Friction Drag Variation via Spanwise Transversal Surface Waves

Abstract

The introduction of spanwise velocity is a promising technique to effect the near-wall turbulent flow field to influence friction drag. However, the essential physical mechanism which significantly reduces friction drag has not been understood, yet. It is the objective of this numerical study to improve the fundamental knowledge on the drag reduction mechanism. The investigation is based on spanwise traveling transversal surface waves which are applied to modify the near-wall flow field and to influence friction drag. Two actuation configurations are analyzed in detail. Compared with an unactuated flat plate boundary layer simulation the first wave setup, which represents a low frequency wave at an amplitude larger than the viscous sublayer, leads to a reduced wall-shear stress resulting in friction drag reduction of up to 9%. The second wave setup, which possesses a higher frequency and an amplitude in the range of the viscous sublayer, yields an increase of friction drag of about 8%. Unlike previous investigations which focus on excitation setups to lower friction drag, the comparison of the two wave setups in this study allows to identify the effects which on the one hand, lead to drag reduction and on the other hand, result in drag increase. That is, due to the pronounced differences the major effects determining the friction distribution are more evident. The two key features for drag reduction are the damping of the wall-normal vorticity fluctuations above the entire surface and the decrease of turbulence production. Furthermore, the effect of rearranging streamwise vorticity, which has been stated to be responsible for drag reduction, is found to occur at increasing and decreasing drag, i.e., it is not the effect that lowers the friction drag.

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

“…On the other hand, with increasing amplitude a higher drag reduction was obtained. Similar to the results of [10] they also found a strong correlation between the drag reduction and the reduction of the wall-normal vorticity component.…”

“…In a more recent analysis extending the previous investigation Tamano and Itoh [21] achieved 13% drag reduction. Klumpp et al [8,10] performed large-eddy simulations (LES) of turbulent boundary layers at Re θ = 887 over transversal traveling surface waves. Through the analysis of two distinct parameter configurations λ + = 174 and λ + = 870 it was possible to reveal a different impact on the skin-friction distribution.…”

Drag reduction in turbulent boundary layers by spanwise traveling waves with wall deformation

“…Quite recently, Klumpp et al [51,52] performed a numerical simulation of a spatially evolving turbulent boundary layer over spanwise traveling transversal sinusoidal surface waves for a compressible flow at a low Mach number. They showed that a drag reduction ratio up to 9% could be obtained, and that two key features for drag reduction were the damping of the wall-normal vorticity fluctuations above the entire surface and the decrease of turbulence production.…”

Turbulent boundary-layer control with plasma spanwise travelling waves

“…This caused the suppression of quasi-streamwise vortices within the boundary layer. A further numerical investigation was conducted by Klumpp et al (2011). In their large-eddy simulation (LES), the outof-plane motion of a spanwise surface wave was implemented in a turbulent channel flow and achieved a skinfriction drag reduction of 9 %.…”