Project Details
Jet noise is a major problem for both military and commercial aircraft, and there is a lot of interest in ways to reduce it. In this research project sponsored by the Office of Naval Research, the objective is to implement active control in rectangular jets to reduce the noise. This is to be done by exciting the jet at a fundamental frequency as well as either a harmonic or subharmonic frequency. The amplitudes of the excitation are small, thus there should be minimal impact of excitation on aircraft performance. In doing this, we can manipulate the large-scale structures in the jet, which is the dominant noise source. The working principle here is that energy from the fundamental mode is transferred to the subharmonic or harmonic, which results in a reduction of the peak noise.
In order to compute the noise sources, High-Fidelity Large Eddy Simulations (LES) is done by modifying a code originally developed by the Air Force Research Laboratory, which uses high-order numerical schemes. However, LES is very computationally expensive and can take weeks to obtain results when running on a supercomputer. Choosing the wrong excitation parameters can result in zero noise reduction or even enhancement of the noise. To predict optimal excitation parameters, a Reduced-Order Model (ROM) has been derived to predict the propagation of noise sources in a jet. Inputs to the ROM can come from linear methods such as Linear Stability Analysis or the Linearized Euler Equations. Once the ROM is set up, a set of nonlinear differential equations can be solved numerically. By comparison, this takes only a matter of seconds and does not require the use of a supercomputing cluster. Using these results, we can observe the damping effect on the dominant noise source, and optimal excitation parameters can be chosen as inputs into LES.
Current work is focused on performing LES on a Mach 1.5 planar jet, which approximates the flow in the minor plane of a rectangular jet. This is being done to validate open-loop control using results from the ROM. Both the symmetric and asymmetric modes will be studied. Future work will involve performing LES on a Three-dimensional rectangular jet, which will be more representative of a real jet. Here, closed-loop control can also be implemented. By measuring the noise signal near the exit of the jet, parameters can be inputted to the ROM to give optimal excitation parameters thereby maximizing the noise reduction.
