA0012 and RAE2822 airfoils and computations of three-dimensional steady and unsteady flows of the LANN Wing. Comparisons with Navier-Stokes results and available experimental data show that the interactive-boundary-layer method provides significant improvement over inviscid calculations by the Euler equations alone. The proposed method is used to predict the flutter boundary for the Isogai wing test case through time domain simulations. The interactive boundary-layer result agrees with that by a Navier-Stokes solver and indicates fundamental differences between the viscous and inviscid solutions in the transonic range.
Thu, 01 Jan 2004 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1172812004-01-01T00:00:00Z
- Computation of shock induced separated flow with a lagged k-ω turbulence modelhttps://scholarbank.nus.edu.sg/handle/10635/117237Title: Computation of shock induced separated flow with a lagged k-ω turbulence model
Authors: Xiao, Q.; Tsai, H.M.; Liu, F.
Abstract: The lag model proposed by Olsen and Coakley1 is incorporated into the baseline two-equation k-ω turbulence model to simulate the transonic and supersonic turbulent separated flow. The performances of the lagged k-ω turbulence model is assessed by computing two transonic airfoil flow cases, the RAE2822 Case 10 and the 18% thick double circular arc airfoil, and a separated nozzle flow. The computational results show that for the flow cases with strong separation, the implementation of the lag model improves the results; while for the attached flow cases, the influence of the lag model is not significant. © 2003 by the authors. Published by the American Institute of Aeronautics and Astronautics, Inc.
Wed, 01 Jan 2003 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1172372003-01-01T00:00:00Z
- Effects of geometric disturbances on the wake of an axisymmetric bluff bodyhttps://scholarbank.nus.edu.sg/handle/10635/117245Title: Effects of geometric disturbances on the wake of an axisymmetric bluff body
Authors: Chng, T.L.; Tsai, H.M.
Abstract: This study assesses the effectiveness of straight-edged and sinusoidal geometric disturbances (skirts) in reducing the base drag of a three dimensional axisymmetric bluff body. The current investigation stems from the previous successful application of this passive technique to a two dimensional bluff body. Wind tunnel tests were conducted on a projectile-like model with a streamlined leading edge and a circular base. The effect of the extension coefficient and of the disturbance wavelength on the base pressure distribution and the shedding frequency were investigated. The results show that the extension coefficient is the dominant parameter influencing the base pressure increase. Proper selection of the extension coefficient can produce an increase in the base pressure of up to 25% (straight-edged disturbance with extension coefficient of 0.16). The effect of the disturbance wavelength however, is less evident. The weak azimuthal dependence of the base pressure distribution as observed from the pressure contours suggests that the influence of the wavelength may be comparatively weaker. There is also evidence that a straight disturbance may outperform a wavy disturbance. Finally, the dominant wake frequency of the baseline is generally retained with the application of the sinusoidal disturbances but displays a marked drop for the straight disturbances. Copyright © 2008 by The Authors.
Tue, 01 Jan 2008 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1172452008-01-01T00:00:00Z
- Euler solution using cartesian grid with least squares techniquehttps://scholarbank.nus.edu.sg/handle/10635/117246Title: Euler solution using cartesian grid with least squares technique
Authors: Koh, E.P.C.; Tsai, H.M.; Liu, F.
Abstract: This paper discusses an approach that uses "gridless" or "meshless" methods to address the boundary or interface while standard structured grid methods are used everywhere else. The present method uses the Cartesian grid to specify and distribute the computational points on the boundary surface but not to define the geometrical properties. Euler fluxes for the neighbors of cut cells are computed using the gridless method involving a local least-squares curve fit to a "cloud" of grid points. The boundary conditions implemented on the surface points are automatically satisfied in the process of evaluating the surface values in a similar least-squares fashion. No halo points are needed. The overall scheme is robust, stable and converges well for a range of Mach numbers tested. Solutions from the proposed approach are computed for the NACA 0012 and RAE 2822 airfoils and the results are compared with those obtained by a standard Euler solver using body-fitted grids. For grids with equal resolution the method is less accurate for capturing shocks but an improvement in resolution of 60% gives a sharper shock front. The approach offers a number of advantages and its extension to three dimensions is straightforward. © 2003 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.
Wed, 01 Jan 2003 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1172462003-01-01T00:00:00Z
- Euler solutions of flow around a rectangular wing with square tiphttps://scholarbank.nus.edu.sg/handle/10635/117247Title: Euler solutions of flow around a rectangular wing with square tip
Authors: Yang, S.; Luo, S.; Liu, F.; Tsai, H.-M.
Abstract: Low speed flows about a high-lift rectangular wing with square lateral tip are computed by a three-dimensional compressible Euler flow solver. Flow separation around the square tip of the wing is studied. The flow is, otherwise, attached to the surface of the wing. The pressure distributions over the upper and lower surfaces of the wing, especially on the outer portion of the wing and the generation and evolution of the tip vortices in the near field of the wing are computed by the Euler method and validated by a comprehensive wind-tunnel test data in the literature. The effects of the wind-tunnel wall are not considered in the computations.
Mon, 01 Jan 2007 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1172472007-01-01T00:00:00Z
- Effects of forebody slot blowing on vortex breakdown and load over a delta winghttps://scholarbank.nus.edu.sg/handle/10635/51590Title: Effects of forebody slot blowing on vortex breakdown and load over a delta wing
Authors: Cui, Y.D.; Lim, T.T.; Tsai, H.M.
Abstract: Previous studies on vortex breakdown control via forebody slot blowing show that this technique can be used to delay vortex breakdown on a delta wing more effectively than some existing blowing techniques. In this paper, we further extended the investigation using a generic delta wing-body model with symmetrical and differential forebody slot blowing by means of flow visualization and force measurement in a water tunnel, at the Reynolds number of 8.5 × 104, and the angles of attack of 17 to 30 degrees. The experimental results show that symmetrical forebody slot blowing leads to a significant delay in the formation of vortex breakdown and an increase in the lift of about 5%. The differential blowing can manipulate the vortex breakdown position and change the roll moment of the wing, which suggests that differential forebody slot blowing can be a potential mean for roll control.
Mon, 01 Jan 2007 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/515902007-01-01T00:00:00Z
- Numerical study of jet plume instability from an overexpanded nozzlehttps://scholarbank.nus.edu.sg/handle/10635/116762Title: Numerical study of jet plume instability from an overexpanded nozzle
Authors: Xiao, Q.; Tsai, H.M.; Papamoschou, D.
Abstract: The compressible jet plume from a planar overexpanded nozzle is investigated by solving the Reynolds-Averaged Navier-Stokes equations with several turbulence models. Computations are conducted for a series of exit-to-throat area ratios (Ae/At,) from 1.0 to 1.8 and a range of nozzle pressure ratios (NPR) from 1.2 to 1.8. The results are compared with available experimental data in a nozzle of the same geometry. The asymmetric jet plume is well predicted by the simulation and is consistent with the experiments. Among the different turbulence models tested, the two-equation Shear Stress Model (SST) is found to agree closest to the experiments. The simulations are able to predict the velocity profiles, total pressure decay, and axial jet thickness distribution in the jet plume reasonably well. Jet mixing is governed by e/At, and to a lesser extent by NPR. Increasing e/At, results in a significant increase of mixing rate. Computations of turbulent kinetic energy (TKE) show that, with increasing e/At, the peak TKE in the plume rises and moves towards the nozzle exit. Significant increase of TKE inside the nozzle results from the asymmetric flow separation.
Mon, 01 Jan 2007 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1167622007-01-01T00:00:00Z
- S-shaped intake duct parametrizationhttps://scholarbank.nus.edu.sg/handle/10635/116786Title: S-shaped intake duct parametrization
Authors: Dhanabalan, S.S.; Won, K.S.; Tsai, H.M.
Abstract: The work attempts to make use of explicit mathematical functions for three-dimensional shape definition of inlet ducts. The motivation is that this facilitates the design optimization of duct shapes for improved aerodynamic performances. The parameters that are known to influence the aerodynamic properties are used to controls the geometrical properties. Of significance are the definitions of the mean flow line (MFL), cross sectional area and shape variations that dictate the general shape of the duct such that generic ducts can be generated. The cross section of the duct by definition is normal to the MFL. To determine the parameters for any generic duct that can be discretized, a novel inverse process is used that makes use of Proper Orthogonal Decomposition (POD). Examples for parameter variation for three different ducts are presented. Details of the grid generation process for flow computations are also discussed. The framework proposed is applicable and relevant for numerical optimization studies of inlet ducts of practical interest.
Sun, 01 Jan 2006 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1167862006-01-01T00:00:00Z
- Computation of turbulent separated nozzle flow by a lag modelhttps://scholarbank.nus.edu.sg/handle/10635/116273Title: Computation of turbulent separated nozzle flow by a lag model
Authors: Xiao, Q.; Tsai, H.-M.; Liu, F.
Abstract: The capability of the lag model for shock-induced separated flows in supersonic nozzles was discussed. The Reynold-averaged Navier-Stokes equations and the baseline Wilcox k-ω turbulence model were used to predict the equilibrium eddy viscosity. It was found that the lag model introduced history effect and relaxation of the eddy viscosity over the equilibrium values predicted by the baseline model. Analysis shows that the inclusion of the lag model significantly improves the results where there is strong shock-induced separation.
Tue, 01 Mar 2005 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1162732005-03-01T00:00:00Z
- Experimental investigation of circular collared jets with inclined and non-inclined exit geometrieshttps://scholarbank.nus.edu.sg/handle/10635/116724Title: Experimental investigation of circular collared jets with inclined and non-inclined exit geometries
Authors: Zeng, Y.; Chng, T.L.; Tsai, H.M.
Abstract: An experimental study was conducted to investigate the general characteristics of circular collared nozzles with inclined and non-inclined collar exit geometries. In particular, the jet mixing response to a variation in collar length was also examined. The Reynolds number based on nozzle diameter and nozzle exit centerline velocity was approximately 30 000 and a collar to nozzle diameter ratio of 3 was chosen for all experiments. The results show that both an increase in collar length, L/d and inclination angle, θ lead to an improvement in the jet mixing behaviour. For both the non-inclined and inclined geometries, an increase in L/d enhances the centerline velocity decay, increases the near field fluctuating intensity and yields larger jet spreads. This is probably due to the higher shear imparted by the presence of the collar wall as compared to the unbounded flow. Similar results are observed with an increase in the inclination angle but this augmentation levels out at θ = 40 °. The effect of collar inclination upsets the axisymmetry of the flow field and causes the flow to deviate towards the azimuthal location where the collar lip protrudes out the furthest. This asymmetry becomes increasingly pronounced as the inclination angle is increased. Finally, the effects of flow reattachment are largely decoupled from any flow instability mechanism and do not lead to any strong form of self-excitation. Instead, there is evidence to show that flow reattachment accelerates the core flow and leads to a poorer jetmixing response.
Mon, 01 Jan 2007 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1167242007-01-01T00:00:00Z
- Mixing enhancement of high-bypass turbofan exhausts via contouring of fan nozzlehttps://scholarbank.nus.edu.sg/handle/10635/116753Title: Mixing enhancement of high-bypass turbofan exhausts via contouring of fan nozzle
Authors: Debiasi, M.; Dhanabalan, S.S.; Tsai, H.M.; Papamoschou, D.
Abstract: The purpose of this paper is to consider experimentally and computationally the use of axial flow to enhance the mixing of the jets from high-bypass turbofan nozzles. Mixing enhancement is caused by flow separation in the convergent-divergent secondary (fan) nozzle operated at overexpanded conditions. The experiments were conducted on a scaleddown version of a typical turbofan engine exhaust. Here the exit-to-throat area ratio of the secondary nozzle is conveniently adjusted via the axial position of the secondary nozzle relative to the primary nozzle exit. Mean velocity surveys of the jet plume were taken at Mach numbers in the range 0.6-1.0 representative of modern turbofan engines. The degree of jet mixing enhancement depends primarily on the nozzle area ratio. The effect of nozzle pressure ratio, which ranged from 1.3 to 1.9, is relatively minor. The high-velocity region of the jet is reduced by about 3 fan diameters and the primary potential core region practically eliminated at nozzle area ratios at or above 1.4. Selected jets were also investigated computationally. The computations capture the salient physics of flow separation and reproduce well the experimentally observed reduction of the potential core region in the vicinity of the nozzle. Further downstream, however, the computations do not capture accurately the experimental trends.
Mon, 01 Jan 2007 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1167532007-01-01T00:00:00Z
- Aerodynamic design of turbine blades using an adjoint equation methodhttps://scholarbank.nus.edu.sg/handle/10635/116688Title: Aerodynamic design of turbine blades using an adjoint equation method
Authors: Wu, H.-Y.; Liu, F.; Tsai, H.-M.
Abstract: Aerodynamic design of turbine blades using an adjoint equation method is studied. Two design cases are tested. The first one is an inviscid design case for a VKI turbine stator, and the design objective is to minimize the entropy generation rate of the blade subject to a prescribed blade loading. The second case is a viscous design case for a standard configuration 4 turbine stator. The design objective is to minimize the entropy generation rate subject to a prescribed mass-averaged exit flow angle. The penalty function method is applied to deal with the constrained optimization problems. A resultant cost function is defined as a weighted sum of the original cost function and the deviation from the constraint. The formulations of the adjoint systems are derived for both cases based on the flow governing equations and the design objectives. Numerical programs are implemented to perform the optimization design. For the inviscid design case, the method is able to effectively reduce the entropy generation rate while the constraint is precisely satisfied. Reduction of shock wave strength is observed. For the viscous design case, results using the Baldwin-Lomax turbulence model and results using laminar flow solutions are presented. The program is effective for both transonic and subsonic conditions, which means the method is able to deal with frictional effects in addition to reducing shock wave strength.
Sat, 01 Jan 2005 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1166882005-01-01T00:00:00Z
- Computation of the flows over flapping airfoil by the euler equationshttps://scholarbank.nus.edu.sg/handle/10635/116703Title: Computation of the flows over flapping airfoil by the euler equations
Authors: Yang, S.; Luo, S.; Liu, F.; Tsai, H.-M.
Abstract: To investigate the mechanism of thrust generation by Happing airfoils, the inviscid version of a three-dimensional unsteady compressible Euler/Navier Stokes flow solver is used to simulate the flow field around flapping airfoil NACA 0012 in an uniform stream of low speeds. Sinusoidally plunging or/and pitching oscillations are studied. The compressible Euler code with a low free-stream Mach number of 0.1 or 0.05 can simulate the incompressible flows without leading-edge separation. The wake-flow structures are visualized by the numerical methods: vorticity filled-contours, perturbation-velocity vector plots, and streamlines of the velocity field relative to the flapping airfoil. The computed wake-vortex structures and the time-averaged thrust coefficients, input-power coefficients, and the efficiency over a period of the oscillation versus the Strouhal number based on the total excursion of the trailing edge of the airfoil for a number of plunging and pitching cases agree well with the test data and the non-linear incompressible potential-flow solutions in the literature. The agreement with the test data decreases when leading-edge vortices appear and become strong enough to interfere with the trailing-edge vortices.
Sat, 01 Jan 2005 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1167032005-01-01T00:00:00Z
- Parallel computation of wing flutter with a coupled Navier-Stokes/CSD methodhttps://scholarbank.nus.edu.sg/handle/10635/116767Title: Parallel computation of wing flutter with a coupled Navier-Stokes/CSD method
Authors: Sadeghi, M.; Yang, S.; Liu, F.; Tsai, H.M.
Abstract: A code is developed for the computation of three-dimensional aeroelastic problems such as wing flutter. The unsteady Navier-Stokes flow solver is based on a flnite-volume approach with centered flux discretization and artificial diffusion. For the structural displacements a modal approach is applied. The temporal discretization is implicit for both the flow equations and the structural equations. An explicit dual-time method is used to integrate the coupled governing equations. A multigrid method is applied to advance the flow solution, and the computation is performed in parallel with a multiblock approach. A supercritical 2-D wing and the AGARD 445.6 wing serve as test cases for flutter investigations. Results for inviscid flow are compared with results obtained by solving the Navier-Stokes equations with the Baldwin-Lomax and k-ω turbulence models, respectively. Inclusion of viscous effects is critical for the 2-D wing. LCO of the 2-D wing is predicted, but with larger amplitude compared to experimental measurements. Predicted flutter boundary for the AGARD wing agrees well with experimental data in subsonic and transonic range but deviates significantly from experimental data in the supersonic range. Inclusion of viscous effects only slightly improves the result for this case. © 2003 by the authors. Published by the American Institute of Aeronautics and Astronautics, Inc.
Wed, 01 Jan 2003 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1167672003-01-01T00:00:00Z
- Trailing-edge flow about unstalled plunging airfoil computed by Euler methodhttps://scholarbank.nus.edu.sg/handle/10635/116798Title: Trailing-edge flow about unstalled plunging airfoil computed by Euler method
Authors: Yang, S.; Luo, S.; Liu, F.; Tsai, H.-M.
Abstract: The flow around the trailing edge of a sinusoidally plunging NACA 0012 airfoil at zero mean angle of attack and low speed is studied by an unsteady compressible Euler flow solver. The Euler-computed load distribution is decomposed into harmonic components. The first component is dominant and compared with the classical linear theory. The Euler computation predicts the load magnitude approaching zero toward the trailing edge as the linear theory predicts when the reduced frequency is below about 0.4. When the reduced frequency is beyond about 0.4, the Euler solution yields a non-zero load magnitude and a load phase lag for the aft-load and the deviation from the linear theory increases with reduced frequency (0.4 - 4.0) and free-stream Mach number (0.05 - 0.2). The Euler solver is validated by a water-tunnel visualization of near wake in the literature. The computed aftload behavior and flow around the trailing edge are qualitatively confirmed with available experimental data and Navier-Stokes computation.
Sun, 01 Jan 2006 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1167982006-01-01T00:00:00Z
- A CFD and CSD interaction algorithm for large and complex configurationshttps://scholarbank.nus.edu.sg/handle/10635/129723Title: A CFD and CSD interaction algorithm for large and complex configurations
Authors: Lai, K.L.; Tsai, H.M.; Lum, K.-Y.
Abstract: A numerical scheme, based on the formulations of the Boundary Element Methods (BEM) for linear elasticity, has been developed to relate the displacements at the boundary of a structure to the prescribed displacements at its interior. The algorithm can be applied in aeroelastic study of flexible structures, where computational fluid dynamics (CFD) and computational structural dynamics (CSD) are involved, to map the force and displacement between the CFD and CSD solvers. The scheme is designed to handle large and complex structures such as an aircraft, and to be in line with the component-based approach in defining such objects. This is facilitated with the use of a multi-region approach, in which, the different regions are considered as separate BEM models during the phase of building the system matrices, handling only one region at a time. With the regional system matrices generated, we can assemble them into a single global matrix for the whole structure. The paper presents the underlying principles and development of the numerical scheme. © 2002 by the author(s).
Tue, 01 Jan 2002 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1297232002-01-01T00:00:00Z
- An overset grid solver for viscous computations with multigrid and parallel computinghttps://scholarbank.nus.edu.sg/handle/10635/129724Title: An overset grid solver for viscous computations with multigrid and parallel computing
Authors: Cai, J.; Tsai, H.-M.; Liu, F.
Abstract: We describe an approach to simulate accurately viscous flows around complex configurations using overset grids. A combination of patched multi-block and overlapping grids is used to discretize the flow domain. A hierarchical grid system with different layers of grids of varying resolution ensures inter-grid connectivity within a multigrid solution acceleration framework. At each stage of the numerical computation, the block boundaries maintain a regular information exchange between neighboring blocks be it patched or overlapping boundaries. Coarse-grain parallel processing is facilitated by the multi-blocking system. Numerical results of flows over multi-element airfoils and three-dimensional turbulent flows around wing-body Aerodynamic configurations show the feasibility and efficiency of the method for large-scale numerical computations. © 2003 by the authors.
Wed, 01 Jan 2003 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1297242003-01-01T00:00:00Z
- Application of spline matrix for mesh deformation with dynamic multi-block gridshttps://scholarbank.nus.edu.sg/handle/10635/129725Title: Application of spline matrix for mesh deformation with dynamic multi-block grids
Authors: Lai, K.L.; Tsai, H.M.; Liu, F.
Abstract: The paper presents a three-dimensional mesh deformation algorithm for dynamic multiple-block moving mesh configurations. The flow domain is modelled as an elastic solid body where the Boundary Element Method (BEM) is applied to formulate a spline matrix that transforms the displacement vectors at a solid boundary to the interior of the field grid. Using a similar approach, a spline matrix for the interaction between the fluid grid and the structure grid of the flexible body can be generated. The BEM-based approach provides an unified treatment for both the flow mesh and the flexible body. For efficient implementation of deforming mesh, the BEM-based algorithm is augmented with a conventional grid deformation method based on transfinite interpolation (TFI). The BEM-based interpolation determines the boundary deformations of each block of the multiple-block flow mesh, while an arclength based TFI deforms the grid within each flow-block. © 2003 by the authors.
Wed, 01 Jan 2003 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1297252003-01-01T00:00:00Z
- Comparison of three geometric representations of airfoils for aerodynamic optimizationhttps://scholarbank.nus.edu.sg/handle/10635/129726Title: Comparison of three geometric representations of airfoils for aerodynamic optimization
Authors: Wu, H.-Y.; Yang, S.; Liu, F.; Tsai, H.-M.
Abstract: Three geometric representation methods for airfoils are compared for the design and optimization of turbomachinery cascades by an adjoint equation method. They are the PARSEC method, the Hicks-Henne shape function method, and the mesh-point method. Two-dimensional, transonic, and inviscid inverse design problems are studied. Comparisons are made for three different configurations including the NACA0015 airfoil, the 10th standard configuration compressor blade, and the VKI turbine nozzle blade. Gradient information for optimization is obtained by using a continuous adjoint equation method based on the Euler equations. Conjugate gradient method is adopted as the optimization scheme. The results suggest that the PARSEC method is not suitable for representing cascade blade shapes, the Hicks-Henne shape function method converges to the optimum faster than the mesh-point method does, but the mesh-point method can reach higher accuracy. © 2003 by the authors.
Wed, 01 Jan 2003 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1297262003-01-01T00:00:00Z
- Control of vortices over slender conical bodies - a theoretical and computational studyhttps://scholarbank.nus.edu.sg/handle/10635/129727Title: Control of vortices over slender conical bodies - a theoretical and computational study
Authors: Cai, J.; Tsai, H.-M.; Luof, S.; Liu, F.
Abstract: Pneumatic controls of vortices over slender conical bodies at high angles of attack and low speeds are studied by a theoretical method developed by Cai, Liu, and Luo (J. of Fluid Mech., vol. 480, 2003, pp.65-94) and verified by Euler computations. The theoretical method is based on an eigenvalue analysis on the motion of the vortices under small perturbations, which pertains to the absolute-type of instability. Steady blowing and suction are simulated by sources and sinks. A modification of the original model to account for the effects of the vortex core is implemented. The theory predicts the positions of stationary conical vortices and their stability. The numerical solver is based on a multi-block, multigrid, finite-volume method and parallel code for the steady and unsteady Euler and Navier-Stokes equations implemented on overset grids. The Euler algorithm has strictly symmetric characteristics, is capable to capture stationary symmetric vortex flows, and can simulate the flow-instability developments under small asymmetric temporal disturbances. Conical slot suction and blowing with a small amount of mass-flow-rate are introduced to stabilize the originally unstable stationary symmetric vortex flow over a circular-cone and a flat-plate delta wing combination, and to manipulate the vortices over a delta wing symmetrically to increase/decrease normal force and antisymmetrically to produce rolling moment. The theoretical predictions agree well with the Euler computations. © 2004 by the authors.
Thu, 01 Jan 2004 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1297272004-01-01T00:00:00Z
- Aerodynamic design optimization by the adjoint equation method on overset gridshttps://scholarbank.nus.edu.sg/handle/10635/115373Title: Aerodynamic design optimization by the adjoint equation method on overset grids
Authors: Liao, W.; Tsai, H.M.
Abstract: This paper deals with the use of the adjoint equation for aerodynamic shape optimization of complex configurations with overset grids methods. The use of overset grid eases the grid generation process, but the nontrivial task of ensuring communication between the overlapping grids needs careful attention. This need is effectively addressed by using a practically useful technique known as Implicit Hole Cutting (IHC) method first introduced by Lee and Baeder 1 and further demonstrated by the authors2 in a previous study. The method depends on the simplicity of the cell selection process based on the main criterion of cell size, and all grid points including hole interior points and hole fringe points are treated indiscriminately in the flow computation. The simplicity of the IHC method is exploited for the adjoint equation solver. Similar to the flow solver, the adjoint equations are solved on the conventional point-matched grids and overlapped grids within a multi-block framework. Parallel computing with MPI is also used to improve the overall efficiency of the design optimization process. The method is successfully applied to several two and three-dimensional shape optimization cases for both external and internal flows problems.
Sun, 01 Jan 2006 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1153732006-01-01T00:00:00Z
- Aircraft configuration design using a multidisciplinary optimization approachhttps://scholarbank.nus.edu.sg/handle/10635/115374Title: Aircraft configuration design using a multidisciplinary optimization approach
Authors: Rao, C.S.; Ray, T.; Tsai, H.M.
Abstract: At the conceptual phase of an aircraft design process, the aim is to determine the set of design features such as configuration arrangement, planform geometry, wing area, engine configuration and weight that meet various performance characteristics. Multidisciplinary design optimization (MDO) at this phase allows the designer to consider various options via simultaneous interactions of aerodynamics, propulsion, structures, stability and flight mechanics to arrive at better designs. Earlier attempts with MDO seek to optimize a design for a predetermined aircraft configuration or optimize a base design for different mission profiles. In this paper, we propose a MDO framework for conceptual aircraft design that considers different aircraft configurations simultaneously. The advantage of this approach is to allow the evolution of various aircraft configurations. The optimization algorithm used in the framework is based on computational intelligence which is a stochastic, zero-order, population based algorithm, especially suitable for multi-objective, constrained optimization problems involving computationally expensive functions. The work is aimed at providing insights in how different mission requirements dictate configuration choices. In the present work we study the evolution of different two-seater, propeller driven aircraft configurations for different mission requirements and we limit it to two configurations namely, conventional wing-tail and canard configurations.
Thu, 01 Jan 2004 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1153742004-01-01T00:00:00Z
- Control of flow separation in S-ducts via flow injection and suctionhttps://scholarbank.nus.edu.sg/handle/10635/115397Title: Control of flow separation in S-ducts via flow injection and suction
Authors: Debiasi, M.; Herberg, M.R.; Yan, Z.; Dhanabalan, S.S.; Tsai, H.M.
Abstract: This work explores the use of injection and suction as a mean to control the separation of flow in S-duct inlets. The overall goal is to reduce the distortion of the S-duct outlet flow and to improve its pressure recovery by using the least expenditure of energy. We also aim to understand to what extent computational means can be practically useful for optimizing this flow-control method. Computations and experiments were used to study the effect of injection and suction of 2% on the main stream in an S-duct of M2129 geometry. The control flow can be recirculated with advantages in term of mass flow and energy management. The benefit of the method is assessed by contrasting controlled flows to corresponding baseline (no-control) flows over a range of subsonic inlet conditions. The experimental measurements substantially validate the results from computation and indicate that the method is highly effective in controlling the flow separation. Copyright © 2008 by Temasek Laboratories - National University of Singapore.
Tue, 01 Jan 2008 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1153972008-01-01T00:00:00Z
- Swarm Algorithm for Single- and Multiobjective Airfoil Design Optimizationhttps://scholarbank.nus.edu.sg/handle/10635/115305Title: Swarm Algorithm for Single- and Multiobjective Airfoil Design Optimization
Authors: Ray, T.; Tsai, H.M.
Abstract: Shape optimization of airfoils involves highly expensive, nonlinear objective(s) and constraint functions often with functional and slope discontinuity that limits the efficient use of gradient-based methods for its solution. Gradient-based methods are not capable of generating a set of pareto solutions as required in multiobjective problems as they work with a single solution and improve it through successive iterations. Population-based, zero-order, stochastic optimization methods are therefore an attractive choice for shape optimization problems as they are easy to implement and effective for highly nonlinear problems. We present a swarm algorithm that is applicable for optimization problems in general, but is here explored for airfoil design optimization studies. The algorithm is based on a sociobehavioral model, and it offers the designer the desired flexibility to solve various unconstrained/constrained, single-/multiobjective forms of the airfoil shape optimization problem. The algorithm handles objectives and constraints separately via pareto ranking and is thus immune to problems of scaling and aggregation that commonly affect penalty-function-based constraint handling schemes. Three different airfoil design optimization problems have been solved to illustrate the algorithm's flexibility and its computational efficiency, which compare favorably with existing stochastic search methods.
Sun, 01 Feb 2004 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1153052004-02-01T00:00:00Z
- Vortical structures in a laminar V-notched indeterminate-origin jethttps://scholarbank.nus.edu.sg/handle/10635/115354Title: Vortical structures in a laminar V-notched indeterminate-origin jet
Authors: New, T.H.; Lim, K.M.K.; Tsai, H.M.
Abstract: A flow visualization investigation using dye-injection and laser-induced fluorescence techniques has been carried out to understand the vortex dynamics resulting from a V-notched indeterminate-origin jet with two peaks and two troughs. The laminar jet (Re=2000) was studied under forcing and nonforcing conditions to investigate the resultant dynamics of coherent large- and small-scale flow structures. Present experimental observations indicated that the effects of the nozzle peaks and troughs differ from those reported previously. Instead of the peaks producing streamwise vortex pairs which spread outwards into the ambient fluid and the troughs generating similar vortex pairs but entraining ambient fluid into the jet flows as indicated by earlier studies, the present experimental observations showed that both peaks and troughs produce outward-spreading streamwise vortex pairs. Laser cross sections further showed that the subsequent formation of azimuthal ring vortices causes these streamwise vortex pairs to be entrained. This entrainment causes the streamwise vortex pairs to "roll-up" together with the ring vortices, leading to intense flow interactions between them. Interestingly, in a comparison with the experimental study reported by Longmire et al. ["Control of jet structure by crown-shaped nozzles," AIAA J. 30, 505 (1992)] using higher Reynolds number air jet (Re=19 000), it was found that forced jet flows with four peaks and four troughs yielded practically the same flow observations as the present nozzles with two peaks and two troughs. An updated flow model based on instantaneous and time-averaged evidence is presented to explain how the interaction of the vortex structures will give rise to the present new observations. © 2005 American Institute of Physics.
Sun, 01 May 2005 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1153542005-05-01T00:00:00Z
- A novel method of flow injection and suction for lift enhancementhttps://scholarbank.nus.edu.sg/handle/10635/115366Title: A novel method of flow injection and suction for lift enhancement
Authors: Chng, T.L.; Zhang, J.; Tsai, H.M.
Abstract: This paper describes a novel method of injection and suction conducted on an airfoil in a manner analogous to Co-Flow Jet (CFJ) control. The objectives of this study are two-fold. First, in relation to the CFJ technique, to assess the effectiveness of a modified design in which the injection and suction slots conform to the surface of the original airfoil geometry. This is done with the aim of maintaining the baseline performance of the airfoil in the absence of any external fluid transfer. The second involves a comparison of two different fluid injection locations. The conformal surface of the modified airfoil is found to maintain its baseline performance and thus eliminates the need for continuous operation of the injection and suction processes required of the CFJ technique. The experimental results for counter-streamwise injection are less favorable than anticipated. Although the momentum coefficient observes a direct relationship with the indicative lift, the lowest value tested leads to a poorer performance than the baseline. Any lift augmentation seems to be largely confined to the immediate vicinity of the injection location and does not translate into an overall increase in suction. Streamwise injection however, produces notable increases in the pressure integral for relatively small momentum coefficients at all angles of attack. Flow separation is also effectively suppressed at high incidence angles of up to 20°. Based on the current experimental results, streamwise injection is clearly a more effective alternative compared to counter-streamwise injection. Copyright © 2008 by the authors.
Tue, 01 Jan 2008 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1153662008-01-01T00:00:00Z
- A numerical study of transonic buffet on a supercritical airfoilhttps://scholarbank.nus.edu.sg/handle/10635/115367Title: A numerical study of transonic buffet on a supercritical airfoil
Authors: Xiao, Q.; Tsai, H.M.; Liu, F.
Abstract: The steady/unsteady flow of the BGK No. 1 supercritical airfoil is investigated by the solution of the Reynolds-Averaged Navier-Stokes equations with a two-equation k-ω turbulence model. The dual-time stepping method is used to march in time. Two steady cases (Ma = 0.71, α = 1.396° and Ma = 0.71, α = 9.0°) and one unsteady case (Ma = 0.71, α = 6.97°), both with a far-stream Re = 20×10 6, are computed. The results are compared with experimental data obtained by Lee et al. The mechanism of the self-excited oscillation for supercritical airfoils is explored. The oscillation period calculated based on the mechanism proposed by Lee is consistent with the Fourier analysis of the computed lift coefficient.
Thu, 01 Jan 2004 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1153672004-01-01T00:00:00Z
- Mixing characteristics of elliptical jets with plughttps://scholarbank.nus.edu.sg/handle/10635/115456Title: Mixing characteristics of elliptical jets with plug
Authors: Debiasi, M.; Herberg, M.R.; Tsai, H.M.; Papamoschou, D.
Abstract: It is known that elliptical jets entrain surrounding air more effectively than round jets. Past studies also show that convergent-divergent nozzles operated at overexpanded conditions create flow instabilities that enhance the jet mixing. A convergent-divergent passage can be created between an elliptical nozzle and a round plug. The exit-to-throat height of the annular passage is non uniform around the nozzle. The aim of the present study is to investigate the characteristics of this arrangement which is expected to produce vigorous mixing. The mean velocity fields of subsonic-speed jets from such nozzle configuration were surveyed using an array of Pitot probes. Comparisons were made between jets using elliptical nozzles with plug at various pressure and exit-to-throat area ratios and jets from round, convergent nozzles with plug. The results indicate that elliptical nozzles with plug split the flow in two cores. This increases the mixing and promotes faster velocity decay in the plume. The flow instabilities associated to overexpanded conditions in the nozzle also contribute to enhance the mixing. Copyright © 2008 by Temasek Laboratories - National University of Singapore.
Tue, 01 Jan 2008 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1154562008-01-01T00:00:00Z
- Vortex dominated flow fields in jets from notched circular nozzleshttps://scholarbank.nus.edu.sg/handle/10635/115515Title: Vortex dominated flow fields in jets from notched circular nozzles
Authors: Cai, J.; Tsai, H.M.
Abstract: The vortex dominated flow field in jets from notched circular nozzles is investigated using numerical simulation. The nozzle denoted as S-shape notched nozzle (S-nozzle) considered in the present study has a lip which is shaped by a sine wave with two peaks and two troughs. A normal circular nozzle with the same circular cross-section is also considered both for comparison and for a better understanding of the vortex formed in the jet flow field. The vortex dynamics resulting from these two different nozzles were visualized by using a novel numerical dye visualization technique. The results indicated that the generation of streamwise vortex-pairs is due to the interaction between two opposite motions; the inward motion of the ambient fluid towards the jet axis and outwardly spreading motion of jet fluid, induced by two neighboring primary vortex-rings. These streamwise vortex-pairs are generated inside and also outside the vortex-rings and spread outward as the vortex-rings convect downstream. Comparison of the S-nozzles flow shows that for the notched S-nozzles, streamwise vortex-pairs are produced only at the peak and trough locations, in the course of the formation of azimuthal vortex-rings. This is unlike the circular nozzle where the number of streamwise vortex-pairs formed would depend on Reynolds number.
Sun, 01 Jan 2006 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1155152006-01-01T00:00:00Z
- Stability of vortex pairs over slender conical bodies - Theory and numerical computationshttps://scholarbank.nus.edu.sg/handle/10635/115499Title: Stability of vortex pairs over slender conical bodies - Theory and numerical computations
Authors: Cai, J.; Tsai, H.-M.; Luo, S.; Liu, F.
Abstract: Theoretical analyses and computational fluid dynamics (CFD) simulations are presented to study the formation and stability of stationary symmetric and asymmetric vortex pairs over slender conical bodies in an inviscid incompressible flow at high angles of attack with and without sideslip. The theoretical analysis is based on an eigenvalue analysis on the motion of the vortices under small perturbations. A three-dimensional time-accurate Euler code is used to compute five typical flows studied by the theoretical method on extraordinarily fine and overset grids with strict convergence criteria. The computational results agree well with the theoretical predictions and corroborate on the conclusion that an absolute type of hydrodynamic instability can be the mechanism for breaking of symmetry of the vortex flow over slender conical bodies at high angles of attack. The presented results demonstrate the usefulness of CFD for stability studies of vortex flow? over slender bodies at high angles of attack.
Thu, 01 Jan 2004 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1154992004-01-01T00:00:00Z
- Effect of traversal jet injections on skin friction in a turbulent channelhttps://scholarbank.nus.edu.sg/handle/10635/115408Title: Effect of traversal jet injections on skin friction in a turbulent channel
Authors: Tay, C.M.J.; Herberg, M.R.; Onorato, M.; Tsai, H.M.
Abstract: The effects of small spanwise inclined jets on the wall shear stress were investigated in a fully developed turbulent 2-D channel flow. Five small jets inclined at 45° in the spanwise direction are introduced only along the upper wall of a Perspex channel measuring 20mm by 300mm. Hot-wire measurements were carried out to measure both the shear stress and the velocity profiles on the upper wall at various positions downstream of the jet injections and for jet flow rates from about 1% to 5% of the that of the main channel flow. Local and spanwise averaged mean shear stress reductions of up to 50% and 17% respectively were measured under optimal conditions. The velocity profile development is tracked downstream at points of minimum shear stress. The results suggest that the spanwise forcing by the inclined jets is able to weaken the turbulence of the flow and increase coherence in the flow. Although different mechanisms seem to be responsible for the reduction in the mean shear stress, its reduction is likely to be largely the result of this increase coherence.
Mon, 01 Jan 2007 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1154082007-01-01T00:00:00Z
- Experimental study on deflagration-to-detonation transition enhancement methods in a PDEhttps://scholarbank.nus.edu.sg/handle/10635/115423Title: Experimental study on deflagration-to-detonation transition enhancement methods in a PDE
Authors: New, T.H.; Panicker, P.K.; Chui, K.F.; Tsai, H.M.; Lu, F.K.
Abstract: An experimental investigation was carried out to study the performance of a pulse detonation engine platform incorporating commercial, off-the-shelf solenoid valve gas injectors and non-conventional deflagration-to-detonation transition enhancing devices. The study made use of stoichiometric propane-oxygen mixtures with low-energy ignition sources. The gas injectors were observed to be sufficiently robust and operated reliably under the high working temperature and pressure conditions normally associated with pulsed detonation operations provided appropriate preventive measures were taken. One of the major motivations in utilizing these gas injectors lies in the ease and accuracy in controlling their injection operations electronically, which allows for tight integration with auxiliary electronic control and measurement systems. This paper reports on the initial success of integrating these gas injectors into a moderate-frequency pulsed detonation engine system as well as the effectiveness of the deflagration-to-detonation transition enhancing devices which included Shchelkin spiral, circumferential and helical grooves, as well as convergent-divergent throats. Lastly, operational insights in the practical use of gas injectors and the impact on pulse detonation operations are highlighted.
Sun, 01 Jan 2006 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1154232006-01-01T00:00:00Z
- Experimental investigations on DDT enhancements by shchelkin spirals in a PDEhttps://scholarbank.nus.edu.sg/handle/10635/115422Title: Experimental investigations on DDT enhancements by shchelkin spirals in a PDE
Authors: New, T.H.; Panicked, P.K.; Lu, F.K.; Tsai, H.M.
Abstract: An experimental investigation was carried out on a multi-cycle pulsed detonation engine, running on a propane-oxygen mixture using a rotary-valve injection system and a low energy ignition source, to study the effectiveness of Shchelkin spiral parameters on the deflagration-to-detonation transition (DDT) phenomenon. Various configurations were tested using spirals with blockage-ratios ranging from 34.7 to 55.6 % and spiral length to diameter ratio of 12.5 and 24.4. The results showed that only spirals with the highest blockage-ratio were able to achieve successful and sustained DDT in the shorter length configuration. However, further studies revealed that lower blockage-ratio spirals were able to achieve successful DDT when their lengths were 24.4 times that of the detonation tube diameter with corresponding higher levels of peak thrust production observed. Lastly, practical operating issues regarding the use of Shchelkin spirals are discussed in this paper.
Sun, 01 Jan 2006 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1154222006-01-01T00:00:00Z
- An algebraic elliptic grid generator with grid spacing controlhttps://scholarbank.nus.edu.sg/handle/10635/116045Title: An algebraic elliptic grid generator with grid spacing control
Authors: Dhanabalan, S.S.; Tsai, H.M.
Abstract: An Elliptic Multiblock grid generation technique is presented. Boundary orthogonality is imposed by floating the grid points along the boundary. A formulation is presented to maintain the grid spacing at the boundaries. For the case of three dimensions an improved search algorithm maintains the grid orthogonality on the surface along with grid spacing. The specification of orthogonal boundary condition as a natural outcome of grid generation process preserves the grid quality obtained by the standard elliptic solver. Grid clustering is done algebraically, thereby de-coupling it from the basic equation in the grid generation process. Elliptic smoothing is used to spread out the clustered grid points adaptively resulting in a very smooth, clustered grid. The advantage of floating grid points at the boundary is exploited to yield a customized clustered grid with smoother grid distribution. User interaction is aided with the help of a TCL (Tool Command Language) interpreter. Since the surface grid is generated along with the volume grid, the grid generation script can be readily reused to generate grids of similar topological structure. The features of grid generator has been demonstrated with typical examples.
Sun, 01 Jan 2006 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1160452006-01-01T00:00:00Z
- Design of wing-body configurations to delay the onset of vortex asymmetryhttps://scholarbank.nus.edu.sg/handle/10635/116061Title: Design of wing-body configurations to delay the onset of vortex asymmetry
Authors: Cai, J.; Tsai, H.-M.; Luo, S.; Liu, F.
Abstract: The stability analysis developed by Cai, Liu, and Luo (J. of Fluid Mech., vol. 480, 2003, pp.65-94) is augmented by a numerical conformal mapping technique in order to extend its applicability to problems with complex geometry. The extended method is applied to investigate the stability of symmetric vortices over wing-body combinations with various designs of the cross-sectional geometry through morphing a basic profile formed by a flat-plate wing and a circular-cone center-body. The effect of the thickness of the wing and the body, bending the wing tips, and contouring the cross-sectional proflies of the wing and the body are studied systematically. Through this process, it is discovered that a cross-sectional profile like that of a disturbed cobra has the largest stability range up to very high angles of attack for the symmetric vortices over a slender conical forebody.
Sun, 01 Jan 2006 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1160612006-01-01T00:00:00Z
- Numerical study of vortical flows shedding from a bluff body with a wavy trailing edgehttps://scholarbank.nus.edu.sg/handle/10635/116105Title: Numerical study of vortical flows shedding from a bluff body with a wavy trailing edge
Authors: Cai, J.; Chng, T.L.; Tsai, H.M.
Abstract: Previous experimental studies have shown that applying a geometric sinusoidal disturbance to the trailing edge of a quasi-streamlined bluff body can result in substantial base pressure recovery when compared against a normal straight edged configuration. Here, a numerical simulation at a Reynolds number of 2500 is used to examine in detail various aspects of the flow such as the base pressure distribution, vortex shedding mechanism and frequency selection in relation to a variation in the wavelength of the sinusoidal disturbance. The concentration of dye released along the upper trailing edge of the body is tracked by solving the scalar transport equation to facilitate visualization and comparison of the flow field analogous to the dye precipitation technique used in previous studies. The results show that a sinusoidally shaped trailing edge with fixed amplitude and a wavelength which is 5 times the base height displays a reduction of more than 36% in the mean drag coefficient when compared to the straight edged case. This reduction is traced to a lengthening of the mean recirculation region, a marked drop in the velocity fluctuation at the dominant frequency of the wake and a reduction in the spanwise component of the wake vorticity. Conversely, this reduction in the drag coefficient is substantially diminished for a sinusoidal trailing edge with a longer wavelength. For such a configuration, the flow is characterized by the formation of vortex dislocations which causes the flow to have a less regular character. Finally, a shorter wavelength of four base heights results in a reduction of the mean recirculation region with an attendant increase in the spanwise vorticity. This leads to a drag coefficient similar to that of the straight trailing edge.
Mon, 01 Jan 2007 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1161052007-01-01T00:00:00Z
- Flutter simulation and prediction with CFD-based reduced-order modelhttps://scholarbank.nus.edu.sg/handle/10635/116085Title: Flutter simulation and prediction with CFD-based reduced-order model
Authors: Lai, K.L.; Tsai, H.M.
Abstract: The paper presents a numerical aeroelastic simulation and prediction environment for applications to complex configurations. The simulation environment embeds a high-order computational fluid dynamic (CFD) code and a reduced-order modeling (ROM) technique. To set up the simulation system, the CFD is applied first to the full-order model to obtain the non-aerodynamically loaded aeroelastic behavior in accordance with the Filter-Impulse Method (FIM). A system identification and reduction scheme is then used to construct the ROMs from the FIM responses. The adoption of ROM permits a black box evaluation of the aeroelastic system in such a way that after a single full order aeroelastic simulation for one flow condition (at a given Mach number) the system state for other flow conditions (at that Mach number) might be estimated. The ROMs are in state space form and so can easily be coupled to a structural model for aeroelastic calculations. Pilot studies?,? have shown that the ROMs are capable of predicting accurately the flutter boundary in a tiny fraction of the time required by full-order CFD code. The crucial first step, and also one major difficulty, in reduced-order modeling is the generation of ROMs that retain the accuracy of the full nonlinear CFD methods. Following previous work,? we present in the paper improved method of model reduction for complex configurations. Applications to fighter-type aircraft, the NORTHROP F-5E TIGER II, will be presented. The reduced-order model is compared to the full-order model, demonstrating the practical suitability of reduced-order model for complex configurations. Computations of flutter boundary using ROMs will also be presented with the AGARD 445.6 test cases.
Mon, 01 Jan 2007 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1160852007-01-01T00:00:00Z
- Flutter simulation and prediction with CFD-based reduced-order modelhttps://scholarbank.nus.edu.sg/handle/10635/116084Title: Flutter simulation and prediction with CFD-based reduced-order model
Authors: Lai, K.L.; Won, K.S.; Koh, E.P.C.; Tsai, H.M.
Abstract: The paper presents an approach for the simulating and predicting flutter of complex configurations. One of the major difficulties in direct numerical simulation is the extensive computational time required. A hybrid approach combining CFD and reduced-order model (ROM) on the other hand takes advantage of the accuracy of CFD-based computations and the efficiency of a ROM model. In this work, the CFD-based aeroelastic computations involve a Cartesian-based Euler solver with embedded multi-grid sequencing for flow computations, using the small perturbation techniques to implement the unsteady boundary conditions on the stationary grid. The structural response of the system is computed using the mode superposition technique. Data communication between the non-matching fluid and structure domains is performed using the Constant Volume Tetrahedron (CVT) interpolation method. For a given set of flight conditions, the CFD solver is first performed to compute the flow solutions for a prescribed input that are needed to construct the reduced-order model of the system. The ROM results are compared with the CFD-solver results. The prediction of flutter boundary for the AGARD 445.6 wing is presented. Copyright © 2006 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.
Sun, 01 Jan 2006 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1160842006-01-01T00:00:00Z
- Flutter computations of complex configurations using cartesian gridshttps://scholarbank.nus.edu.sg/handle/10635/116083Title: Flutter computations of complex configurations using cartesian grids
Authors: Lai, K.L.; Koh, E.P.C.; Tsai, H.M.
Abstract: We present in the paper a practical approach to compute the fluid-structure interaction of complex configurations. A Cartesian-based Euler solver with embedded multi-grid sequencing is used for the flow computation. The Cartesian approach significantly simplifies the process of grid generation for complex configuration. The unsteady flow computation is performed on a set of stationary Cartesian grids using the small perturbation techniques. The full boundary conditions for the Euler equations on the moving surface are replaced by approximate boundary conditions on the stationary grid. On the structural side, the modal superposition technique is applied to analyse the structural response under the action of the aerodynamic forces. The coupling between the fluid and structure domains is implemented using the Constant Volume Tetrahedron (CVT) transformation method, by which the aerodynamic forces are transferred to the structural domain and the structural deformation to the fluid domain. This enables a tight fluid-structure coupling. The present numerical scheme is applied in flutter analysis of an AGARD 445.6 wing and of a complete aircraft. In the former case, results are compared with published results in order to demonstrate the present numerical scheme. In the latter case, the complex shape of the NORTHROP F-5E TIGER II with wing-tip missiles is used to demonstrate the overall capability of the present scheme.
Sun, 01 Jan 2006 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1160832006-01-01T00:00:00Z