This thesis studies active control for suppression of flow-induced noise from open cavities. When a strong wind flows over a cavity, loud noise and powerful flow disturbance are generated. This phenomenon is a cavity resonance, in which a flow disturbance induced by instability of flow and an acoustic pressure caused by bumping the flow disturbance on the downstream edge of the cavity are amplified by each other. This cavity resonance causes many troubles such as break down of landing gear of the airplane, explosion accidents in the weapons bay of fight-bombers, and noise from the sunroof of a vehicle.
Two kinds of active control methods can suppress the cavity resonance, which is a feedback mechanism between acoustic waves of the cavity and fluctuations in the pressure of the cavity flow. One method is active flow suppression, which diminishes the flow disturbance in a flow field. The other method is active noise canceling, which interferes with the cavity noise in an acoustic field. Consequently these two active control methods can respectively weaken the cavity resonance. But most research on cavity resonance control is focused on suppressing the flow disturbance. Hence, the various schemes of a flow control are poorly understood.
This thesis proposes a time-delayed phase control method to reduce the global noise of the cavity. The acoustic feedback of the cavity noise, which amplifies the flow disturbance, is reduced in this control method. Thus, regardless of the complex flow physics, the feedback of the cavity noise is limited. The control algorithm is a feedback algorithm that uses phase shifter and filter. In the phase analysis, the positions of the sensor and the actuator are determined in relation to a comparison of the control efficiency.
The experiments verify that the proposed control method is fairly effective to reduce resonance sound. These empirical results confirm that the time-delayed phase control method can regulate the suppression o...