Characteristics of fluid thrust acting on valve stem are studied by experiment in close relation to flow patterns of a supersonic air flow expanding through a valve of simplified geometries. The valve consists of a conical plug and a plane seat, and is mounted at a nozzle exit of a blow-down facility. The experimental pressure ratio is up to twenty. Changes of fluid thrust are closely related to transition of flow patterns. As the pressure ratio increases, the flow which, separated due to shock-boundary layer interaction, reattaches to plug surface, is abruptly altered into a flow pattern where the separation opens into atmosphere, and at the same time the fluid thrust increases significantly. From the measured pressure distributions along plug surface, it is clear that this change of fluid thrust is mainly caused by the reattached separation with negative pressure opening into atmosphere. This transition of flow patterns is simulated by a numerical analysis.
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