Secondary wake instability of a bridge model and its application in wake control

Three-dimensional spanwise-varying control, which is known to be related to secondary wake instability, has recently been reported as a highly efficient method to control vortex shedding. With the aim of suppressing Kármán vortex shedding and the underlying aerodynamic forces in a bridge model using three-dimensional spanwise-varying control, Floquet stability analysis was applied to determine the most unstable secondary instability of a period vortex shedding wake at low Reynolds numbers (i.e. Re = 516, 774, and 1032). The numerical results showed that there was only a single peak of Floquet multipliers ranging from 1H to 6H, with H being the height of the bridge model; additionally, all Floquet modes were found to be analogous to the Mode-A type. Subsequently, spanwise-varying passive vortex generators (PVGs), which can trigger secondary instability of a wake by activating pairs of streamwise counter-rotating vortical structures, were applied to implement wake control. Large eddy simulation was used to verify the wake control effects with PVGs arranged on the lower surface of the bridge model. The results showed that, overall, the case of λ/H = 2 yields improved control effects on the fluctuating aerodynamic forces (an approximately 60% reduction in fluctuating lift and moment, 10% reduction in fluctuating drag). In addition, the PVG could effectively trigger and enhance the Mode-A-like flow pattern and suppress spanwise vortices.