ScholarBank@NUShttps://scholarbank.nus.edu.sgThe DSpace digital repository system captures, stores, indexes, preserves, and distributes digital research material.Thu, 13 Aug 2020 17:38:36 GMT2020-08-13T17:38:36Z50131- Second Taylor vortex flow: Effects of radius ratio and aspect ratiohttps://scholarbank.nus.edu.sg/handle/10635/61268Title: Second Taylor vortex flow: Effects of radius ratio and aspect ratio
Authors: Xiao, Q.; Lim, T.T.; Chew, Y.T.
Abstract: This paper is motivated by our earlier investigation on the stability of Taylor-Couette flow in which we discovered a previously unidentified flow regime, which we refer to as "Second Taylor vortex flow" (STVF) when an inner cylinder is subjected to some critical acceleration [Lim, Chew, and Xiao, Phys. Fluids 10, 3233 (1998)]. The aim here is to explore how the STVF regime is affected by changes in radius ratio and aspect ratio. Results show that the STVF regime is sensitive to the gap size between the two cylinders, and does not exist for some radius ratios, whereas it increases with decreasing aspect ratio. © 2002 American Institute of Physics.
Mon, 01 Apr 2002 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/612682002-04-01T00:00:00Z
- 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
- Effect of acceleration on the wavy Taylor vortex flowhttps://scholarbank.nus.edu.sg/handle/10635/85036Title: Effect of acceleration on the wavy Taylor vortex flow
Authors: Xiao, Q.; Lim, T.T.; Chew, Y.T.
Abstract: In this paper, we use a laser optical technique to investigate the characteristics of a wavy Taylor vortex flow between two concentric cylinders, with the inner cylinder subjected to a wide range of predetermined acceleration and the outer one at rest. We focus on the inner/outer radius ratio of 0.894, with an acceleration (dRe/dt*) from 0.1123 to 2,247, and Reynolds number from Re/Rec = 1.0 to 36. The results show that, with increasing Reynolds number, there is an initial increase in the wavelength of the wavy vortex flow (λ), and a decrease in the wave speed (c) before they asymptote to a constant value, which is a function of the acceleration. As for the wave amplitude (A), it is found that the effect of acceleration is significant only in a very narrow range of Reynolds numbers.
Sat, 01 Jun 2002 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/850362002-06-01T00:00:00Z
- A flow visualization study of acceleration effect on the Taylor-Couette flowhttps://scholarbank.nus.edu.sg/handle/10635/92818Title: A flow visualization study of acceleration effect on the Taylor-Couette flow
Authors: Xiao, Q.; Lim, T.T.; Chew, Y.T.
Abstract: The acceleration effect on the Taylor-vortex and later developed wavy vortices was investigated from flow visualization techniques in circular Couette system (with the inner cylinder rotating and outer cylinder at rest) as a function of Reynolds number Re. Our results show that when the acceleration is higher than a critical value of about 2.2 s-1, there exists a new flow regime in which the flow pattern shows remarkable resemblance to the regular Taylor vortex flow with shorter axial wavelength. One of the most surprising aspects of this flow is that it occurs in the range of Reynolds number in which the wavy vortex flow is supposed to occur.
Thu, 01 Jan 1998 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/928181998-01-01T00:00:00Z
- 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
- A new flow regime in a Taylor-Couette flowhttps://scholarbank.nus.edu.sg/handle/10635/92651Title: A new flow regime in a Taylor-Couette flow
Authors: Lim, T.T.; Chew, Y.T.; Xiao, Q.
Abstract: In this Brief Communication, we report a new finding on a Taylor-Couette flow in which the outer cylinder is stationary and the inner cylinder is accelerated linearly from rest to a desired speed. The results show that when the acceleration (dRe/dt) is higher than a critical value of about 2.2s-1, there exists a new flow regime in which the flow pattern shows remarkable resemblance to regular Taylor vortex flow but is of shorter wavelength. However, when the acceleration is lower than 2.2 s-1, a wavy flow is found to occur for the same Reynolds number range. To our knowledge, this is probably the first time that such a phenomenon has been observed. For completeness, the case of a decelerating cylinder is also investigated, and the results are found to be almost the same. © 1998 American Institute of Physics.
Thu, 01 Jan 1998 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/926511998-01-01T00:00:00Z
- 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
- 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
- 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
- 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
- Influence of a "slip" splitter plate on transonic shock oscillation on airfoilshttps://scholarbank.nus.edu.sg/handle/10635/111595Title: Influence of a "slip" splitter plate on transonic shock oscillation on airfoils
Authors: Xiao, Q.; Tsai, H.M.
Abstract: The effect of a "slip" splitter plate to control the buffeting phenomena at transonic speed is reported. The aim is to examine the wake vortex of airfoils when there is large flow unsteadiness arising from self-excited shock induced oscillation. While the use of a "slip" splitter plate is a hypothetical one, its use here is as a tool to better understand the flow phenomena involved. The study is made using unsteady computation of Reynolds-Average Navier-Stokes equation with a two-equation k-ω turbulent model with lagged effect. Two different airfoils are investigated; a symmetric non-lifting 18% thick bi-convex airfoil and a lifting BQK No.1 supercritical airfoil. The effect of locating the splitter plate placed downstream from the airfoil trailing edge reveals that the influence from the interaction between the upper and lower surfaces is persistent. Our preliminary study here shows that although the vortex size can be decreased with a splitter plate the communication between the top and bottom surfaces at the trailing edge is still important and present with small separation distances. The size of the required splitter plate is approximately a function of the size of the vortex formed.
Thu, 01 Jan 2004 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1115952004-01-01T00:00:00Z
- Viscous computation of steady/unsteady transonic diffuser flowshttps://scholarbank.nus.edu.sg/handle/10635/130104Title: Viscous computation of steady/unsteady transonic diffuser flows
Authors: Xiao, Q.; Tsai, H.M.; Liu, F.
Abstract: The lag model proposed by Olsen and Coakley (2001) is applied to simulate the steady and unsteady transonic flows in a diffuser. The unsteady flows are induced by imposing fluctuating back-pressures. A fully implicit time-accurate multigrid algorithm is used for solving the Navier-Stokes equations and the coupled k-ω turbulence model equations. Two test cases are investigated, one with a weak shock in the channel corresponding to a static-to-total pressure ratio Rp=0.82 at the diffuser exit and the other with a strong shock corresponding to Rp=0.72. The results with the implementation of the lag model are closer to the experimental data for the strong-shock case. However, the computational results are almost the same with and without the lag model for the weak-shock case. © 2002 by the author.
Tue, 01 Jan 2002 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1301042002-01-01T00:00:00Z