Nozzle geometry effects on supersonic wind tunnel studies of shock–boundary-layer interactions

Many supersonic wind tunnel experiments investigate shock–boundary-layer interactions by measuring the response of tunnel wall boundary layers to an incident shock wave. To generate the supersonic flow, these facilities typically use two-dimensional contoured converging–diverging nozzles which can be arranged in two different ways. One configuration is symmetric about the centre height, whereas this symmetry plane defines the tunnel floor in the other asymmetric arrangement. In order to determine whether these nozzle configurations, which are widely thought to be equivalent, can influence experiments on shock–boundary-layer interactions, two different nozzle geometries are compared with one another in a single facility with rectangular cross section. For each setup, a full-span 8-degree wedge introduces an oblique shock to a Mach 2.5 flow. The two setups exhibit quite dissimilar behaviour, both in the corner regions and on the tunnel’s centre span, with a difference in central separation length of as much as 35% suggesting that nozzle geometry can have a profound impact on these interactions. The observed behaviour is caused by known secondary flows in the sidewall boundary layers which are driven by vertical pressure gradients in the nozzle region. The subsequent impact on the response of the floor boundary layer is consistent with expectations based on local flow momentum affecting corner separation size and on the displacement effect of this corner separation influencing the wider flow