State switching in the wake of a transverse circular cylinder in the transonic regime

The flow past a circular cylinder in a cross-flow configuration was investigated using a wind tunnel experiment across a range of Mach numbers M ∞ from 0.30 to 0.85 and corresponding Reynolds numbers, based on diameter, ranging from 2 × 10 5 to 5 × 10 5 . The boundary layer at the cylinder surface is either free or fixed turbulent using artificial tripping at azimuth 12.5 ° , 25 ° , and 50 ° . Measurements combine temporal recordings of the wall pressure and schlieren high-speed visualizations of the flow past and downstream of the cylinder. First, by attaching the tripping strips to the cylinder at the different azimuthal positions, the effect of the boundary layer state on the cylinder wake is found to be strong at subsonic speeds but much smaller in the transonic regime. Multiple flow states are observed depending on the value of M ∞ . Of particular interest was the narrow region between M ∞ = 0.80 and = 0.85 , which yields three different states, namely the vortex shedding state, the parallel shear layer state, and the crossed shear layer state. By precisely managing back and forth travels in Mach number with the wind tunnel, it is found that the transitions between these states have a hysteresis behavior. This sub-critical behavior could explain the surprisingly variable threshold values between these different states reported in the literature.