Compliant surfaces - Introduction

It is noted that the essence of the compliant surface question is whether or not wall motion can provide a decrease in skin friction for either transitional or turbulent flows. This wall motion is capable of being externally excited or flow driven, with varying amounts of fluid motion-wall motion phasing (depending primarily upon wall structural configuration and damping). It is concluded that current research has eroded still further the experimental data base which indicates drag reduction on compliant surfaces under turbulent boundary layers in air. The need for further clarification and research is stressed for the small wavelength liquid case.