Kevin W. Kinzie

Measurements of supersonic helium/air mixture jets

An enhanced method of using helium/air mixture jets to simulate the aeroacoustic properties of hot jets is presented. By using helium to reduce the jet density and increase the jet acoustic speed, unheated nominal Mach 1.5 jets are tested which have jet-to-ambient density and acoustic speed ratios which approximately match those from a hot jet with a jet-to-ambient static temperature ratio of 1.2. The jets are operated at a reduced Reynolds numbers (approximately 27,000) which allows the use of diagnostic measurement tools such as hot-wire anemometry and active control via glow discharge excitation. Mean and fluctuating flowfield and acoustic measurements from a near perfectly expanded Mach 1.5 elliptic and round jet are presented. Direct comparisons of the cold and simulated heated jets are made. Compared to the pure air jets, the helium/air mixture jets showed increased instability wave phase speeds near or exceeding the ambient acoustic speed, increased noise levels, and increased coupling between the flowfield fluctuations and the radiated acoustic field. These features are consistent with the theory of Mach wave radiation, the dominant noise source in high speed jets. The data presented show that the helium/air simulation is able to capture the dominant noise characteristics of actual heated jets. The use of this group of diagnostic measurement techniques is an added benefit of the simulation which is not available in conventional heated jet experiments.

COMPUTATIONAL ANALYSIS OF A PYLON-CHEVRON CORE NOZZLE INTERACTION

In typical engine installations, the pylon of an engine creates a flow disturbance that interacts with the engine exhaust flow. This interac tion of the pylon with the exhaust flow from a dual stream nozzle was studied computationally. The dual stream nozzle simulates an engine with a bypass ratio of five. A total of five configurations were simulated all at the take -off operating point. All computations were performed using the structured PAB3D code which solves the steady, compressible, Reynolds-averaged Navier- Stokes equations. These configurations included a core nozzle with eight chevron noise reduction devices built into the nozzle trai ling edge. Baseline cases had no chevron devices and were run with a pylon and without a pylon. Cases with the chevron were also studied with and without the pylon. Another case was run with the chevron rotated relative to the pylon. The fan nozzle did not have chevron devices attached. Solutions showed that the effect of the pylon is to distort the round jet plume and to destroy the symmetrical lobed pattern created by the core chevrons. Several overall flow field quantities were calculated that migh t be used in extensions of this work to find flow field parameters that correlate with changes in noise.

Impact of Fluidic Chevrons on Jet Noise

The impact of alternating fluidic core chevrons on the production of jet noise is investigated. Core nozzles for a representative 1/9th scale, bypass ratio 5 model system were manufactured with slots cut near the trailing edges to allow for air injection into the core and fan streams. The injectors followed an alternating pattern around the nozzle perimeter so that the injection alternated between injection into the core stream and injection into the fan stream. For the takeoff condition and a forward flight Mach number of 0.10, the overall sound pressure levels at the peak jet noise angle decrease with increasing injection pressure. Sound pressure levels increase for observation angles less than 110o at higher injection pressures due to increases in high frequency noise. Greater increases in high frequency noise are observed when the number of injectors increases from 8 to 12. When the forward flight Mach number is increased to 0.28, jet noise reduction (relative to the baseline) is observed at aft angles for increasing injection pressure while significant increases in jet noise are observed at forward observation angles due to substantial acoustic radiation at high frequencies. A comparison between inflow and alternating injectors shows that, for equal mass injection rates, the inflow nozzle produces greater low frequency noise reduction (relative to the baseline) than the alternating injectors at 90o and aft observation angles and a forward flight Mach number of 0.28. Preliminary computational fluid dynamic simulations indicate that the spatial decay rate of the hot potential core flow is less for the inflow nozzle than for the alternating nozzles which indicates that gentle mixing may be preferred over sever mixing when fluidic chevrons are used for jet noise reduction.

Turbulent Flow Field Measurements of Separate Flow Round and Chevron Nozzles with Pylon Interaction Using Particle Image Velocimetry

Particle Image Velocimetry (PIV) measurements for six separate flow bypass ratio five nozzle configurations have recently been obtained in the NASA Langley Jet Noise Laboratory. The six configurations include a baseline configuration with round core and fan nozzles, an eight-chevron core nozzle at two different clocking positions, and repeats of these configurations with a pylon included. One run condition representative of takeoff was investigated for all cases with the core nozzle pressure ratio set to 1.56 and the total temperature to 828 K. The fan nozzle pressure ratio was set to 1.75 with a total temperature of 350 K, and the freestream Mach number was M = 0.28. The unsteady flow field measurements provided by PIV complement recent computational, acoustic, and mean flow field studies performed at NASA Langley for the same nozzle configurations and run condition. The PIV baseline configuration measurements show good agreement with mean flow field data as well as existing PIV data acquired at NASA Glenn. Nonetheless, the baseline configuration turbulence profile indicates an asymmetric flow field, despite careful attention to concentricity. The presence of the pylon increases the upper shear layer turbulence levels while simultaneously decreasing the turbulence levels in the lower shear layer. In addition, a slightly shorter potential core length is observed with the addition of the pylon. Finally, comparisons of computational results with PIV measurements are favorable for mean flow, slightly over-predicted for Reynolds shear stress, and under- predicted for Reynolds normal stress components.

Jet-Pylon Interaction of High Bypass Ratio Separate Flow Nozzle Configurations

** An experimental investigation was performed of the acoustic effects of jet-pylon interaction for separate flow and chevron nozzles of both bypass ratio five and eight. The models corresponded to an approximate scale factor of nine. Cycle conditions from approach to takeoff were tested at wind tunnel free jet Mach numbers of 0.1, 0.2 and 0.28. An eight-chevron core nozzle, a sixteen-chevron fan nozzle, and a pylon were primary configuration variables. In addition, two orientations of the chevrons relative to each other and to the pylon were tested. The effect of the pylon on the azimuthal directivity was investigated for the baseline nozzles and the chevron nozzles. For the bypass ratio five configuration, the addition of the pylon reduces the noise by approximately 1 EPNdB compared to the baseline case and there is little effect of azimuthal angle. The core chevron produced a 1.8 EPNdB reduction compared to the baseline nozzle. Adding a pylon to the chevron core nozzle produces an effect that depends on the orientation of the chevron relative to the pylon. The azimuthal directivity variation remains low at less than 0.5 EPNdB. For the bypass ratio eight configuration the effect of adding a pylon to the baseline nozzle is to slightly increase the noise at higher cycle points and for the case with a core chevron the pylon has little additional effect. The azimuthal angle effect continues to be very small for the bypass ratio eight configurations. A general impact of the pylon was observed for both fan and core chevrons at both bypass ratios. The pylon reduces the typical low frequency benefit of the chevrons, even eliminating it in some cases, while not impacting the high frequency. On an equal ideal thrust basis, the bypass ratio eight baseline nozzle was about 5 EPNdB lower than the bypass ratio five baseline nozzle at the highest cycle condition, however, with a pylon installed the difference decreased to about 4 EPNdB.

Aeroacoustic properties of supersonic elliptic jets

The aerodynamic and acoustic properties of supersonic elliptic and circular jets are experimentally investigated. The jets are perfectly expanded with an exit Mach number of approximately 1.5 and are operated in the Reynolds number range of 25 000 to 50 000. The reduced Reynolds number facilitates the use of conventional hot-wire anemometry and a glow discharge excitation technique which preferentially excites the varicose or flapping modes in the jets. In order to simulate the high-velocity and low-density effects of heated jets, helium is mixed with the air jets. This allows the large-scale structures in the jet shear layer to achieve a high enough convective velocity to radiate noise through the Mach wave emission process. Experiments in the present work focus on comparisons between the cold and simulated heated jet conditions and on the beneficial aeroacoustic properties of the elliptic jet. When helium is added to the jet, the instability wave phase velocity is found to approach or exceed the ambient sound speed. The radiated noise is also louder and directed at a higher angle from the jet axis. In addition, near-field hot-wire spectra are found to match the far-field acoustic spectra only for the helium/air mixture case. These results demonstrate that there are significant differences between unheated and heated asymmetric jets in the Mach 1.5 speed range, many of which have been found previously for circular jets. The elliptic jet was also found to radiate less noise than the round jet at comparable operating conditions.

Measurements of Kelvin-Helmholtz instabilities in a supersonic shear layer

Experiments have been performed in a new supersonic shear layer facility in which two streams of air are produced in adjacent supersonic sliding block nozzles. Each stream is about 2.5 cm high and 12.5 cm wide. In the present experiments, the high-speed Mach number ranges from 3 to 4 whereas the low-speed stream was held to M=1.2. Running at low-to-moderate Reynolds numbers allows the mixing layer to be studied in the laminar to turbulent transition region as well as under fully turbulent conditions. Low dynamic pressures permit the use of standard hot-wire anemometry without damage to the fragile sensors. Also, the low pressure in the test section permits the use of glow discharge techniques as a means of exciting the shear layer

Structure of coherent instabilities in a supersonic shear layer

Detailed measurements have been made of the instabilities present in supersonic shear layers. A high-speed stream of Mach number 3 or 4 and a low-speed stream of Mach number 1.2 are produced and begin mixing at the trailing edge of the dividing centerbody. Glow discharge excitation is used to excite either two-dimensional or oblique instability waves. Mach-number profiles for the Mach 3 case show little effect of excitation on the growth rate, whereas the higher Mach number case shows enhanced mixing with both exaltation geometries. Four hot wires are used simultaneously to measure the axial and spanwise wavelengths for each case. With these wavelengths, the propagation angles of the instabilities are calculated.

Turbulence Measurements of Separate-Flow Nozzles with Pylon Interaction Using Particle Image Velocimetry

Particle image velocimetry measurements for separate-flow nozzles with bypass ratio five have recently been obtained in the NASA Langley Jet Noise Laboratory. The six configurations tested include a baseline configuration with round core and fan nozzles, an eight-chevron core nozzle at two different clocking positions, and repeats of these configurations with a pylon included. One run condition representative of takeoff was investigated for all cases. The unsteady flowfield measurements complement recent computational, acoustic, and mean flowfield studies performed at NASA Langley for the same nozzle configurations and run condition. The baseline configuration measurements show good agreement with existing mean and turbulent flowfield data. Nonetheless, the baseline configuration turbulence profile indicates an asymmetric flowfield, despite careful attention to concentricity. The presence of the pylon increases the upper shear layer turbulence levels while simultaneously decreasing the turbulence levels in the lower shear layer. In addition, a slightly shorter potential core length is observed with the addition of the pylon. Finally, comparisons of computational results with current measurements are favorable for mean flow, slightly overpredicted for Reynolds shear stress, and underpredicted for Reynolds normal stress components.

Azimuthal mode measurements of elliptic jets

A procedure to quantitatively measure the relative amplitudes of azimuthal modes in the acoustic field of an elliptic jet is presented. The work describes how the azimuthal modes in an elliptic jet can be represented by a linear combination of Mathieu function modes and how the amplitude coefficients of each individual mode can be determined through an orthogonal decomposition based on Mathieu functions. The modal decomposition is performed in an elliptic cylindrical coordinate system natural to the elliptic jet geometry. The procedure is first tested on an artificially excited, perfectly expanded Mach 1.5 elliptic jet with preferential varicose and flapping mode excitation of discrete frequencies. The excitation was provided with a four electrode glow discharge system with phase control of the individual electrodes. Following that, the procedure was applied to naturally excited Mach 1.5 jets with both air and a helium/air mixture as the jet working gas. The helium/air jets simulate the higher jet velocit...