We investigated the flow and bubble dynamics in a stopper control valve as used in continuous casting of steel under flow conditions where cavitation or quick degassing is expected to occur. We compare the flow of a typical control stopper with four archetypical stoppers. The effect of stopper geometry on cavitation dynamics is studied experimentally in a downscaled two-dimensional water model. High-speed imaging in combination with pressure data sheds light on the performance of the various stopper geometries under cavitating and non-cavitating conditions. It is found that a blunt-ended stopper shape induces, under all conditions, a strong flow separation leading to a large pressure loss coefficient, K. In return we find that the controllability of the flow improves while K changes only marginally when entering a cavitating flow regime. In contrast, more streamlined stoppers show a lower K that yet sharply increases at the onset of cavitation; possibly causing instability in valve throughput. ... Read more