Turbulent Friction Drag Reduction: From Feedback to Predetermined, and Feedback Again

Abstract

We overview the recent attempts for turbulent friction drag reduction and related studies, by focusing on those conducted in our research group. While the earlier studies for friction drag reduction mainly targeted at suppression of quasi-streamwise vortices using feedback control, predetermined control methods using streamwise traveling waves or a uniform blowing have also been extensively investigated in the last decade. For both the streamwise traveling wave of wall deformation and the uniform blowing, their drag reduction capabilities have been confirmed well by direct numerical simulation at relatively low Reynolds numbers. Prediction of their drag reduction capabilities at higher Reynolds numbers and attempts for experimental confirmation are also intensively ongoing toward their practical implementation. We also introduce our practice on the application of resolvent analysis for designing a more effective feedback control law. In addition, we briefly introduce some recent attempts on the applications of machine learning to turbulent flows, which may be utilized for a better design of flow control in future.