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ISSN オンライン: 2642-0554

LES AND UNSTEADY RANS OF BOUNDARY-LAYER TRANSITION INDUCED BY PERIODICALLY PASSING WAKES

Frank Ham
Department of Mechanical Engineering, University of Waterloo Waterloo, Ontario, N2L 3G1, Canada; Center for Turbulence Research, Stanford University 488 Escondido Mall, Stanford, California 94305, USA

Fue-Sang Lien
Department of Mechanical Engineering, University of Waterloo Waterloo, Ontario, N2L 3G1, Canada

要約

Results from 3-dimensional Large-Eddy Simulation (LES) and 2-dimensional unsteady Reynolds-Averaged Navier Stokes (RANS) simulation of a spatially-evolving flat-plate boundary-layer undergoing transition induced by periodically passing wakes are presented and compared.
The LES simulations used a novel kinetic-energy conserving finite-volume discretization of the incompressible Navier-Stokes equations and the standard dynamic Smagorinsky subgrid-scale model. When compared to the direct numerical simulation (DNS) of Wu et al (1999), the LES was able to correctly predict the onset of transition. Inspection of the instantaneous flow field in the transition region confirmed that intermittent turbulent spots were being distinctly resolved. A close inspection of the fluctuating velocities near the top of the boundary layer just prior to spot formation confirmed the presence of the "backward jet" inflectional velocity profile proposed by Wu et al and Jacobs and Durbin (2000) as the precursor to turbulent spot formation, suggesting that the LES is actually capturing the bypass transition mechanism, at least in these initial stages. The transition length predicted by LES, however, was consistently shorter than the DNS result.
Unsteady RANS simulations were based on the STREAM code of Lien and Leschziner (1994), with the v2 − f turbulence model of Lien and Durbin (1996). RANS was able to correctly predict the onset of transition. The transition length predicted by RANS was also in agreement with the DNS, however the overshoot of average skin friction relative to the flat plate correlation (seen in both the DNS and present LES) was not observed.