Selected Publications
Kent L. Gee (et al.)
Kent L. Gee (et al.)
Kent Gee (et al.)
K. L. Gee (et al.)
The role of nonlinearity in the propagation of noise radiated from high-performance jet aircraft is currently not well understood, despite past studies such as those by Blackstock and Howell and Morfey (see Refs. 2 and 3). More recently, Norum et al. found that F-15 flyover data exhibited anomalously high spectral levels above 1.5kHz; although nonlinear effects were mentioned as a possible cause for the High levels, the issue was not pursued further. As a finite amplitude noise waveform propagates, it undergoes amplitude-dependent steepening, which results in a transfer of spectral energy from mid- to higher frequencies. If competing processes, such as atmospheric absorption and geometrical spreading, are not sufficient to prevent shock formation, coalescence of shocks will also transfer energy to lower frequencies. Significant nonlinear spectral broadening, if present in high-speed jet noise, would cause perceived levles to vary from those predicted via linear methods.
Kent L. Gee (et al.)
The overall sound pressure levels of noise radiated by military jet aircraft along certain
angles are such that nonlinearity is likely to influence the propagation. Bispectral analysis of
noise data from the F/A-18E Super Hornet has been carried out in order to provide further
evidence that nonlinear effects are indeed present. The bicoherence, which is a normalized
form of the bispectral density, has been previously used in a variety of applications to detect
quadratic phase coupling (QPC) in a signal. In this case, the results of the bicoherence
calculations indicate that QPC is indeed present at high-thrust conditions along the peak
radiation angles, which means that nonlinearity does play a role. However, additional
investigations are still needed to more fully understand the physical interpretation of the
bispectral results for a random noise signal, which will also help better quantify the role of
nonlinearity in jet noise propagation.
angles are such that nonlinearity is likely to influence the propagation. Bispectral analysis of
noise data from the F/A-18E Super Hornet has been carried out in order to provide further
evidence that nonlinear effects are indeed present. The bicoherence, which is a normalized
form of the bispectral density, has been previously used in a variety of applications to detect
quadratic phase coupling (QPC) in a signal. In this case, the results of the bicoherence
calculations indicate that QPC is indeed present at high-thrust conditions along the peak
radiation angles, which means that nonlinearity does play a role. However, additional
investigations are still needed to more fully understand the physical interpretation of the
bispectral results for a random noise signal, which will also help better quantify the role of
nonlinearity in jet noise propagation.
Kent L. Gee (et al.)
An algorithm has been developed to study the nonlinear propagation of high-amplitude
jet noise. The hybrid time-frequency domain algorithm employs a split-step solution to a Mendousse-Burgers equation that includes the effects of quadratic nonlinearity, atmospheric absorption and dispersion, and geometrical spreading. Spectral predictions generated using the algorithm are compared to recent F/A-18E engine run-up noise measurements made at afterburner and military thrust conditions at distances of 74 and 150 m from the engine nozzles. The agreement between the predicted and measured spectra is such that the results help confirm that energy transfer is occurring to higher frequencies. However, the differences between the model and the measurement raise important issues regarding some of the physical phenomena likely associated with the measurement but not accounted for in the model. Among these are the substantial multipath interference effects in the measured spectra and the finite extent of the aeroacoustic sources within the jet.
jet noise. The hybrid time-frequency domain algorithm employs a split-step solution to a Mendousse-Burgers equation that includes the effects of quadratic nonlinearity, atmospheric absorption and dispersion, and geometrical spreading. Spectral predictions generated using the algorithm are compared to recent F/A-18E engine run-up noise measurements made at afterburner and military thrust conditions at distances of 74 and 150 m from the engine nozzles. The agreement between the predicted and measured spectra is such that the results help confirm that energy transfer is occurring to higher frequencies. However, the differences between the model and the measurement raise important issues regarding some of the physical phenomena likely associated with the measurement but not accounted for in the model. Among these are the substantial multipath interference effects in the measured spectra and the finite extent of the aeroacoustic sources within the jet.