The current measurements show that the position of the transonic separation shock on the hemisphere surface moves upstream when the reverse velocities in the recirculation region are strong and is located downstream when they are weaker. No correlation was detected between the incoming boundary layer and the shock position. These observations are consistent with recent studies concluding that for large strong separation regions, the dominant mechanism is the instability of the separated flow rather than a direct influence of the incoming boundary layer. However, it is possible that the upstream boundary layer influences the shock position by affecting the shear layer instability at separation, which would not be captured by the present experiment if the response time is lengthy.
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