Nicholas S. Heeb

Numerical Study of Noise Characteristics in Overexpanded Jet Flows

Abstract : Noise characteristics in shock-containing jets at an overexpanded jet condition have been investigated. Total temperature ratios of 1.0 (cold), 2.0, and 3.0 are considered. The cold jet is a highly screeching jet. Frequency-wavenumber Fourier analysis is employed to examine the wave characteristics of pressure waves along the lip line and also along a near-field conical surface. It is found that the radiating portion of the pressure wave intensity increases with the jet temperature, but the hydrodynamic portion is much less sensitive to the change of the jet temperature. The near-field noise intensity associated with the Mach wave radiation is observed over a large axial distance, and the Mach wave radiation extends to much higher frequencies in heated jets. The peak radiation direction in the cold jet is dominated by the axisymmetric mode, but the directions around the sideline show a much weaker azimuthal dependence. Furthermore, the axial locations of lip-line pressure peak intensities at the screech frequency are near the axial locations of shock-cell tips. A reinforcing loop between upstream/downstream propagating waves and the induced shock-cell coherent oscillatory motion is observed in the highly screeching jet. The formation of this reinforcing loop requires a match of the peak phase velocities between upstream and downstream propagating waves. This phase velocity match exists in the highly screeching cold jet, but not in the weakly screeching heated jets. It appears that the phase velocity match that sustains the reinforcing loop is important to the screech generation, and the phase velocity mismatch in heated jets is believed to be an important cause of the screech intensity reduction.

Investigation of the Noise from a Rectangular Supersonic Jet

An experimental investigation of a low aspect ratio rectangular nozzle’s flow field was completed. Over, ideally, and under expanded operating conditions were presented for nozzle temperature ratios of between unity and three. The aim of this study was threefold, first to validate the newly constructed heated jet noise rig at the University of Cincinnati, second to provide data for future computational validation, and third to compile acoustic baselines for future noise reduction studies. Validation of the experimental results was achieved through comparison to jet noise theories and empirical relations, particularly the fine and large scale similarity spectrum, screech frequency predictions, and scaling of acoustic intensity with powers of the expansion factor β. Excellent comparisons were achieve with limited deficiencies identified, such as possible nonlinear propagation and/or rig noise contamination at high frequencies for elevated temperatures. It was found for the current experimental setup that screech was eliminated at highly elevated temperatures. This phenomena that is still currently debated and bears further investigation. Additionally, the noise source known as crackle was briefly investigated, and it was found that at elevated temperature and pressure ratios the measured acoustics met the criteria for a crackling jet.

Computational Study of Shock-Associated Noise Characteristics Using LES

Abstract : The shock-associated noise characteristics of an underexpanded jet at three jet temperatures were investigated using large-eddy simulations (LES). The impact of shock cells on the flow field and near- and far-field noise characteristics were examined. The impact of shock-associated noise is confined to one and a half jet core length in the near field. The near-field shift of the shock-associated peak frequency matches well with the inverse of the shock-cell size, indicating that the variation of the shock-cell size is largely responsible for this shift. In the far field, the variation of the shock-associated peak frequency agrees well with the available empirical model over a large range of the radiation angle, but the model under-predicts the fast increase near the end of the shock-associated noise region. In addition, the distributed nature of the shock-associated noise source impacts the far-field noise characteristic if the distance is not sufficiently large. Heating decreases the shock-cell impact on the total noise, but the heating impact on the shock-associated noise level in the upstream direction is opposite to that in the downstream direction. On the other hand, heating greatly increases the mixing noise, reducing the difference between the shock-containing jets and the shock-free jets.

Computational Study of the Impact of Chevrons on Noise Characteristics of Imperfectly Expanded Jet Flows

The impact of chevrons on flow field and noise characteristics at an underexpanded jet condition with three jet temperatures has been studied by using large-eddy simulations. It is found that chevrons reduce shock-cell size and shock-cell strength, and also produce additional shock waves with a wavelength almost half the original shock cell size. Increasing the jet temperature reduces the vorticity of induced vortices. In addition, chevrons increase shear-layer thickness, reduce turbulence level in the jet wake region, but have little impact on the jet core length. Double broadband peaks of shock-associated noise are observed in both the nearand far-field sound pressure level distributions. In addition, a large increase of high-frequency components is found near the nozzle exit, and this high-frequency increase is elevated by the heating effect. In the far field, this high-frequency increase is found at both upstream and downstream shallow radiation angles. Chevrons are effective in reducing the broadband shock-associated noise at this underexpanded jet condition, but the effectiveness is reduced as the jet temperature increases, and the least effective region is at the downstream shallow radiation angles, which are upstream of the peak radiation direction. On the other hand, chevrons show a more consistent effectiveness in reducing the peak mixing noise level at those three heating conditions.

Characterization of supersonic jet noise and its control

As supersonic aircraft and their turbojet engines become more powerful they emit more noise. The principal physical difference between the jets emanating from supersonic jets and those from subsonic jets is the presence of shocks in the supersonic one. This paper summarizes a study of noise reduction technologies applied to supersonic jets. The measurements are performed with a simulated exhaust of a supersonic nozzle representative of supersonic aircraft. The nozzle has a design Mach number of 1.56 and is examined at design and off-design conditions. Several components of noise are present including mixing noise, screech, broadband shock associated noise, and crackle. Chevrons and fluidic injection by microjets and a combination of them are shown to reduce the noise generated by the main jet. These techniques provide significant reduction in jet noise. PIV provides detailed information of the flow and brings out the physics of the noise production and reduction process.