ScholarBank@NUShttps://scholarbank.nus.edu.sgThe DSpace digital repository system captures, stores, indexes, preserves, and distributes digital research material.Fri, 03 Apr 2020 21:49:34 GMT2020-04-03T21:49:34Z50951- Effects of noncircular collars on an axisymmetric jethttps://scholarbank.nus.edu.sg/handle/10635/115080Title: Effects of noncircular collars on an axisymmetric jet
Authors: New, T.H.; Tan, K.S.; Tsai, H.M.
Abstract: An experimental investigation was carried out to study the effects of noncircular collars on an axisymmetric jet using surface flow visualization and hot-wire anemometry. Circular, square, and triangular collars with expansion ratios of 1.20, 1.35, and 1.54, respectively, with collar lengths of up to two jet diameters were used. Flow visualization shows that circular collars led to equidistant flow reattachments along the collar wall, while square and triangular collars resulted in the formation of a pair of counter-rotating vortex-pairs on each side of the collar wall. These vortex-pairs are caused by the presence of the three-dimensional velocity gradients between locations of minimum and maximum step-heights, which drove fluid from the collar wall centerlines towards the corners. Time-averaged velocity measurements show that the circular collar required the shortest collar length to achieve maximum centerline velocity decay, followed by square and triangular collars. Centerline turbulence intensity and velocity spectra results reveal that all three collars were able to suppress vortex-pairing events when they were sufficiently long with the triangular collar being the most effective. Furthermore, the triangular collar also produces the widest overall jet-spread, ahead of square and circular collars, respectively, even though it demonstrates significantly different jet-spreads along planes of minimum and maximum step-height. Self-excitation frequencies from all three collars could be distinguished into distinct frequency bands with changes in the collar lengths. Within each band, the circular collar results in the largest variation of excitation frequency over the square and triangular collars, respectively. Lastly, square and triangular collars require wider ranges of collar lengths for each of the frequency bands, as compared to the circular collar. © 2007 American Institute of Physics.
Wed, 01 Aug 2007 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1150802007-08-01T00:00:00Z
- Numerical investigation of supersonic nozzle flow separationhttps://scholarbank.nus.edu.sg/handle/10635/115209Title: Numerical investigation of supersonic nozzle flow separation
Authors: Xiao, Q.; Tsai, H.M.; Papamoschou, D.
Abstract: Separation of supersonic flow in a planar convergent-divergent nozzle with moderate expansion ratio is investigated by solving the Reynolds-averaged Navier-Stokes equations with a two-equation k-ω turbulence model. The focus of the study is on the structure of the fluid and wave phenomena associated with the flow separation. Computations are conducted for an exit-to-throat area ratio of 1.5 and for a range of nozzle pressure ratios. The results are compared with available experimental data in a nozzle of the same geometry. The flow separates by the action of a lambda shock, followed by a succession of expansion and compression waves. For 1.5 < NPR < 2.4, the computation reveals the possibility of asymmetric flow structure. The computationally obtained asymmetric flow structures are consistent with previous experimental flow visualizations studies. In addition, other flow features such as shock location and wall pressure distributions are also in good agreement with the experimental data. The present study provides new information that confirms earlier conjectures on the flow-wave structure relevant to the instability of the separated flow in convergent-divergent nozzles of moderate expansion ratio.
Thu, 01 Mar 2007 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1152092007-03-01T00:00:00Z
- Numerical study of transonic buffet on a supercritical airfoilhttps://scholarbank.nus.edu.sg/handle/10635/111616Title: Numerical study of transonic buffet on a supercritical airfoil
Authors: Xiao, Q.; Tsai, H.M.; Liu, F.
Abstract: The flow of the Bauer-Garabedian-Korn (BGK) No. 1 supercritical airfoil is investigated by the solution of the unsteady Reynolds-averaged Navier-Stokes equations with a two-equation lagged k-ω turbulent model. Two steady cases (M = 0.71, α = 1.396 deg and M = 0.71, α = 9.0 deg) and one unsteady case (M = 0.71, α = 6.97 deg), all with a far-stream Reynolds number of 20 × 106, are computed. The results are compared with available experimental data. The computed shock motion and the evolution of the concomitant flow separation are examined. Space-time correlations of the unsteady pressure field are used to calculate the time for pressure waves to travel downstream within the separated region from the shock wave to the airfoil trailing edge and then back from the trailing edge to the shock outside the separated region. The reduced frequency so calculated agrees well with the computed buffet frequency, supporting the signal propagation mechanism for buffet proposed by Lee (Lee, B. H. K., "Oscillation Shock Motion Caused by Transonic Shock Boundary-Layer Interaction," AIAA Journal, Vol. 28, No. 5, 1990, pp. 942-944).
Wed, 01 Mar 2006 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1116162006-03-01T00:00:00Z
- A multigrid overset grid flow solver with implicit hole cutting methodhttps://scholarbank.nus.edu.sg/handle/10635/111312Title: A multigrid overset grid flow solver with implicit hole cutting method
Authors: Liao, W.; Cai, J.; Tsai, H.M.
Abstract: In the computation of flow over complex configurations, the use of overset grids eases the grid generation process, but the non-trivial task of ensuring communication between the overlapping grids must first be established. This need is efficiently addressed by using a new practically useful implicit hole cutting (IHC) method introduced by Lee and Baeder. This approach is a 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. Potentially these features ease the implementation of the multigrid algorithms which are rarely used in overset methods. The present study addresses the use of IHC method as an inter-grid communication method within a hybrid multi-block framework amenable for the implementation of a multigrid method and parallel computation. The convergence, accuracy and efficiency of the current solver are assessed particularly for the large-scale computations of realistic aerodynamic configurations. © 2006 Elsevier B.V. All rights reserved.
Thu, 01 Feb 2007 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1113122007-02-01T00:00:00Z
- Aerodynamic shape optimization on overset grids using the adjoint methodhttps://scholarbank.nus.edu.sg/handle/10635/114990Title: Aerodynamic shape optimization on overset grids using the adjoint method
Authors: Liao, W.; Tsai, H.M.
Abstract: This paper deals with the use of the continuous adjoint equation for aerodynamic shape optimization of complex configurations with overset grids methods. While the use of overset grid eases the grid generation process, the non-trivial task of ensuring communication between overlapping grids needs careful attention. This need is effectively addressed by using a practically useful technique known as the implicit hole cutting (IHC) method. The method depends on a simple cell selection process based on the criterion of cell size, and all grid points including interior points and fringe points are treated indiscriminately in the computation of the flow field. This paper demonstrates the simplicity of the IHC method for the adjoint equation. Similar to the flow solver, the adjoint equations are solved on conventional point-matched and overlapped grids within a multi-block framework. Parallel computing with message passing interface is also used to improve the overall efficiency of the optimization process. The method is successfully demonstrated in several two-and a three-dimensional shape optimization cases for both external and internal flow problems. Copyright © 2009 John Wiley & Sons, Ltd.
Thu, 01 Apr 2010 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1149902010-04-01T00:00:00Z
- Aeroelastic simulations using gridless boundary condition and small perturbation techniqueshttps://scholarbank.nus.edu.sg/handle/10635/111322Title: Aeroelastic simulations using gridless boundary condition and small perturbation techniques
Authors: Lai, K.L.; Carolina, L.; Tsai, H.M.
Abstract: This paper examines the use of stationary Cartesian mesh for non-linear flutter computations involving complex geometries. The surface boundary conditions are implemented using reflected points which are determined via a gridless approach. The method uses a cloud of nodes in the vicinity of the surface to get a weighted-average of the flow properties using radial basis functions. To ensure computational efficiency and for local grid refinements, multigrid computations within an embedded grids framework are used. As the displacements of moving surfaces from their original position are typically small for flutter problems, a small perturbation boundary condition method is used to account for the moving surfaces. The method therefore does not require repeated grid re-generation for the deforming surfaces. The overall method is both accurate and robust. Computations of the well-known Onera M6 wing, RAE wing-body configuration, the AGARD 445.6 wing flutter test case show good accuracy and efficiencies. Simulations of the aeroelastic behavior of a complete fighter-type aircraft with wing tip missiles at high transonic speeds further demonstrate the practical usefulness of the present boundary conditions technique. © 2010 Elsevier Ltd. All rights reserved.
Sat, 01 May 2010 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1113222010-05-01T00:00:00Z
- Determining the critical condition for turbulent transition in a full-developed annulus flowhttps://scholarbank.nus.edu.sg/handle/10635/84971Title: Determining the critical condition for turbulent transition in a full-developed annulus flow
Authors: Dou, H.-S.; Khoo, B.C.; Tsai, H.M.
Abstract: Axial flow in an annulus between two concentric cylinders is commonly seen in various flow devices used in chemical processing industries and petroleum science and engineering. The flow state in the annulus strongly influences the performance of fluid transportation in the devices. Therefore, the determination of flow state which is laminar flow or turbulent flow is an important task to predict the performance of the flow devices. In previous works, we have proposed an energy gradient method for studying the flow instability and turbulent transition. In this method, it is shown that the flow instability and turbulent transition in wall-bounded shear flows depend on the relative magnitude of the gradient of the total mechanical energy in transverse direction and the rate of loss of the total mechanical energy along the streamwise direction for a given imposed disturbance. For pipe and plane Poiseuille flows, it has been demonstrated that the transition to turbulence for these wall-bounded parallel flows occurs at a consistent value of the energy gradient parameter (Kmax). In the present study, the critical condition for turbulent transition in annulus flow is calculated with the energy gradient method for various radius ratios. The critical flow rate and critical Reynolds number are given for various radius ratios. Then, the analytical results are compared with the experiments in the literature. Finally, the implication of the result is discussed in terms of the drag reduction and mixing as well as heat transfer in practical industrial applications of various fluid delivery devices. © 2010 Elsevier B.V.
Sun, 01 Aug 2010 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/849712010-08-01T00:00:00Z
- A combined experimental and analytical investigation of the vortex stability over sharp-edged slender bodieshttps://scholarbank.nus.edu.sg/handle/10635/111309Title: A combined experimental and analytical investigation of the vortex stability over sharp-edged slender bodies
Authors: Cai, J.S.; Cui, Y.D.; Tsai, H.M.
Abstract: A combined experimental and analytical investigation is conducted to assess the stability of the symmetric vortex pair over three different sharp-edged slender bodies at high angles of attack. The analysis is based on a previously developed eigenvalue stability method. Corresponding experimental studies involving flow visualization in the water tunnel, force and pressure measurement in the wind tunnel are carried out. The angles of attack tested range from 15° to 40°. Overall, the experiments confirm the stability behaviors as determined from the analysis. The analytical solutions indicate that an absolute type of instability is the mechanism for the breakdown of symmetry of the vortex flow over slender conical bodies at high angles of attack. Both the experiments and the analysis reveal that a sharp-edged slender body with a smaller volume on the leeward side and a bigger volume on the windward side increases the overall stability of the symmetric vortex pair. A shape with extended sharp edges in the outboard direction on its sides further enhances the stability of the vortex pair. The analytical method can be a useful tool for exploring shapes that favor stability or instability. © 2007 American Institute of Physics.
Wed, 01 Aug 2007 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1113092007-08-01T00:00:00Z
- Forebody slot blowing on vortex breakdown and load over a delta winghttps://scholarbank.nus.edu.sg/handle/10635/85220Title: Forebody slot blowing on vortex breakdown and load over a delta wing
Authors: Cui, Y.D.; Lim, T.T.; Tsai, H.M.
Abstract: Previous study (Cui, Y. D., Lim, T. T., and Tsai, H. M., "Control of Vortex Breakdown Over a Delta Wing Using Forebody Slot Blowing," AIAA Journal, Vol. 45, No. 1, 2007, pp. 110-117.) shows that a forebody slot blowing technique significantly delays vortex breakdown over a delta wing. Blowing only on one side while delaying vortex breakdown has an opposite effect on the nonblowing side. Although the authors provided a plausible explanation for the observed behavior, no concrete evidence was given. In this paper, we address this issue and further evaluate the effects of symmetric and differential blowing on the aerodynamic loads. Flow visualization and force measurement were carried out in a water tunnel at a Reynolds number of 8.5 x 104. Our study shows that the differing effect of differential blowing can be traced to a combination of forebody slot blowing itself and the interaction of the vortices from both sides resulting in a side-slip-like effect Moreover, symmetrical forebody slot blowing, apart from producing a significant delay in the formation of vortex breakdown, increases the lift by more than 5 %. Differential blowing can be used to manipulate the vortex breakdown position and change the roll moment of the wing, which suggests that the method can be a potential means for roll control.
Sat, 01 Mar 2008 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/852202008-03-01T00:00:00Z
- On the use of notched collars on an axisymmetric jethttps://scholarbank.nus.edu.sg/handle/10635/115213Title: On the use of notched collars on an axisymmetric jet
Authors: Zeng, Y.; New, T.H.; Tsai, H.M.
Abstract: This paper reports an experimental study conducted on the use of circular, V- and A-notched collars with the expansion-ratio of 3 for a Re = 20,000 axisymmetric jet by using hot-wire anemometry. Results show that limited specific differences in the centerline velocity decay and turbulent intensity levels arise from the use of notched collars. The overall flow field of the collared-jets and their self-excitation mechanisms are also not significantly influenced by modifications to the axial distance distribution of the collars, at least for the notch configurations used in the present study. Observations gathered in the present study suggest that altering the collar cross-sectional geometry, length and expansion-ratio remains to be more effective control techniques for collared-jets. © 2009 Elsevier Inc.
Tue, 01 Sep 2009 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1152132009-09-01T00:00:00Z
- Euler calculations with embedded Cartesian grids and small-perturbation boundary conditionshttps://scholarbank.nus.edu.sg/handle/10635/115098Title: Euler calculations with embedded Cartesian grids and small-perturbation boundary conditions
Authors: Liao, W.; Koh, E.P.C.; Tsai, H.M.; Liu, F.
Abstract: This study examines the use of stationary Cartesian mesh for steady and unsteady flow computations. The surface boundary conditions are imposed by reflected points. A cloud of nodes in the vicinity of the surface is used to get a weighted average of the flow properties via a gridless least-squares technique. If the displacement of the moving surface from the original position is typically small, a small-perturbation boundary condition method can be used. To ensure computational efficiency, multigrid solution is made via a framework of embedded grids for local grid refinement. Computations of airfoil wing and wing-body test cases show the practical usefulness of the embedded Cartesian grids with the small-perturbation boundary conditions approach. © 2010 Elsevier Inc.
Sat, 01 May 2010 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1150982010-05-01T00:00:00Z
- Experimental investigations on indeterminate-origin V- and A-notched jetshttps://scholarbank.nus.edu.sg/handle/10635/115103Title: Experimental investigations on indeterminate-origin V- and A-notched jets
Authors: New, T.H.; Tsai, H.M.
Abstract: Experimental studies on aspect ratio of 1.5 indeterminate-origin V- and A-notched nozzles using flow visualization and hot-wire anemometry techniques are reported here. Time-sequenced flow visualization images of forced water jets at Reynolds number of 2000 show that the general vortex dynamics of the two nozzle types are largely similar despite the differences in the peak and trough configurations. In addition to vortex lines emanating from both nozzles which result in streamwise vortex pairs forming along both peak and trough locations, consistent bending of the large-stale ring vortices is also observed at some downstream distances away from both nozzle exits, which suggests some limited form of axis-switching behavior. Velocity measurements for unforced air jets issuing from these nozzle types at Reynolds number of 20,500 show that there exist subtle yet discernible quantitative flow differences depending on whether the peak and trough contours are smooth or sharp. Sharp peaks and smooth troughs result in wider jet widths over smooth peaks and sharp troughs, respectively, although the differences become smaller with increasing downstream distance. Jet-width comparisons also reveal crossover locations and hence distortions to the jet cross sections, which further supports the notion of axis-switching behavior observed during flow visualizations. Two axis-switching locations have been found for each of the two nozzle types with the gross general flow behavior similar to those observed earlier for free elliptic jets. Copyright © 2007 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.
Sun, 01 Apr 2007 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1151032007-04-01T00:00:00Z
- Experimental and numerical study on indeterminate-origin V-notched jethttps://scholarbank.nus.edu.sg/handle/10635/115421Title: Experimental and numerical study on indeterminate-origin V-notched jet
Authors: New, T.H.; Cai, J.; Tsai, H.M.
Abstract: An experimental and numerical study has been carried out on an indeterminate-origin V-notched jet to understand the pertinent vortex dynamics involved in the resultant flow. Laser cross-sections showed that outward-spreading streamwise vortices are formed at both peak and trough locations. The numerical simulation was also able to reproduce the general flow features observed in the same study by employing numerical dye-visualization. In view of the observations, a vortex flow model is proposed to account for the phenomenon. © World Scientific Publishing Company.
Tue, 20 Dec 2005 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1154212005-12-20T00:00:00Z
- Simulations of detonation wave propagation in rectangular ducts using a three-dimensional WENO schemehttps://scholarbank.nus.edu.sg/handle/10635/61313Title: Simulations of detonation wave propagation in rectangular ducts using a three-dimensional WENO scheme
Authors: Dou, H.-S.; Tsai, H.M.; Khoo, B.C.; Qiu, J.X.
Abstract: This paper reports high resolution simulations using a fifth-order weighted essentially non-oscillatory (WENO) scheme with a third-order TVD Runge-Kutta time stepping method to examine the features of detonation front and physics in square ducts. The simulations suggest that two and three-dimensional detonation wave front formations are greatly enhanced by the presence of transverse waves. The motion of transverse waves generates triple points (zones of high pressure and large velocity coupled together), which cause the detonation front to become locally overdriven and thus form "hot spots." The transversal motion of these hot spots maintains the detonation to continuously occur along the whole front in two and three dimensions. The present simulations indicate that the influence of the transverse waves on detonation is more profound in three dimensions and the pattern of quasi-steady detonation fronts also depends on the duct size. For a "narrow" duct (4 L × 4 L where L is the half-reaction length), the detonation front displays a distinctive "spinning" motion about the axial direction with a well-defined period. For a wider duct (20 L × 20 L), the detonation front exhibits a "rectangular mode" periodically, with the front displaying "convex" and "concave" shapes one following the other and the transverse waves on the four walls being partly out-of-phase with each other. © 2008 The Combustion Institute.
Mon, 01 Sep 2008 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/613132008-09-01T00:00:00Z
- Control of vortex breakdown over a delta wing using forebody slot blowinghttps://scholarbank.nus.edu.sg/handle/10635/59793Title: Control of vortex breakdown over a delta wing using forebody slot blowing
Authors: Cui, Y.D.; Lim, T.T.; Tsai, H.M.
Abstract: In this paper we study the effectiveness of forebody slot blowing to control vortex breakdown over a generic delta wing-body configuration. The motivation is to exploit the benefits of both slot blowing and canards to control vortex breakdown over a delta wing. Parameters investigated include slot length, slot width, single and double-sided blowing, and the Reynolds number. Time-averaged flow images show that Reynolds number has little effect on vortex breakdown location, at least for the range of conditions studied here, and a single-sided blowing has favorable effects on the blowing side and unfavorable effects on the nonblowing side. It is postulated that when fluid is discharged from the slot at the forebody in the spanwise direction, it produces a vortex sheet that interacts with the freestream and is rolled up to form a trailing vortex further downstream. The rolled-up vortex sheet produces a downwash effect on the blowing side and a sideslip effect on the opposite side. The downwash modifies the flowfield around the leading edge of the wing, causing a delay in vortex breakdown. To compensate for the unfavorable effects of blowing on the opposite side, a much higher blowing momentum (more than two times the single-sided slot case) is needed to delay vortex breakdown on both sides of the wing when double-sided blowing is employed. Our results also show that for a given Reynolds number and angle of attack, increasing the blowing momentum leads to a substantial delay in the vortex breakdown position. In addition, for the same blowing momentum coefficient, varying the slot width has little effect on the breakdown location, whereas increasing the slot length has favorable effect, particularly at lower angles of attack.
Mon, 01 Jan 2007 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/597932007-01-01T00:00:00Z
- Numerical simulation of vortical flows in the near field of jets from notched circular nozzleshttps://scholarbank.nus.edu.sg/handle/10635/115840Title: Numerical simulation of vortical flows in the near field of jets from notched circular nozzles
Authors: Cai, J.; Tsai, H.M.; Liu, F.
Abstract: The vortex dominated flows in the near field of jets from notched circular nozzles are investigated using direct numerical simulation. The nozzles studied include a normal circular nozzle, a V-shaped notched nozzle, and an A-shaped notched nozzle, all with the same circular cross-section. The vortical structures resulting from these different circular nozzles are visualized by using a numerical dye visualization technique. Results for the V-shaped notched nozzle are compared with available experimental measurements using laser-induced fluorescence techniques. In addition to azimuthal vortex rings created because of the shear-layer between the jet and the ambient fluid, the computations also reveal streamwise vortex pairs both inside and outside the vortex rings that spread outward as the vortex rings move downstream. Comparisons of the three different nozzles show that, unlike in the case of the circular nozzle where the streamwise vortex pairs emerge evenly along the nozzle lip, streamwise vortex pairs for the notched circular nozzles are produced only at peak and trough locations. Analysis of the mixing characteristics of the three types of nozzles shows that the notches in the nozzle exit significantly enhance jet mixing. © 2009 Elsevier Ltd. All rights reserved.
Mon, 01 Mar 2010 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1158402010-03-01T00:00:00Z
- The evolution of a detonation wave in a variable cross-sectional chamberhttps://scholarbank.nus.edu.sg/handle/10635/61493Title: The evolution of a detonation wave in a variable cross-sectional chamber
Authors: Qu, Q.; Khoo, B.C.; Dou, H.-S.; Tsai, H.M.
Abstract: A two-dimensional numerical simulation has been performed to study the interaction of a gaseous detonation wave with obliquely inclined surfaces in a variable cross-sectional chamber. The weighted essentially non-oscillatory (WENO) numerical scheme with a relatively low resolution grid is employed. A detailed elementary chemical reaction model with 9 species and 19 elementary reactions is used for a stoichiometric oxy-hydrogen mixture diluted with argon. In this work, we study the effect of area expansion and contraction on the main/gross features of the detonation cellular structures in the presence of detonation reflection, diffraction and localized explosion. The result shows that there exists a transition region as the detonation wave propagates through the converging/diverging chamber. Within the transition region, the initial regular detonation cells become distorted and irregular before they re-obtain their regularity. While the ultimate regular cell size and the length of the transition region are strongly affected by the converging/diverging angle, the width/length ratio of the cells is fairly independent of it. A localized explosion near the wall is found as the detonation wave propagates in the diverging chamber. © 2008 Springer-Verlag.
Fri, 01 Aug 2008 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/614932008-08-01T00:00:00Z
- On vortical flows shedding from a bluff body with a wavy trailing edgehttps://scholarbank.nus.edu.sg/handle/10635/115845Title: On vortical flows shedding from a bluff body with a wavy trailing edge
Authors: Cai, J.; Chng, L.T.; Tsai, H.M.
Abstract: Studies have shown that shaping the trailing edge of a quasistreamlined bluff body in the form of a sinusoidal wave can result in substantial base pressure recovery. In this paper, direct numerical simulations at a Reynolds number based on base height of 2500 are used to understand the flow characteristics associated with the observed drag reduction. In particular, the effects of the wavelength of the sinusoidal disturbance on the drag coefficient, vortex shedding mechanism, and frequency selection are examined. Numerical flow visualizations compare favorably to previous experimental observations and the results confirm that there is a range of wavelengths where significant drag reduction is possible. A sinusoidal trailing edge with fixed amplitude and a wavelength which is five times the base height produces the largest reduction of more than 30% in the mean drag coefficient compared to a straight trailing edge. The drag reduction is associated with known observations such as a lengthening of the mean recirculation region, a marked drop in the velocity fluctuation at the dominant frequency of the wake and a relative increase in streamwise vorticity. Although the presence of vortex dislocations is linked with a reduction in drag, the occurrences of these structures do not yield the optimum configuration. Rather, the maximum reduction in drag appears to coincide with the transition of the vortex shedding process from a spanwise-irregular to a spanwise-regular mode characterized by spatially well-organized structures along the span. © 2008 American Institute of Physics.
Sun, 01 Jun 2008 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1158452008-06-01T00:00:00Z
- Engineering notes performance of coflow jet airfoils with conformal slot geometrieshttps://scholarbank.nus.edu.sg/handle/10635/60171Title: Engineering notes performance of coflow jet airfoils with conformal slot geometries
Authors: Zhang, J.-M.; Chng, T.L.; Tsai, H.M.
Sun, 01 May 2011 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/601712011-05-01T00:00:00Z
- Stability of vortex pairs over slender conical bodies: Analysis and numerical computationhttps://scholarbank.nus.edu.sg/handle/10635/115947Title: Stability of vortex pairs over slender conical bodies: Analysis and numerical computation
Authors: Cai, J.; Tsai, H.-M.; Luo, S.; Liu, F.
Abstract: Analytical studies and computational fluid dynamics 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. The analytical method 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 analytical method on extraordinarily fine grids with strict convergence criteria. Both the theory and the computation show that the vortices over a delta wing are stable and those over a wing-body configuration at the low angle of attack are symmetric and stable, but become asymmetric and bistable at higher angles of attack; that is, the vortices shift to one of two stable mirror-imaged asymmetric configurations. The computational results agree well with the analytical predictions, demonstrating the existence of a global inviscid hydrodynamic instability mechanism responsible for the asymmetry of separation vortices over slender conical bodies.
Sat, 01 Mar 2008 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1159472008-03-01T00:00:00Z
- Experimental and numerical study of jet mixing from a shock-containing nozzlehttps://scholarbank.nus.edu.sg/handle/10635/111399Title: Experimental and numerical study of jet mixing from a shock-containing nozzle
Authors: Xiao, Q.; Tsai, H.M.; Papamoschou, D.; Johnson, A.
Abstract: The compressible jet plume emerging from a planar convergent-divergent nozzle containing a separation shock is investigated experimentally and numerically. The investigation encompasses exit-to-throat area ratios (A e/A t) from 1.0 to 1.8 and nozzle pressure ratios from 1.2 to 1.8. Experiments were conducted in a variable-geometry nozzle facility, and computations solved the Reynolds-averaged Navier-Stokes equations with several turbulence models. The computed mean velocity field outside the nozzle compares reasonably well with the experimental data. Among the different turbulence models tested, the two-equation shear stress transport model is found to provide the best agreement with the experiments. Jet mixing is governed by A e/A t and, to a lesser extent, by nozzle pressure ratios. Increasing A e/A t results in an increased growth rate and faster axial decay of the peak velocity. The experimental trends of jet mixing versus A e/A t and nozzle pressure ratios are captured well by the computations. Computations of turbulent kinetic energy show that, with increasing A e/A t, the peak turbulent kinetic energy in the plume rises and moves toward the nozzle exit. The significant increase of turbulent kinetic energy inside the nozzle is associated with asymmetric flow separation.
Fri, 01 May 2009 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1113992009-05-01T00:00:00Z
- 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