Ecological and economical considerations motivate the search for ways to reduce the skin friction drag in turbulent flows. Several numerical studies have shown that wall shear stress can be lowered at low Reynolds numbers by applying a small amount of wall transpiration. In this study it is investigated how another type of actuation, active wall deformation, could be used to yield a similar effect.
First, discrete time-dependent wall deformation is studied in laminar flow. Lacking background turbulence, the baseflow allows clear identification of the flow perturbation. The analysis reveals that a downward moving actuator is surrounded by a region of negative wall-normal velocity and vice versa. Comparably less intuitive are the vorticity fields which often display complicated structures. A similar, subsequent study in turbulent flow shows that, indeed, active wall deformation can restructure wall turbulence.
Based on these findings, a series of experiments were conducted on opposition control. This scheme aims at opposing the velocity sensed away from the wall by imposing velocity of opposite direction at the wall. By locally deforming the wall accordingly, skin friction reductions of up to 15% are observed. Parameters critical to the performance of the control scheme, such as actuation scales and deformation limiters, are identified and analysed. As Reynolds number and actuation scales are much smaller than in practical applications, the results are of limited applicability but encouraging for prospective drag reduction at higher Reynolds numbers.
In a separate study a novel control method based on non-linear global stabilisation of the perturbed Navier-Stokes equations was tested. Using body forcing over the entire domain as actuation, the flow relaminarised.