Nick Heeb

Supersonic Jet Noise Reduction Technologies for Gas Turbine Engines

This paper presents observations and simulations of the impact of several technologies on modifying the flow-field and acoustic emissions from supersonic jets from nozzles typical of those used on military aircraft. The flow-field is measured experimentally by shadowgraph and particle image velocimetry. The acoustics are characterized by near- and far-field microphone measurements. The flow- and near-field pressures are simulated by a monotonically integrated large eddy simulation. Use of unstructured grids allows accurate modeling of the nozzle geometry. The emphasis of the work is on “off-design” or nonideally expanded flow conditions. The technologies applied to these nozzles include chevrons, fluidic injection, and fluidically enhanced chevrons. The fluidic injection geometry and the fluidic enhancement geometry follow the approach found successful for subsonic jets by employing jets pitched 60 deg into the flow, impinging on the shear layer just past the tips of the chevrons or in the same axial position when injection is without chevrons.

ACOUSTIC EFFECT of CHEVRONS on JETs Exiting Conical C-D NOZZLES

This paper describes a joint study of the acoustics of conical convergent-divergent (C-D) nozzles such as those found on high-performance military aircraft. In this paper we examine the influence of chevrons on a nozzle with an area ratio corresponding to a design Mach number of 1.5 (design pressure ratio of 3.67). The nozzle is tested at its design condition and at pressure ratios of 2.5, 3.0 and 3.5 representing overexpanded conditions and 4.0, 4.5 and 5.0 representing underexpanded conditions. Each case is compared to a baseline nozzle at the same condition without chevrons. Shadowgraph images show that chevrons reduce the shock cell spacing, and introduce additional weak shock waves. Far-field acoustic measurements show that the application of chevrons reduces screech, broad-band shock-associated noise and mixing noise except for frequencies above the broad-band shock-associated noise peak for over-expanded and perfectly expanded conditions. Near-field acoustic reveal that chevrons produce significant noise near the nozzle exit across the whole range of frequencies, but reduce mixing noise, and broad-band shock-associated noise elsewhere and they all but eliminate screech.

Impact of Heat on the Pressure Skewness and Kurtosis in Supersonic Jets

Mach wave radiation and crackle are dominant noise components from high-speed jets, found in both high-power engines and scale nozzles. The statistics of the pressure signal and its time derivative (dP/dt) have been widely studied to identify and quantify crackle. In this paper, we investigate the impact of operating condition on the overall sound pressure level, skewness, and kurtosis of the pressure and dP/dt signals of a jet issuing from an Md=1.5 converging–diverging conical nozzle. The effect of temperature and nonideal expansion were independently investigated. An increase in convective Mach number Mc, achieved by increasing either jet temperature or nozzle pressure ratio, proved to be related to elevated values of overall sound pressure level, skewness, and kurtosis, in both the near and far fields. The peak values of overall sound pressure level, skewness levels, and kurtosis levels were found to propagate at different angles for cold jets, but at elevated temperature, the directivity was more sim...

An experimental investigation of the flow dynamics of streamwise vortices of various strengths interacting with a supersonic jet

The results of an experimental study involving the introduction of streamwise vorticity into a supersonic jet are presented. Both streamwise and cross stream PIV measurements of a baseline jet and three vortex generator configurations of varied penetration were acquired in the overexpanded, ideally expanded, and underexpanded regimes. Streamwise vortex pairs were shown to persist no further than two diameters downstream independent of initial magnitude. Integration of the modulus of streamwise vorticity was shown to better correlate with secondary flow field modifications than maximum values. Reductions in shock cell spacing and downstream turbulence, and increases in initial spread rate and upstream integrated turbulence were correlated with increases in streamwise vorticity for all operating conditions. Streamwise vorticity was shown to achieve opposite effects on shock strength in the over/under expanded regimes due to introduction of secondary shock structures in the underexpanded case. Additionally, limited effect on potential core length was quantified.

SUPERSONIC JET NOISE FROM A CONICAL C-D NOZZLE WITH FORWARD FLIGHT EFFECTS

Flow and far-field noise measurements are taken on a conical ConvergentDivergent nozzle similar to the nozzles employed on high-performance tactical jets. Matching flow and far-field computations are presented, produced by Large Eddy Simulation and the Kirchhoff integral method. The conditions examined are those in which the nozzle is operated at its design Mach number of 1.56 while forward flight is simulated at Mach numbers of 0.1, 0.3 and 0.8. Both measurement and LES show that increasing forward flight Mach number to the high subsonic range shortens the initial shock cell size, and weakens the shock cells induced by the nozzle throat relative to the shock cells induced by the nozzle lip. LES shows that high forward flight speed substantially reduces the noise radiated into the forward quadrant where shock noise is dominant. It also removes the screech tone entirely.

Near- and Far-Field Pressure Skewness and Kurtosis in Heated Supersonic Jets From Round and Chevron Nozzles

When the turbulent structures in the shear layer of high-speed jets travel at supersonic convective speeds relative to the ambient speed of sound, they radiate Mach waves that become the dominant component of the overall perceived noise. This is consistent with the OASPL in the far field reaching a maximum in same direction as the Mach wave angle. When the speed of the supersonic jet exceeds a certain level, the steepening of the wave-front in the near field produces a noise feature called “crackle.” Both pressure wave steepening and crackle cannot be recognized in the spectrum of the pressure signal, but in the temporal waveform of the pressure. The statistics of the pressure signal and its time derivative, particularly skewness, have become standard measures of crackle in heated supersonic jets. Previous studies showed that it is possible to reduce far-field pressure skewness with the implementation of notched and chevron nozzles, and to mitigate Mach Wave radiation with secondary flow techniques. In this paper, we investigate the effect of chevrons on the pressure and dP/dt high-order statistics of a Md = 1.5 converging-diverging round conical nozzle, both in the near and far fields. Cold and heated jets, To = 300 K and 600 K, are tested at over, design, and under-expanded conditions. Far-field results of the heated jet showed that chevrons effectively reduce elevated levels of skewness and kurtosis of the pressure and dP/dt. These reductions are remarkable especially around the Mach Wave angle, the region in which high-order statistics tend to propagate. Near-field results corroborated the effectiveness of chevrons in the skewness reduction.Copyright

Effect of Scale on the Far-Field Pressure Skewness and Kurtosis of Heated Supersonic Jets

In heated supersonic jets, Mach wave radiation and crackle have been identified as dominant noise components that propagate to the downstream region of the jet, in a direction noted as the Mach wave angle. At certain conditions, the Mach waves coalesce in the near field causing steepening of the wavefront, which exceeding a certain level produces a noise feature called “crackle.” The skewness and kurtosis of the pressure and its time derivative (dP/dt) have been widely studied as a measurement of crackle. In this paper, we investigate the impact of different test conditions and different nozzle exit diameters on the far-field high-order statistics of the pressure and dP/dt signals of three converging-diverging conical nozzles, with a design Mach number of 1.5 and jet exit diameters of 0.542, 0.813, and 1.085. Results are compared to a smooth contoured nozzle designed by the Method of Characteristics, with the same design Mach number. For all nozzles, cold and heated jets, TR=1.0 to 3.0, are tested at over, design, and under-expanded conditions. Second, third, and fourth order statistics are examined in three far-field arrays positioned at a nondimensionalized constant radial distance of r=40De. The OASPL, skewness, and kurtosis magnitudes and their propagation angles are proportional to the jet temperature and the NPR, and have peak amplitudes near the Mach wave angle. The pressure skewness and kurtosis plots collapsed for all three scaled nozzles when the pressure signals were not filtered. The dP/dt statistics collapsed when the signals were downsampled proportional to the nozzle exit diameters, applying beforehand a low-pass filter at a proportional cutoff frequency, to avoid aliasing effects.

Supersonic Jet Noise Reduction by Chevrons Enhanced with Fluidic Injection

Fluidically enhanced mechanical chevrons are experimentally investigated for over, under, and ideally expanded operating conditions. Results show appreciable modification to the velocity and turbulent kinetic energy fields as well as a reduction in shock cell spacing due to fluidics in the overexpanded operating range. Negligible influence on the flow field is seen for underexpanded and ideally expanded operating conditions due to insufficient momentum flux ratios. Modification to the flow field is related to the far field acoustic field and the mechanical chevron’s acoustic benefit is seen to improve by over 1dB in terms of overall sound pressure level.

Supersonic Jet Noise Reduction Using Fluidics, Mechanical Chevrons and Fluidically Enhanced Chevrons

This paper presents observations and simulations of the impact of several technologies on modifying the flow field and acoustic emissions from supersonic jets from nozzles typical of those used on military aircraft. The flowfield is measured experimentally by shadowgraph and particle image velocimetry (PIV). The acoustics are characterized by near and far-field microphone measurements. The flow and near-field pressures are simulated by monotonically integrated large-eddy simulation (MILES). Use of unstructured grids allows accurate modeling of the nozzle geometry. The nozzle geometries used in this research are representative of practical engine nozzles. The emphasis of the work is on “off-design” or non-ideally expanded flow conditions. The technologies applied to these nozzles include chevrons, fluidic injection and fluidically enhanced chevrons. The fluidic injection geometry and fluidic enhancement geometry follow the approach found successful for subsonic jets by Alkislar, Krothapalli & Butler [1] employing jets pitched 60° into the flow, impinging on the shear layer just past the tips of the chevrons, or in the same axial position when injection is without chevrons.© 2010 ASME

Supersonic Jet Noise Reduction by Chevrons and Fluidic Injection

This work focuses on the noise reduction capabilities of chevrons, fluidic injection, and a combination of both. The control mechanisms were experimentally investigated for over, under, and ideally expanded operating conditions. Acoustic far-field data was collected to quantify noise reductions and changes to the flow field through use of theoretical relations. Results show appreciable noise reduction by both fluidic injection and chevrons. Optimal performance of the two technologies occured at opposite ends of the operation envelope, with the chevrons performing better in the underexpanded regime and the fluidics in the overexpanded regime. The combination of the two technologies was also shown to achieve substantial noise reduction, but direct additive benefits were not always gained. This result is still important as it shows fluidic injection can be employed on complex geometry nozzles, such as that on the F-35, and still achieve noise reduction.