Experimental Investigation of Flow Control Techniques To Reduce Hydroacoustic Rotor-Stator Interaction Noise
Control of radiated acoustic noise is vital to the survivability and the detectability of submersible watercraft. Two primary sources of radiated fluid noise in submersible vessels are the boundary layer turbulence along the forebody and propulsor fluid-structure interaction. The propulsor contains several locations of such interaction, one of which was investigated in this research. Specifically, this research focused on experimentally investigating active flow control techniques to reduce rotor-stator interaction noise sources.
Two of the three flow control configurations applied to the flow involved the application of active flow control to the leading edge of a single exit guide vane (EGV) mounted downstream of a seven-bladed rotor. The leading edge blowing configuration (LEB) consisted of a single jet expelled from the leading edge of the EGV against the oncoming flow. This interaction between the wake and jet should offset or disrupt the coherency of any incoming flow structures. The second active flow control method applied to the EGV involved a tangential blowing configuration (TB) where two symmetric tangential jets were used to create an insulating fluid layer that reduced the effect of passing flow structures on the EGV. The final flow control design was the implementation of trailing edge wake filling on a three bladed rotor. A rotor was designed to ingest lower velocity flow from the hub and pump the fluid out of a blowing slot at the blade trailing edge. The blowing slot was concentrated on the outer third of the blade span in order to maximize pumping effect.
In order to quantify the effects of the active flow control techniques on rotor-stator interaction, the fluctuating lift force on the EGV was measured. Since this fluctuating force serves as a primary acoustic source, the effects of the active flow control on the radiated interaction sound can be estimated. These active flow control techniques were intended for reduction of blade passing frequency tonal sound radiation. The LEB configuration showed minor changes in overall spectral response; however, there was no significant reduction in forcing at the BPF measured. Similarly the TB configuration also yielded no measurable change in BPF tonal forcing. The first generation design of the self-pumping rotor also proved to have problems. Experiments showed that the application of the flow control on the self-pumping rotor did not generate the expected increase in torque demand or changes in the tonal forcing on the EGV. Field alterations to the rotor were unable to improve the performance; therefore, the conclusion became that the initial design was unable to pump fluid due to excessive pressure losses. Further design iterations are required to perfect the functionality of the self-pumping rotor.