HM Tsai - Academia.edu (original) (raw)

Papers by HM Tsai

Detonation waves in rectangular ducts are simulated with a fifth-order WENO (Weighted Essentially... more Detonation waves in rectangular ducts are simulated with a fifth-order WENO (Weighted Essentially NonOscillatory) scheme and a 3rd order TVD Rouge-Kutta method. The simulation shows that under an initial disturbance, the detonation front finally develops to an unsteady three-dimensional distorted pattern. After sufficiently developing time, the detonation front becomes a quasi-steady periodic motion. For the wide duct, the flow front shows a quasi-steady “rectangular mode” periodically, with the front assuming variation of “convex” and “concave” shapes continuously. It is shown that the detonation mechanism depends on the coupling between the pressure and the velocity. The transverse wave plays an important role by facilitating this coupling via collisions.

Modern Physics Letters B, 2005

An experimental and numerical study has been carried out on an indeterminate-origin V-notched jet... more 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.

International Journal for Numerical Methods in Fluids, 1990

A spectral code has been used to simulate a developing turbulent boundary layer at low Reynolds n... more A spectral code has been used to simulate a developing turbulent boundary layer at low Reynolds number Reθ (based on free stream velocity and momentum thickness) between 353 and 576. The starting field was generated by allowing a step change of temperature to diffuse outwards from one wall in a fully developed channel flow. The thermal boundary layer so created was conditionally sampled to convert it into a momentum boundary layer with an irrotational free stream region, a process which is justified by appeal to experiments. This initial field was allowed to develop until the momentum boundary layer thickness δ995 had grown to about 1·5 times its original thickness. The results of the simulation have been compared with a wide range of experimental data. The outcome of this comparison is generally very satisfactory; the main trends of the experiments are well reproduced and our simulation supplements and extends the existing sets of experimental data. The simulation also gives pressure statistics which cannot be obtained experimentally. In particular, it gives the contribution of pressure diffusion to the balance equations for the Reynolds stress and indicates the error produced by omitting this term.

Experimental Thermal and Fluid Science, 2009

This paper reports an experimental study conducted on the use of circular, V-and A-notched collar... more 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.

International Journal of Computer Applications in Technology, 2000

A viscous simulation of the £ow over an F-16A model with inlet and the ventral ¢ns, which were no... more A viscous simulation of the £ow over an F-16A model with inlet and the ventral ¢ns, which were normally not considered in previous studies, is described. The method uses a patch grid multi-block, multi-grid Navier-Stokes solver. A novel method to grid the intake and diverter is used. The arrangement of the gridding system adopted minimizes the number of grids needed particularly for viscous computations. A modi¢ed form of the Baldwin^Lomax algebraic turbulence model was used for the high AoA studies. To simplify the incorporation of turbulence model, the blocking arrangement is such that it allows a thin-layer Navier-Stokes equation formulation in the calculation of the eddy viscosity. A series of four £ow conditions were studied; M ¼ 0.9, AoA ¼ 4 , 6.0 ; and M ¼ 1.2 at AoA ¼ 6.0 ; and M ¼ 0.85 at AoA ¼ 16. Extensive comparisons with experimental data of the pressure distribution on the wing and on different streamwise stations show the modelling to be reasonably accurate. For the results presented here, the wing tip missile is ignored.

14th AIAA/AHI Space Planes and Hypersonic Systems and Technologies Conference, 2006

An experimental investigation was carried out to study the performance of a pulse detonation engi... more An experimental investigation was carried out to study the performance of a pulse detonation engine platform incorporating commercial, off-the-shelf solenoid valve gas injectors and non-conventional deflagration-to-detonation transition enhancing devices. The study made use of stoichiometric propane-oxygen mixtures with low-energy ignition sources. The gas injectors were observed to be sufficiently robust and operated reliably under the high working temperature and pressure conditions normally associated with pulsed detonation operations provided appropriate preventive measures were taken. One of the major motivations in utilizing these gas injectors lies in the ease and accuracy in controlling their injection operations electronically, which allows for tight integration with auxiliary electronic control and measurement systems. This paper reports on the initial success of integrating these gas injectors into a moderate-frequency pulsed detonation engine system as well as the effectiveness of the deflagration-to-detonation transition enhancing devices which included Shchelkin spiral, circumferential and helical grooves, as well as convergentdivergent throats. Lastly, operational insights in the practical use of gas injectors and the impact on pulse detonation operations are highlighted.

Physics of Fluids, 2007

An experimental investigation was carried out to study the effects of noncircular collars on an a... more 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.

AIAA Journal, 2004

Shape optimization of airfoils involves highly expensive, nonlinear objective(s) and constraint f... more Shape optimization of airfoils involves highly expensive, nonlinear objective(s) and constraint functions often with functional and slope discontinuity that limits the efficient use of gradient-based methods for its solution. Gradient-based methods are not capable of generating a set of pareto solutions as required in multiobjective problems as they work with a single solution and improve it through successive iterations. Population-based, zero-order, stochastic optimization methods are therefore an attractive choice for shape optimization problems as they are easy to implement and effective for highly nonlinear problems. We present a swarm algorithm that is applicable for optimization problems in general, but is here explored for airfoil design optimization studies

AIAA Journal, 2007

ABSTRACT

44th AIAA Aerospace Sciences Meeting and Exhibit, 2006

AIAA Journal, 2001

A novel parallel dynamic moving mesh algorithm designed for multiblock parallel unsteady ow calcu... more A novel parallel dynamic moving mesh algorithm designed for multiblock parallel unsteady ow calculations using body-tted grids is presented. The moving grid algorithm within each block uses a method of arc-length-based trans nite 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 ow calculations of airfoils and wings with deforming shapes as found in utter simulations.

Fluid Mechanics and Its Applications, 1991

45th AIAA Aerospace Sciences Meeting and Exhibit, 2007

Physics of Fluids, 2005

A flow visualization investigation using dye-injection and laser-induced fluorescence techniques ... more A flow visualization investigation using dye-injection and laser-induced fluorescence techniques has been carried out to understand the vortex dynamics resulting from a Vnotched indeterminate-origin jet with two peaks and two troughs. The laminar jet was studied under forcing and non-forcing conditions to investigate the resultant dynamics of coherent large-and small-scale flow structures. Present experimental observations indicated that the effects of the nozzle peaks and troughs differ from those reported previously. Instead of the peaks producing streamwise vortex-pairs which spread outwards into the ambient fluid and the troughs generating similar vortex-pairs but entrain ambient fluid into the jet flows as indicated by earlier studies, the present experimental observations showed that both peaks and troughs produce outward-spreading streamwise vortex-pairs. Laser cross-sections further showed that the subsequent formation of azimuthal ring-vortices causes these streamwise vortex-pairs to be entrained. This entrainment causes the streamwise vortex-pairs to "rollup" together with the ring vortices, leading to intense flow interactions between them.

Journal of Propulsion and Power, 2003

ABSTRACT

Journal of Aircraft, 2009

Computers & Fluids, 2010

This paper examines the use of stationary Cartesian mesh for non-linear flutter computations invo... more 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

45th AIAA Aerospace Sciences Meeting and Exhibit, 2007

Detonation waves in rectangular ducts are simulated with a fifth-order WENO (Weighted Essentially... more Detonation waves in rectangular ducts are simulated with a fifth-order WENO (Weighted Essentially NonOscillatory) scheme and a 3rd order TVD Rouge-Kutta method. The simulation shows that under an initial disturbance, the detonation front finally develops to an unsteady three-dimensional distorted pattern. After sufficiently developing time, the detonation front becomes a quasi-steady periodic motion. For the wide duct, the flow front shows a quasi-steady “rectangular mode” periodically, with the front assuming variation of “convex” and “concave” shapes continuously. It is shown that the detonation mechanism depends on the coupling between the pressure and the velocity. The transverse wave plays an important role by facilitating this coupling via collisions.

Modern Physics Letters B, 2005

An experimental and numerical study has been carried out on an indeterminate-origin V-notched jet... more 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.

International Journal for Numerical Methods in Fluids, 1990

A spectral code has been used to simulate a developing turbulent boundary layer at low Reynolds n... more A spectral code has been used to simulate a developing turbulent boundary layer at low Reynolds number Reθ (based on free stream velocity and momentum thickness) between 353 and 576. The starting field was generated by allowing a step change of temperature to diffuse outwards from one wall in a fully developed channel flow. The thermal boundary layer so created was conditionally sampled to convert it into a momentum boundary layer with an irrotational free stream region, a process which is justified by appeal to experiments. This initial field was allowed to develop until the momentum boundary layer thickness δ995 had grown to about 1·5 times its original thickness. The results of the simulation have been compared with a wide range of experimental data. The outcome of this comparison is generally very satisfactory; the main trends of the experiments are well reproduced and our simulation supplements and extends the existing sets of experimental data. The simulation also gives pressure statistics which cannot be obtained experimentally. In particular, it gives the contribution of pressure diffusion to the balance equations for the Reynolds stress and indicates the error produced by omitting this term.

Experimental Thermal and Fluid Science, 2009

This paper reports an experimental study conducted on the use of circular, V-and A-notched collar... more 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.

International Journal of Computer Applications in Technology, 2000

A viscous simulation of the £ow over an F-16A model with inlet and the ventral ¢ns, which were no... more A viscous simulation of the £ow over an F-16A model with inlet and the ventral ¢ns, which were normally not considered in previous studies, is described. The method uses a patch grid multi-block, multi-grid Navier-Stokes solver. A novel method to grid the intake and diverter is used. The arrangement of the gridding system adopted minimizes the number of grids needed particularly for viscous computations. A modi¢ed form of the Baldwin^Lomax algebraic turbulence model was used for the high AoA studies. To simplify the incorporation of turbulence model, the blocking arrangement is such that it allows a thin-layer Navier-Stokes equation formulation in the calculation of the eddy viscosity. A series of four £ow conditions were studied; M ¼ 0.9, AoA ¼ 4 , 6.0 ; and M ¼ 1.2 at AoA ¼ 6.0 ; and M ¼ 0.85 at AoA ¼ 16. Extensive comparisons with experimental data of the pressure distribution on the wing and on different streamwise stations show the modelling to be reasonably accurate. For the results presented here, the wing tip missile is ignored.

14th AIAA/AHI Space Planes and Hypersonic Systems and Technologies Conference, 2006

An experimental investigation was carried out to study the performance of a pulse detonation engi... more An experimental investigation was carried out to study the performance of a pulse detonation engine platform incorporating commercial, off-the-shelf solenoid valve gas injectors and non-conventional deflagration-to-detonation transition enhancing devices. The study made use of stoichiometric propane-oxygen mixtures with low-energy ignition sources. The gas injectors were observed to be sufficiently robust and operated reliably under the high working temperature and pressure conditions normally associated with pulsed detonation operations provided appropriate preventive measures were taken. One of the major motivations in utilizing these gas injectors lies in the ease and accuracy in controlling their injection operations electronically, which allows for tight integration with auxiliary electronic control and measurement systems. This paper reports on the initial success of integrating these gas injectors into a moderate-frequency pulsed detonation engine system as well as the effectiveness of the deflagration-to-detonation transition enhancing devices which included Shchelkin spiral, circumferential and helical grooves, as well as convergentdivergent throats. Lastly, operational insights in the practical use of gas injectors and the impact on pulse detonation operations are highlighted.

Physics of Fluids, 2007

An experimental investigation was carried out to study the effects of noncircular collars on an a... more 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.

AIAA Journal, 2004

Shape optimization of airfoils involves highly expensive, nonlinear objective(s) and constraint f... more Shape optimization of airfoils involves highly expensive, nonlinear objective(s) and constraint functions often with functional and slope discontinuity that limits the efficient use of gradient-based methods for its solution. Gradient-based methods are not capable of generating a set of pareto solutions as required in multiobjective problems as they work with a single solution and improve it through successive iterations. Population-based, zero-order, stochastic optimization methods are therefore an attractive choice for shape optimization problems as they are easy to implement and effective for highly nonlinear problems. We present a swarm algorithm that is applicable for optimization problems in general, but is here explored for airfoil design optimization studies

AIAA Journal, 2007

ABSTRACT

44th AIAA Aerospace Sciences Meeting and Exhibit, 2006

AIAA Journal, 2001

A novel parallel dynamic moving mesh algorithm designed for multiblock parallel unsteady ow calcu... more A novel parallel dynamic moving mesh algorithm designed for multiblock parallel unsteady ow calculations using body-tted grids is presented. The moving grid algorithm within each block uses a method of arc-length-based trans nite 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 ow calculations of airfoils and wings with deforming shapes as found in utter simulations.

Fluid Mechanics and Its Applications, 1991

45th AIAA Aerospace Sciences Meeting and Exhibit, 2007

Physics of Fluids, 2005

A flow visualization investigation using dye-injection and laser-induced fluorescence techniques ... more A flow visualization investigation using dye-injection and laser-induced fluorescence techniques has been carried out to understand the vortex dynamics resulting from a Vnotched indeterminate-origin jet with two peaks and two troughs. The laminar jet was studied under forcing and non-forcing conditions to investigate the resultant dynamics of coherent large-and small-scale flow structures. Present experimental observations indicated that the effects of the nozzle peaks and troughs differ from those reported previously. Instead of the peaks producing streamwise vortex-pairs which spread outwards into the ambient fluid and the troughs generating similar vortex-pairs but entrain ambient fluid into the jet flows as indicated by earlier studies, the present experimental observations showed that both peaks and troughs produce outward-spreading streamwise vortex-pairs. Laser cross-sections further showed that the subsequent formation of azimuthal ring-vortices causes these streamwise vortex-pairs to be entrained. This entrainment causes the streamwise vortex-pairs to "rollup" together with the ring vortices, leading to intense flow interactions between them.

Journal of Propulsion and Power, 2003

ABSTRACT

Journal of Aircraft, 2009

Computers & Fluids, 2010

This paper examines the use of stationary Cartesian mesh for non-linear flutter computations invo... more 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

45th AIAA Aerospace Sciences Meeting and Exhibit, 2007