Pablo A. Mora

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...

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

Acoustics from a Rectangular C-D Nozzle Exhausting Over a Flat Surface

In futuristic high-speed aircraft, jets in propulsion systems that are closely integrated with the aircraft would have to be exhausted over airframe surfaces. Also, during military take-off and landing operations, intense noise waves are reflected from the ground and the jet-surface interaction becomes another source of noise generation. To simulate these scenarios, far-field acoustics are investigated on a supersonic rectangular jet that is exhausted over a parallel flat surface, at different proximities to the jet plume (h). An Md = 1.5 converging-diverging rectangular nozzle of 2:1 aspect ratio was tested with and without a flat plate that extends 30De downstream of the nozzle exit and has a width of 21De. The plate is moved from h/ De = 0 (when it touches the inner side of the nozzle exit lip,) to different ‘stand-off’ positions (away from the jet axis,) up to h/ De = 3. The jet is predicted to impinge on the plate, in all configurations. Far-field cold and heated jet results at design, overexpanded and under-expanded conditions are shown at jet azimuthal angles of φ = 0 o

Impact of Scale on the Acoustics from a Conical C-D Nozzle Interacting with a Flat Surface

Advances in jet technology have pushed towards faster aircraft, leading to more streamlined designs and configurations, pushing engines closer to the aircraft frame. This creates additional noise sources stemming from interactions between the jet flow and surfaces on the aircraft body, as well as interactions between the jet and the ground during takeoff and landing. This paper studies how the presence of a flat plate affects the flow and acoustics in the supersonic jet from a circular C-D nozzle, and how these scale to other nozzle diameters. Comparisons are drawn between baseline cases without a plate and varying nozzle diameters to properly assess the scaling impact of the plate at NPRs of 2.5, 3.0, 3.67, 4.0 and 4.5, and ultimately temperature ratios up to 3.0. At an azimuthal angle of 180° the flat surface causes a shielding effect that mitigates jet mixing noise, and broadband shock-associated noise. Conversely, the sideline position at the azimuthal angle of 90° enhances OASPL magnitudes across all conditions. The study found that plate acoustics scale very well with increasing nozzle diameter, although plate shielding at 180° was the one outlier, causing less dampening of noise across all angles relative to the medium nozzle. Screech tones were not adversely affected by the plate presence, producing significant spikes even at the design condition at an azimuthal angle of 90° (largest spike ~15dB). The frequency of peaks in the upstream and downstream spectra, as well as the frequency of screech tones was matched regardless of nozzle scale. The high screech case, NPR of 3.0, even produces screech for shielded and reflected cases (up to ~30dB), though temperature mitigates these tones across all test conditions. Ultimately these results are compared to a nozzle designed through method of characteristic and a rectangular nozzle of equivalent diameter.

Investigation of a Heated Supersonic Jet Chevrons Nozzle

Jet noise radiated from high-speed supersonic military aircrafts operating at high-power and after-burner conditions is highly dominated by Mach wave radiation and crackle. This paper studies how adding chevrons (0.08D penetration) to an Md = 1.5 converging-diverging conical nozzle impacts the jet noise intensity and directivity. Results are shown for jet temperature ratios of 2.0, 2.5, and 3.0, at design, over-expanded and under-expanded conditions. The selected chevrons configuration collapses the set of double-shocks from the baseline into an stronger single set, increases the shock-cell spacing, and reduces the potential core length. The chevrons also increased TKE levels near the nozzle exit and reduced levels downstream. The chevrons were effective at decreasing OASPL, pressure skewness, and dP/dt skewness in the near and far fields, and therefore Mach wave radiation and crackle are expected to be effectively mitigated. Chevrons had a greater degree of impact on the statistics of the TR 2.0 condition as compared to the TR = 3.0, and at the design and under-expanded conditions, as compared to the over-expanded case. Also, while chevrons reduced mixing in the aft quadrant of the jet, shock noise was increased in the sideline.

Acoustic field of a shielded rectangular supersonic jet

The impact of a flat surface installed parallel to a supersonic rectangular jet of AR = 2 on the far field noise is studied. Far-field cold jet results at design, over-expanded and under-expanded conditions are compared between the free-field jet, the nozzle with the surface matching with the exit lip (h = 0De), and the surface at different stand-off positions away from the jet axis, h = 1, 2, 3 De. Results are shown for all jet azimuthal angles. When the surface is installed at h = 0De, broadband shock-associated noise intensity was decreased and its peak frequency shifted. At NPR = 2.5, shock noise appears to be entirely mitigated. Also, a low frequency noise component is observe below St = 0.15, and assumed to be related to the trailing edge/jet plume interaction. At NPR = 3.0, strong screech tones are mitigated with the surface installed at h = 0De. In the shielded region, noise levels are significantly lower for all plate positions. Mixing noise and Mach wave radiation are affected by the plat stand-...

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.