Daily Archives: April 26, 2013

The Prediction and Reduction of Jet Noise from Tactical Fighter Aircraft

date April 26, 2013
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Friday, April 26, 2013 4:00 p.m. in ETC 4.150

Professor Philip J. Morris
Department of Aerospace Engineering
The Pennsylvania State University

The noise generated by modern tactical fighter aircraft can cause noise-induced hearing loss in personnel located near the aircraft and annoyance in communities surrounding military bases. This is a special problem for the Navy as both landing and takeoff involve high power engine settings and personnel on a carrier deck are located very close to the aircraft. The jet noise generated by the hot supersonic jet exhausts involves two source mechanisms. The dominant noise is turbulent mixing noise generated by the supersonic convection of the large turbulent structures in the jet exhaust. This generates the highest levels and radiates in the downstream arc. The interaction of the turbulence with shock cells in the jet plume results in broadband shock-associated noise. This is important at larger angles to the jet downstream axis. This talk will describe different ways to predict these two noise sources and their radiation. Broadband shock-associated noise is predicted on the basis of an acoustic analogy and steady Reynolds averaged Navier-Stokes simulations. The mixing noise is predicted using unsteady Navier-Stokes simulations coupled to an acoustic analogy for wave extrapolation to a far field observer. Finally, a new method to reduce the strength of these jet noise sources is described. Results of simulations and experiments to demonstrate the effectiveness of this noise reduction method will be given.

Acoustics from High-Speed Jets with Crackle

date April 26, 2013
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Friday, April 26, 2013 1:00 p.m. in ACES 4.304

Woutijn J. Baars
Department of Aerospace Engineering
The University of Texas at Austin
A scaling model based on the Gol’dberg number is proposed for predicting the presence of cumulative nonlinear distortions in acoustic waveforms produced by high speed jets. Two acoustic length scales, the shock formation distance and the absorption length are expressed in terms of jet exit parameters. This approach allows one to compute the degree of cumulative nonlinear distortion in full-scale scenarios, from laboratory-scale observations, or vice versa. Surveys of the acoustic pressure waveforms emitted by a laboratory-scale, shock free and unheated Mach 3 jet are used to support model predictions. These acoustic waveforms are acquired on a planar grid in an acoustically treated and range-restricted environment. Various statistical metrics are employed to examine the degree of local and cumulative nonlinearity in the measured waveforms. This includes skewness, kurtosis, the number of zero crossings in the waveform, a wave steepening factor, the Morfey-Howell nonlinearity indicator and an application of the generalized Burgers’ equation.

Based on findings of the model and the spatial topography of the metrics, it is concluded that cumulative nonlinear steepening effects are absent in the current data set. This implies that acoustic shock-structures in the waveforms are generated by local mechanisms in, or in close vicinity to, the jet’s hydrodynamic region. Furthermore, these shock-structures induce the crackle noise component. The research aims to quantify crackle in a temporal and spectral fashion, and is motivated by the fact that (1) it is perceived as the most annoying component of jet noise, (2) no unique measures of crackle exist, and (3) significant reductions in jet noise will be achieved when crackle can be controlled. A detection algorithm is introduced which isolates the shock-structures in the temporal waveform that are responsible for crackle. Ensemble-averages of the identified waveform sections are employed to gain an in-depth understanding of these structures. Moreover, PDF’s of the temporal intermittence of these shocks reveal modal trends and show evidence that crackling shock-structures are present in groups of multiple shocks. A spectral measure of crackle is considered by using wavelet-based time-frequency analyses. The increase in sound energy is computed by considering the global pressure spectra and the ones that represent the spectral behavior during instances of crackle. This energy-based metric is postulated to be an appropriate metric for the level of crackle.