Development of High Frequency Excitation Devices for Noise Reduction

The noise level of new and existing aircraft need to be reduced to meet the increasingly stringent US and international airport standards. High frequency excitation (wavelengths much shorter that amplified instabilities) of jet flows is a new technology that has the potential to reduce acoustic levels significantly in addition to lowering turbulence levels. In application, forcing is applied at frequencies that are much higher than ones associated with the large-scale, two-dimensional structures associated with the jet, but much lower than the Kolmogorov scale. The development of the jet is substantially altered through two mechanisms. First, the high-frequency forcing increases the turbulent dissipation rate. Second, it reduces the turbulent production. These two mechanisms together lead to a significant decrease in the energy in the large-scale two-dimensional motion and subsequently lowering turbulent and noise levels. One benefit of the control is that the forcing needs only to possess frequencies that are above a certain range. This single frequency requirement encompasses a wide array of potentially suitable devices.