Control of Wall-Bounded Turbulence Through Closed-Loop Wall Transpiration

Turbulent flow control aimed at drag reduction has the potential to deliver enormous energetic and economic savings, but many challenges remain despite active research for well over a century. In this talk I will discuss our work towards two open questions of the field: first, what are suitable controller design tools for high Reynolds number wall-bounded flows? And second, how does actuation through closed-loop wall transpiration change the flow physics? We investigate aspects of these questions through direct numerical simulation (DNS) and modal analyses of an example control scheme, which is applied to a low Reynolds number turbulent channel flow.

In the first part of the talk we show that a low-order model based on the resolvent framework is able to approximate the drag reduction results of DNS over the entire parameter space considered, and we present a strategy based on subsampling of the wave number space and analytical scaling laws that enables model-based flow control design at technologically relevant Reynolds numbers. In the second part of the talk we demonstrate that the physics of the controlled flows can be understood from transpiration with a few select spatial scales and phase shifts relative to the background flow. Comparisons to other forms of wall transpiration and broader implications for future flow control design will also be discussed.

Live Zoom Event: <https://caltech.zoom.us/j/84737586094>

Box Recordings for Caltech: <https://caltech.box.com/s/ktk4t67cwdgqp2eky4duoxcahqqmt9hk>