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Direct numerical simulation of turbulent channel flow over a compliant surface has been performed to evaluate the friction drag reduction effect. It is found that a slight drag reduction is observed over a compliant surface. This observation indicates that drag reduction is possible using a compliant surface that is deformed passively by wall pressure fluctuation. A small wall displacement and velocity are observed in the present study, however, a large-scale pressure field becomes dominant. The typical shape of displacement of the compliant surface is a wave, which is almost homogeneous in the spanwise direction.

Control valves of the main steam flow for power plants are operated under wide range of valve openings and pressure ratios. In the present paper, experimental and numerical investigations are described in order to clarify mechanisms of the valve head vibration. Experiments are conducted with two types of the valve head support. One is a flexible support and the other one is with an exciter. Results show that valve head vibrations with large amplitude appear with the flexibly supported valve head under certain range of valve openings and the pressure ratio. With the valve head exciter, dynamic fluid forces are measured. Results show that the added damping force becomes negative around the condition where the valve head oscillation is observed with flexible support. Numerical analyses are carried out in order to observe the flow field. In the simulations, forced vibrations of valve head are assumed. Results show that the pressure distribution on the valve head surface changes depending on the excitation frequencies, and as a result, the negative damping force occurs.

Main steam control valves in power plants are required to operate under wide ranges of valve openings and pressure ratios. In the present paper, experimental and numerical investigations are conducted with rigid and flexible valve head supports in order to clarify mechanisms of valve head vibrations caused by unsteady flows around the valve. Results obtained with the rigid support without valve head vibration show that the unsteady flow around the valve head causes pressure fluctuations on the valve head surface with random and impulsive wave forms. With flexible support, the valve head vibrates at near the natural frequency of the valve head support system and the vibrations are excited around the operation condition where the pressure fluctuation becomes large with rigid support valve head. When the valve head vibration becomes large, the pressure fluctuation becomes periodic with the same frequency of the valve head vibration. The numerical result shows that the response of the separated jet delays behind the valve head motion. As a result, the lateral fluid force fluctuations on the valve head provide negative damping on the vibration.