Turbulent flow over a flexible wall undergoing a streamwise travelling wave motion

Direct numerical simulation is used to study the turbulent flow over a smooth wavy wall undergoing transverse motion in the form of a streamwise travelling wave. The Reynolds number based on the mean velocity U of the external flow and wall motion wavelength lambda is 10 170; the wave steepness is 2pia/lambda 0.25 where a is the travelling wave amplitude. A key parameter for this problem is the ratio of the wall motion phase speed c to U, and results are obtained for c/U in the range of -1.0 to 2.0 at 0.2 intervals. For negative c/U, we find that flow separation is enhanced and a large drag force is produced. For positive cl U, the results show that as cl U increases from zero, the separation bubble moves further upstream and away from the wall, and is reduced in strength. Above a threshold value of c/U approximate to 1, separation is eliminated; and, relative to small- c/U cases, turbulence intensity and turbulent shear stress are reduced significantly. The drag force decreases monotonically as c/U increases while the power required for the transverse motion generally increases for large c/U; the net power input is found to reach a minimum at c/U approximate to 1.2 (for fixed U). The results obtained in this study provide physical insight into the study of fish-like swimming mechanisms in terms of drag reduction and optimal propulsive efficiency.