Reverse Karman street via lock-in of a turbulent wake

It has long been long known that the periodic forcing of a bluff body wake can induce significant alterations to both the large-scale structures that are emanating from the body and to the mean-flow wake topology. These effects are greatly accentuated during ‘lock-in’, i.e. when the forcing frequency is made approximately equal to that of the naturally occurring vortex shedding instability.

Here, we present an experimental investigation of a turbulent wake lock-in via periodic oscillation of a solid plate. Our combined PIV and drag measurements show that, during lock-in, a drop in drag can be observed, associated with a reversal of the regular Karman street pattern.