Optical sensing of laminar to turbulent transition and boundary layer turbulence (Conference Presentation)

Abstract

Characterizing turbulent instabilities in transitional and developed turbulent boundary layers is of critical importance to the development of methods to suppress such instabilities, which has important implications for research into drag reduction and energy-efficient propulsion methods. This includes the cancellation of Tollmien-Schlichting (TS) waves in the transitional and the counteracting of large-scale coherent structures in the developed turbulent boundary layer. To efficiently respond to instabilities near a boundary, the instabilities have to be sensed and described with a high level of accuracy. We visualize and measure velocity fluctuations in a laminar to turbulent flow tank to study the development of TS waves for active cancellation and identify boundary layers streaks for the development of stabilization methods. The flow structure in the tank is described through dye experiments, Particle Image Velocimetry (PIV), novel high-frequency fiber-optics flow sensors, and Acoustic Doppler Velocimeters (ADV), with the goal of identifying TS waves in transitional turbulence, as well as boundary layer streaks in developed turbulence. The impact of passive elastic boundary materials on the flow is investigated to aid the development of active actuated membranes aimed at reducing boundary layer turbulence and drag. In addition to flow measurements from various sensors, we employ computational fluid dynamics (CFD) to emulate the laboratory setting and complement the measurements. The CFD representation of the laboratory tank is implemented as high-resolution large-eddy simulation with elastic boundary conditions simulating the compliant boundary. The combined setup is a critical tool in the ongoing development of methods for active boundary later control.