Direct Conversion of Acoustic Energy to Flow Energy

The research focused on the study of the direct conversion of acoustic energy into flow kinetic energy. The energy conversion has the potential to provide noise reduction if applied as an acoustic liner in a turbo-fan engine while also energizing its internal flow field. The energy levels of a sound wave were compared to the kinetic energy deficit of a turbulent boundary layer. The results indicated the energy levels of a sound wave from a typical turbo-fan are greater than the kinetic energy deficit in the turbulent boundary layer. Devices that were capable of directly converting acoustic energy into flow kinetic energy were designed and studied. The devices were a combination of a Helmholtz resonator with a pitched orifice and a stretched membrane. The membrane was exposed to an incident sound field to harvest its energy and the pitched orifice functioned as a synthetic jet. Hot-wire and Laser Doppler Velocimetry (LDV) measurements in an anechoic wind tunnel facility under quiescent conditions revealed a device was capable of producing a synthetic jet at its natural frequencies. The natural frequencies of the device were found to be different from the natural frequencies of the membrane and the Helmholtz resonator. A 2-degree-of-freedom model was used to predict the frequency response of a device. The model was successful at predicting the value of the natural frequencies of a device. Hot-wire measurements with a turbulent boundary layer over a device indicated a significant increase in mean and RMS velocity in the region downstream of the pitched orifice when the device was exposed to an incident tonal acoustic pressure that coincided with one of its natural frequencies. The Advanced Noise Control Fan (ANCF) was used to study the effect of an acoustic liner constructed from 37 individual devices on both the acoustic far-field and the internal flow field. The acoustic measurements showed a sound pressure level decrease of 23 dB at a speed of 1540 RPM. Overall, the findings suggest the direct conversion of acoustic energy into flow kinetic energy is worth exploring for the development of noise reduction and turbo-fan engine performance-enhancing technologies.