Scale Interactions of Turbulent Boundary Layer Flows Under Local Dynamic Wall Disturbance

This paper analysed the relationships between various scales by considering the condition of the upstream flow fields. The feasibility of flow control for drag reduction through intermittent energy input from active drag reduction systems was discussed. In the experiment, a piezoelectric oscillator was used to impose local disturbances on a turbulent boundary layer. Flow field information was collected using fixed probes upstream of the piezoelectric oscillator and moving probes downstream (moving along the wall-normal direction). Correlation analyses were performed on velocity signals of different scales both upstream and downstream of the piezoelectric oscillator to determine their spatiotemporal relationships. Distur- bance signals and their higher-order harmonics were identified through the pre-multiplied energy spectrum, and different signal scales were categorized. The interactions between large scales and disturbance scales, as well as between disturbance scales and small scales, were particularly discussed under large-scale high-speed/low-speed inflow backgrounds. It is ob- served that the large-scale high-speed background modulates the disturbance signals. Under both large-scale high-speed and low-speed inflow backgrounds, there exists a fixed phase correspondence between the disturbance signals and small-scale signals, which remains unaffected by the inflow background. It is explicitly stated that when actively controlling the flow field intermittently using a piezoelectric oscillator, it is advisable to operate under a large-scale high-speed inflow back- ground.