Drag Reduction Via Spanwise Transversal Surface Waves at High Reynolds Numbers

Dependence of turbulent wall-shear stress on the amplitude of spanwise transversal surface waves

“…To be more precise, Touber and Leschziner [9] found the drag reduction to be proportional to Re −0.2 τ for channel flow at Re τ ≤ 1000. A similar analysis performed by Koh et al [23] for streamwise evolving turbulent boundary layers led to the result that the drag 6 reduction is proportional to Re −1 τ for Re τ ≤ 2250. The latter correlation was obtained under the assumption that the wall excitation in inner coordinates,…”

Impact of transversal traveling surface waves in a non-zero pressure gradient turbulent boundary layer flow

“…For the trough, a reduction in the wall-normal vorticity component was only found for the drag-reduction configuration. This hypothesis was further confirmed by analyses of Koh et al [11,12] for increased Reynolds numbers and amplitudes. The effect of different pressure gradients is illustrated in Fig.…”

“…Through the analysis of both setups, excited by the same mechanism, it was shown, that a crucial parameter indicating drag reduction is the reduction of the near-wall distribution of the wall-normal vorticity component at the crest and trough. More recently, Koh et al [11,12] analyzed the impact of the Reynolds number ranging from 1000 ≤ Re θ ≤ 7000 and amplitude 30 ≤ A + ≤ 70 on the wall-shear stress distribution of a turbulent boundary layer undergoing transversal spanwise surface waves. It was shown that with increasing Reynolds number, the drag reduction effect weakens.…”

Drag Reduction and Energy Saving by Spanwise Traveling Transversal Surface Waves for Flat Plate Flow

“…The numerical method has thoroughly been validated by computing a wide variety of internal and external flow problems [43,3,39,46]. Analyses of drag reduction have been performed for riblet structured surfaces [24] and for traveling transversal surface waves in canonical turbulent boundary layer flow [26,27,28,34] and in turbulent airfoil flow [2]. The quality of the results confirms the validity of the approach for the current flow problem.…”

“…Instead of directly introducing spanwise velocity, the surface is wavily deflected in the wall-normal direction to generate a secondary flow field of periodic wallnormal and spanwise fluctuations. Positive drag reduction using this technique was achieved experimentally [20,47,30] and numerically for channel flow [48], boundary layer flow [26,28,27,19], and airfoil flow [2]. Tomiyama and Fukagata [48] observed a possible shielding effect of quasi-streamwise vortices from the wall by the wave-like deformations and showed that a combination of the thickness of the Stokes layer, i.e., the actuation period, and the actuation velocity amplitudes scales reasonably well with drag reduction.…”