Current Computational Fluid Dynamics Methods Research Articles

Numerical simulation of wing vortex generators – methodologies and validation

ABSTRACTThis paper presents the results of a study of different Vortex Generator (VG) modelling approaches that have been performed in order to develop a better understanding of the current Computational Fluid Dynamics (CFD) capability to simulate transonic flows where wing-mounted VGs are present.The practicality of using CFD methods commonly employed in the aerospace industry to predict the influence of VGs on wing performance is studied. It is hoped that presenting the experience gained will be of value to aerodynamicists working on similar problems in industry. An approach, using fully resolved, conformal mesh around the VGs, has been investigated through studying Reynolds-Averaged Navier Stokes (RANS) simulations with two alternate turbulence models, the Spalart-Allmaras (SA) model and the Speziale-Sarkar-Gatski (SSG) Reynolds Stress Model. An initial assessment of two alternative VG modelling techniques, use of the Chimera overset meshing and a reduced-order VG model has also been performed. In addition, an investigation of the impact of the wing deformation under aerodynamic loading was conducted. The results obtained were compared with the wind-tunnel measurements acquired in the Aircraft Research Association Transonic Wind Tunnel, using the N47-05 half model with installed VGs.It was observed that the VGs significantly modify the flow behaviour at sufficiently high incidence, which leads to higher lift coefficient values. While the SA turbulence model was unable to capture the complicated nature of the flow when VGs were present, SSG simulations yielded promising results.Each of the VG modelling approaches has shown some strengths and weaknesses. Further study on the subject is suggested in order to develop best practices that can be applied for solutions of industrial-scale problems.

Isothermal Air-Ingress Validation Experiments

Idaho National Laboratory has conducted air-ingress experiments as part of a campaign to validate computational fluid dynamics (CFD) calculations for very high-temperature gas-cooled reactor (VHTR) analysis. An isothermal test loop was designed to recreate exchange or stratified flow that occurs in the lower plenum of VHTR after a break in the primary loop allows helium to leak out and reactor building air to enter the reactor core. The experiment was designed to measure stratified flow in the inlet pipe connecting to the lower plenum of the General Atomics gas turbine-modular helium reactor (GT-MHR). Instead of helium and air, brine and sucrose were used as heavy fluids, and water was used as the lighter fluid to create, using scaling laws, the appropriate flow characteristics of the lower plenum immediately after depressurization. These results clearly indicate that stratified flow is established even for very small density differences.Corresponding CFD results were validated with the experimental data. A grid sensitivity study on CFD models was also performed using the Richardson extrapolation and the grid convergence index method for the numerical accuracy of CFD calculations. The calculated current speed showed very good agreement with the experimental data, indicating that current CFD methods are suitable for simulating density gradient stratified flow phenomena in an air-ingress accident.

Detached-Eddy Simulations of Vortex Breakdown over a 70-Degree Delta Wing

An understanding of the vortical structures that comprise the vortical flowfield around slender bodies is essential for the development of highly maneuverable and high angle-of-attack flight. This is primarily due to the physical limits this phenomenon imposes on aircraft and missiles at extreme flight conditions. Demands for more maneuverable air vehicles have pushed the limits of current computational fluid dynamics methods in the high Reynolds number regime. Simulation methods must be able to accurately describe the unsteady, vortical flowfields associated with fighter aircraft at Reynolds numbers more representative of full-scale vehicles. One of the goals of this paper is to demonstrate the ability of detached-eddy simulation, a hybrid Reynolds-averaged Navier―Stokes large-eddy simulation method, to accurately predict the vortical flowfield over a slender delta wing at Reynolds numbers above 1 × 10 6 . Although detached-eddy simulation successfully predicted the location of the vortex breakdown phenomenon in previous work, the goal of the current effort is to further validate the method with additional experimental data from the Office National d'Etudes et Recherches Aerospatiales, such as surface pressures and turbulent kinetic energy in the vortex core. The effect of grid density and an adaptive mesh refinement technique is also assessed through comparisons with the experiment. Detailed wind-tunnel geometry, such as tunnel walls and the sting mount system, are simulated and found to make a measurable difference. Finally, modeling the laminar-to-turbulent transition is demonstrated to have a significant effect on the vortical flowfield.

Analysis of Delta-Wing Vortical Substructures Using Detached-Eddy Simulation

An understanding of the vortical structures that comprise the vortical flowfield around slender bodies is essential for the development of highly maneuverable and high-angle-of-attack flight. This is primarily because of the physical limits these phenomena impose on aircraft and missiles at extreme flight conditions. Demands for more maneuverable air vehicles have pushed the limits of current computational fluid dynamics methods in the high-Reynolds-number regime. Simulation methods must be able to accurately describe the unsteady, vortical flowfields associated with fighter aircraft at Reynolds numbers more representative of full-scale vehicles. It is the goal here to demonstrate the ability of detached-eddy simulation (DES), a hybrid Reynolds-averaged Navier-Stokes/large-eddy-simulation method, to accurately model the vortical flowfield over a slender delta wing at Reynolds numbers above one million. DES has successfully predicted the location of the vortex breakdown phenomenon, and the goal of the current effort is to analyze and assess the influence of vortical substructures in the separating shear layers that roll up to form the leading-edge vortices. Very detailed experiments performed at ONERA using three-dimensional laser-Doppler-velocimetry measurement will be used to compare simulations utilizing DES turbulence models. The computational results provide novel insight into the formation and impact of the vortical substructures in the separating shear layers on the entire vertical flowfield.

Perspective: Flow at High Reynolds Number and Over Rough Surfaces—Achilles Heel of CFD

The law of the wall and related correlations underpin much of current computational fluid dynamics (CFD) software, either directly through use of so-called wall functions or indirectly in near-wall turbulence models. The correlations for near-wall flow become crucial in solution of two problems of great practical importance, namely, in prediction of flow at high Reynolds numbers and in modeling the effects of surface roughness. Although the two problems may appear vastly different from a physical point of view, they share common numerical features. Some results from the ’superpipe’ experiment at Princeton University are analyzed along with those of previous experiments on the boundary layer on an axisymmetric body to identify features of near-wall flow at high Reynolds numbers that are useful in modeling. The study is complemented by a review of some computations in simple and complex flows to reveal the strengths and weaknesses of turbulence models used in modern CFD methods. Similarly, principal results of classical experiments on the effects of sand-grain roughness are reviewed, along with various models proposed to account for these effects in numerical solutions. Models that claim to resolve the near-wall flow are applied to the flow in rough-wall pipes and channels to illustrate their power and limitations. The need for further laboratory and numerical experiments is clarified as a result of this study.