Frequency effect of an oscillating plate immersed in a turbulent boundary layer

A phase-averaged technique was employed to investigate the flow structures downstream of an oscillating bluff plate immersed in a turbulent boundary layer. The Reynolds number based on the maximum plate height, was kept at 7.8 x 10³ throughout the investigation. During the experiments the reduced frequency of the oscillating plate was varied in a range from 0.0018 to 0.013. The experimental results obtained clearly indicate that a critical reduced frequency near 0,009 can be identified, which distinguishes two types of flows with different characteristics. Under the subcritical condition (i.e. the reduced frequency less than 0.009), the flow shows quasi-steady behaviors. Whereas under the supercritical condition, the disturbances generated by the oscillating plate is concentrated in a narrow region convecting downstream. An additional vortical structure is developed on the top of this region, which enhances the downwash motion toward the wall, ft is also seen that pressure fluctuations measured on the wall response faithfully to the passage of the vortical structures. These experimental observations further lead us to suggest the criteria for the determination of the critical condition. According to these criteria the critical reduced frequency estimated is about 0.02.