The impact of pressure gradient history on flow structures in High Reynolds number rough wall turbulence

High Reynolds number experiments are conducted over a rough wall with strong non-equilibrium pressure gradients. The boundary layer is exposed to different pressure gradient histories via an aerofoil mounted above the boundary layer. Particle image velocity (PIV) allows for the flow development from one chord upstream of the leading edge to one chord downstream of the trailing edge to be captured (3.75 m or $\approx 22{\delta }_0$). The freestream speed upstream of the aerofoil is set to 20 m/s, and the resulting $R{e}_{\tau }$ varies from 12900 to 18500. The integral pressure gradient history parameter is seen to have a second-order relationship with the wake strength, $\Pi$. Furthermore, the extent to which the upstream history has to be accounted for is considered. The structures within the flow are examined first through the mean turbulence intensity profiles as well as quadrant analysis. These results show that the effect of flow events on adverse and favourable pressure gradients varies depending on the position within the boundary layer. Some success is achieved in matching the turbulence profiles and flow structure length scales through the integral of the pressure gradient history. The agreement is improved for cases where the local pressure gradient history parameter is also matched.