Zhixiang Xiao

Numerical Dissipation Effects on Massive Separation Around Tandem Cylinders

Three spatial schemes, the original Roe scheme and two high-order symmetric total variation diminishing schemes,whose dissipations aremultiplied by a constant parameter or a function (called ), are coupledwith delayed detached eddy simulation to investigate the numerical dissipation effects on the massive separation flow around tandem cylinders. From the comparisons between the computations and the available measurements, the numerical dissipation has a significant influence on the mean and instantaneous flowfields. The original Roe scheme is too dissipative to predict the small scale turbulent structures, and it strongly suppresses the growth of resolved turbulence. The S6WENO5 schemes with constantand adaptivetimes of dissipation have similar performances and they well match the measurements. However, the S6WENO5 with constanttimes (0.12 here) of dissipation is too empirical, and the small constanttimes of dissipation cannot generally suppress the numerical oscillations near the wall and in the far fields. The S6WENO5 with adaptive dissipation provides the best performance.

Investigation of Flows Around a Rudimentary Landing Gear with Advanced Detached-Eddy-Simulation Approaches

Unsteady and massively separated flows past the rudimentary landing gear are investigated using delayed detached-eddy-simulation and improved delayed detached-eddy-simulation based on shear stress transport model. To eliminate the unfavorable influence of large numerical dissipation, a high order symmetric total variation diminishing scheme with adaptive dissipation approach is implemented. Three sets of grid, including the coarse, medium and locally refined grids are applied. It is observed that the grid density effect is weak on the mean flows, but significant on the instantaneous quantities. Both approaches present acceptable agreements with the available experiments. Due to its wall-modeled large-eddy-simulation mode, improved delayed detached-eddy-simulation can deliver slightly larger secondary separation and smaller horseshoe vortex on the aft wheels, predict the shear layer instability a little more upstream and resolve smaller instantaneous structures. There are strong interactions between vortic...

Study of Delayed-Detached Eddy Simulation with Weakly Nonlinear Turbulence Model

This paper presents a parallel computation with hybrid Reynolds-averaged Navier-Stokes and large eddy simulation methods. Two types of hybrid approaches, referred to as detached eddy simulation and delayed-detached eddy simulation, are investigated through introducing length scales into the weakly nonlinear k-w turbulence model. Before the implementation of hybrid methods, the baseline weakly nonlinear model and k-w shear-stress-transport model are validated in the transonic RAE-2822 airfoil and ONERA-M6 wing cases. The numerical results show satisfactory agreement with the available experimental data. The Reynolds-averaged and hybrid methods based on the weakly nonlinear turbulence model are then applied to calculate the high Reynolds number transonic separation flows around NASA TN D-712 wing fulesage For the higher angle of attack case (a = 26.2 deg), both hybrid methods deliver good results when compared with the experiment; for the moderate angle of attack case (a = 12.5 deg), the results obtained with the delayed detached eddy simulation are satisfactory, whereas the starting point of the wing vortex breakdown predicted with the detached eddy simulation is too far upstream.

Comparisons of Three Improved DES Methods on Unsteady Flows Past Tandem Cylinders

Three advanced DES-type models coupled with adaptive dissipation scheme, DDES-2003/2006 and IDDES, are applied to predict the unsteady flow past tandem cylinders. The main differences among them are the shield functions and length scales, which leads to significant differences in mean turbulence kinetic energy, root mean square of pressure, instantaneous vorticity, and so on. The computational results are compared with almost all available measurements. These three models show good capability for the massive separation flows. IDDES performs relatively better than other two SST-DDES models. Furthermore, IDDES matches well with the measurements with trips on the rear cylinder surface.

Computations with k-g Model for Complex Configurations at High-Incidence

The two-equation k-g turbulence model, a derivation of the k-w model, with its advantages such as no use of normal-to-wall distance, simple source terms, and straightforward boundary conditions, can be easily applied in complex computational fluid dynamics applications. Transonic flows around the ONERA M6 wing and the NASA TN D-712 wing-fuselage model at angles of attack of 12.5 and 26.2 deg are first used to validate the codes. Then the transonic flows around an F-22-like complete aircraft configuration are numerically simulated at angles of attack from 0 to 48 deg

Direct Numerical Simulation of Hypersonic Transition Induced by Ramp Roughness Elements

Hypersonic boundary layer transition from laminar to turbulent induced by ramp roughness element, is investigated using Direct Numerical Simulation based on finite volume formulation. To explore the transition mechanism and capture the shock wave at the same time, high order MDCD (Minimum Dispersion and Controllable Dissipation) scheme is used. The simulation results show that the transition is dominated by the instability of the counterrotating streamwise vortices generated by the roughness. The qualitative comparison is made between simulation and experiment and they show a good agreement. Furthermore, the effects of the space between roughness elements are investigated. The horseshoe vortices of two ramp roughness elements interact with each other in the interval region and are strengthened due to the counter-rotating effect.

Simulation of Transition Triggered by Isolated Roughness in Hypersonic Boundary Layer

Hypersonic boundary layer transition from laminar to turbulent triggered by isolated roughness, is investigated using IDDES (Improved Delayed Detached Eddy Simulation) model, which combines DDES and WMLES (Wall-Modeled Large Eddy Simulation). To explore the transition mechanism and capture the shock wave at the same time, the numerical dissipation of the upwind scheme is dexterously designed with adaptive function. The original upwind scheme with full dissipation is applied near the wall, the shock wave and in the irrotational region to suppress the numerical oscillation, and the scheme becomes almost zero dissipation in the separation region downstream of the roughness element to resolve the small-scale structures. From the preliminary results, IDDES with adaptive dissipation can successfully simulate the hypersonic boundary layer transition triggered by isolated roughness element. For the diamond-shape roughness, the transition is mainly caused by the shear layer instability; for the cylinder-shape case, the transition is mainly dominated by the instability of both the horseshoe vortex and the shear layer.

Calculations of massive separation around landing-gear-like geometries

The massive separating flows around landing-gear-like configurations, such as Rudimentary landing gear (RLG) and tandem cylinders (TC), are calculated using unsteady Reynolds averaged Navier-Stokes (URANS) and delayed-detached-eddy simulation (DDES) based on k-ω-SST model. A number of numerical schemes and dissipation are applied in an effort to compare the averaged as well as the instantaneous flow-fields with the available measurements. It is shown that high-order and low-dissipation scheme is necessary to calculate the small scale structures. Furthermore, DDES delivered better results than URANS.

Vortex Breakdown Flows Around a Double-Delta Wing During Pitching Motion Based on DDES

A solver based on rigid moving mesh and DDES techniques is implemented to investigate the unsteady flows around an 80°/65°double delta wing during a sinusoidal pitching motion with reduced frequency equal to 0.4. We focus on the behavior of burst point, helical mode instability, pressure fluctuations and dynamic pitching stability. The response of burst point is nearly a simple harmonic motion and locked in the frequency of pitching motion associated with a phase lag. The time-averaged flow after breakdown regions is still approximately a conical flow, whose cone angle is different from the steady state. Cm lags behind with the variation of incidence, which means dynamic pitching stability is obtained.

A modular RANS approach for modeling hypersonic flow transition on an air-breathing configuration

In this study, the laminar-turbulent flow transition scenario on a blunted double ramp, and further on a 20%-scale X-51A forebody configuration, has been simulated with a recently proposed RANS (Reynolds-Averaged Navier-Stokes) transition/turbulence model, which takes into account of the rational effects of compressibility, crossflow and flowseparation characteristics on different instability modes in 3-D boundary-layer flows. The model is based on the K-ω-γ three-equation eddy-viscosity concept with K representing the fluctuating kinetic energy, ω the specific dissipation rate and γ the intermittency factor. This model performs pretty well in the blunted double ramp case, where the hypersonic flow transition is caused by flow separation at the compression corner. For the X-51A forebody configuration, computed results are in good agreement with experimental data under both quiet and noisy conditions.