LES and RANS Studies of Oscillating Flows over Flat Plate

Abstract

Oscillatory flows over a flat plate are studied by using Large Eddy Simulation (LES) and Reyn- olds-Average Navier-Stokes (RANS) methods. A dynamic subgrid scale (SGS) model is employed in LES, while the Saffman's turbulence model in RANS. The mean velocity profile, the turbulence intensity, and the wall shear stress are computed and compared with earlier experimental and numerical works. The results indicate that the flow behaviors are quite different during the accelerating and decelerating phases of the oscillating cycle. The transition from laminar to turbulent is also investigated as a function of the Reynolds number, R, which represents the square of the ratio of the oscillation amplitude at free stream to the thickness of the Stokes layer at the plate. The present results both from LES and RANS show that the transition occurs in the range of 5 3 10 4 < R <5 3 10 5 . The evolution of the flow structure in the Stokes layer during the transition from laminar to turbulent is clearly demonstrated from the numerical results. The friction coefficient of the amplitude of oscil- latory surface shear stress varies as R 20.5 with a phase angle of 457 in laminar regime and transition to R 20.23 with a phase angle of about 107 in turbulence regime. These variations in the surface shear stress with the Reynolds number are in excellent agreement with the earlier experimental results of Kamphuis and the numerical results of Blondeaux. The excellent agreement between the LES and RANS demonstrated that Saffman's tur- bulence model, as originally intended by Saffman, is applicable for unsteady flows.