Structural Changes due to Varying-Phase Opposition Control in Turbulent Channel Flow*

Recent work by Luhar et al (J Fluid Mech, 2014) and the present authors has shown that the attainable drag reduction of opposition control (Choi et al, J Fluid Mech, 1994) strongly depends on the phase between the sensor measurement and the actuator response.
From a resolvent model perspective (McKeon \& Sharma, J Fluid Mech, 2010), a nonzero phase changes the linear amplification mechanisms described by the resolvent operator of the controlled flow, but it is unknown how a nonzero phase changes the structure of the real flow to bring about the observed changes in amplification and drag reduction.
The present study uses direct numerical simulation of a turbulent channel flow to observe the structural changes of the full nonlinear system under varying-phase opposition control.
A combination of flow visualizations, statistical analysis and low-order modeling is used to provide insights into the role of the phase and the physics of drag reduction.
Our findings may help guiding the design of future control schemes from both, a low-order model and physical perspective.