Dennis K. McLaughlin

Biasing correction for individual realization of laser anemometer measurements in turbulent flows

In turbulent flow situations the histograms constructed in the individual realization mode of laser anemometry are biased. The biasing occurs because a larger than average volume of fluid, and hence a larger than average number of scattering centers, pass through the probe volume during periods when the velocity is faster than the mean. Similarly, a smaller volume of fluid and a smaller number of scattering particles pass through the probe volume during periods when the velocity is slower than the mean. The proper weighting function needed to correct the biased data is the inverse of the instantaneous velocity vector. However, an analysis using turbulent flow models show that corrections based only upon the streamwise component of the velocity vector are adequate for many flow situations.

Experiments on the flow and acoustic properties of a moderate-Reynolds-number supersonic jet

An experimental investigation of the flow and acoustic properties of a moderate-Reynolds-number ( Re = 70000), Mach number M = 2·1, axisymmetric jet has been performed. These measurements extended the experimental studies conducted previously in this laboratory to a higher-Reynolds-number regime where the flow and acoustic processes are considerably more complex. In fact, mean-flow and acoustic properties of this jet were determined to be closely comparable to published properties of high-Reynolds-number jets. The major results of the flow-field measurements demonstrate that the jet shear annulus is unstable over a broad frequency range. The initial growth rates and wavelengths of these instabilities as measured by a hot wire were found to be in reasonable agreement with linear stability theory predictions. Also, in agreement with subsonic-jet results, the potential core of the jet was found to be most responsive to excitation at frequencies near a Strouhal number of S = 0·3. The overall development of organized disturbances around S = 0·2 seems to agree in general with calculations performed using the instability theory originally developed by Morris and Tam. The acoustic near field was characterized in terms of sound-pressure level and directivity for both natural and excited (pure-tone) jets. In addition, propagation direction and azimuthal character of dominant spectral components were also measured. It was determined that the large-scale flow disturbances radiate noise in a directional pattern centred about 30° from the jet axis. The noise from these disturbances appears from simple ray tracing to be generated primarily near the region of the jet where the coherent fluctuations saturate in amplitude and begin to decay. It was also determined that the large-scale components of the near-field sound are made up predominately of axisymmetric ( n = 0) and helical ( n = ±1) modes. The dominant noise-generation mechanism appears to be a combination of Mach-wave generation and a process associated with the saturation and disintegration of the large-scale instability. Finally, the further development of a noise-generation model of the instability type appears to hold considerable promise.

Flow field and acoustic properties of a Mach number 0·9 jet at a low Reynolds number

Abstract An experimental study of the flow field and acoustic properties of a low Reynolds number (Re ≅ 3600), M = 0·9 jet has been performed in our low pressure anechoic test chamber. The mean flow field was surveyed with a conventional Pitot pressure probe and flow fluctuations were detected with a normal hot wire probe. Also, condenser microphone measurements were made in the acoustic field. The major goal of the study was to develop a better understanding of the noise generation mechanisms of subsonic jets. The flow fluctuations within the jet were found to be dominated initially by a relatively discrete, large-scale, wave-like instability centered around a Strouhal number of 0·44. The axial wavelength of this instability was determined to be 1·45 jet diameters and its azimuthal character includes the n = 0 and n = ± 1 modes. The growth of this instability coupled with its non-linear breakdown are major contributors to the termination of the potential core region of the jet. The acoustic field of the jet, in contrast to the flow field, has a broad frequency spectrum with a peak amplitude near a Strouhal number of St = 0·2. The results indicate that a non-linear mechanism involving the large scale flow instability is responsible for a dominant portion of the noise generated from this jet.

Experiments on the instability waves in a supersonic jet and their acoustic radiation

An experimental investigation of the instability and the acoustic radiation of the low Reynolds number axisymmetric supersonic jet has been performed. Hot-wire measurements in the flow field and microphone measurements in the acoustic field were obtained from different size jets at Mach numbers of about 2. The Reynolds number ranged from 8000 to 107000, which contrasts with a Reynolds number of 1·3 × 10 6 for similar jets exhausting into atmospheric pressure. Hot-wire measurements indicate that the instability process in the perfectly expanded jet consists of numerous discrete frequency modes around a Strouhal number of 0·18. The waves grow almost exponentially and propagate downstream at a supersonic velocity with respect to the surrounding air. Measurements of the wavelength and wave speed of the St = 0·18 oscillation agree closely with Tams theoretical predictions. Microphone measurements have shown that the wavelength, wave orientation and frequency of the acoustic radiation generated by the dominant instability agree with the Mach wave concept. The sound pressure levels measured in the low Reynolds number jet extrapolate to values approaching the noise levels measured by other experimenters in high Reynolds number jets. These measurements provide more evidence that the dominant noise generation mechanism in high Reynolds number jets is the large-scale instability.

EXPERIMENTS ON THE INSTABILITIES OF A SWIRLING JET

Instabilities present in a swirling jet in the Reynolds number range from 20 000 to 60 000 and a nominal swirl number of 0.5 were studied experimentally, using smoke visualization, hot‐wire anemometry and acoustic excitation. Flow visualization photographs of the natural jet show vortex breakdown at the core and rolling up of the shear layer around the jet into weak, irregular, large‐scale organized structures. When forced by acoustic excitation these structures became energetic and periodic. Axisymmetric and helical instability waves in the Strouhal number range 0.75 to 1.5 were excited and their evolution along the axial direction were measured from velocity spectra and ensemble averaged measurements. Compared to a nonswirling jet, the overall growth of the instability waves is considerably smaller, and vortex pairing is suppressed in a swirling jet. However, the overall spread and mass entrainment rates are higher in the latter. Measurements of the mean velocity components and turbulence fluctuations s...

Noise generation by instabilities in low Reynolds number supersonic jets

Abstract An experimental investigation of noise generation by instabilities in low Reynolds number supersonic air jets has been performed. Sound pressure levels, spectra and acoustic phase fronts were measured with a traversing condenser microphone in the acoustic field of axisymmetric, perfectly expanded, cold jets of Mach numbers 1·4, 2·1 and 2·5. Low Reynolds numbers in the range from Re = 3700 to Re = 8700 were obtained by exhausting the jets into an anechoic vacuum chamber test facility. This contrasts with Reynolds numbers of over 106 for similar jets exhausting into atmospheric pressure. The flow fluctuations of the instability in all three jets have been measured with a hot-wire and the results are documented in a previous paper by Morrison and McLaughlin. Acoustic measurements show that the major portion of the sound radiated by all three jets is produced by the instabilitys rapid growth and decay that occurs near the end of the potential core. This takes place over a relatively short distance (less than two wavelengths of the instability) in the jet. In the lower two Mach number jets the instability has a phase velocity less than the ambient acoustic velocity. In the Mach number 2·5 jet the instability phase speed is 1·11 times the ambient acoustic velocity. In this case the acoustic phase fronts indicate the possibility of a Mach wave component. It was also determined that low level excitation at the dominant frequency of the instability actually decreased the radiated noise by suppressing the broad band component.

Instability process in low Reynolds number supersonic jets

The objective of the present research was to characterize the instability of supersonic jets of Mach numbers 1.4, 2.1, and 2.5 in the Reynolds number range around 8000. In this Reynolds number range the jet instability has its maximum coherence so that its properties as well as the acoustic properties can be most clearly identified. Growth rate, wavelength, and wave orientation of dominant spectral components of the instability for the three Mach number jets were measured in order to characterize the instability. The saturation and subsequent decay on the instability are coincident with a drastic decrease in the coherence of the instability. Associated research shows that the phenomenon of rapid growth and decay is of fundamental importance in the noise generation process.

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.

Acoustic and mean flow measurements of high-speed, helium-air mixture jets

The effectiveness of using helium-air mixture jets to simulate heated jets at both subsonic and supersonic velocities is investigated. Particular emphasis is placed on replicating the aeroacoustic properties of heated jets. Acoustic directivity and spectral measurements of helium-air mixture jets are compared to existing heated jet data. Results typically indicate agreement within 2 dB with some discrepancy attributable to facility and run condition differences. In addition, mean flowfield measurements indicate a shortening of the potential core and a slight decrease in jet spreading with the addition of helium – the same trends observed for heated jets. The similarities in acoustic and mean flow behavior indicate that helium-air mixture jet experiments provide a viable low-cost alternative to heated jet experiments.

Reynolds Number Dependence in Supersonic Jet Noise

An experimental study of the noise production by high speed jets over a wide range of Reynolds numbers has been performed. Two jets of nominal Mach numbers 1.5 and 2.3 were run over a Reynolds number range from 5300 to 107,000. Microphone measurements of the radiated noise and hot-wire measurements of the flow fluctuations demonstrate that at low Reynolds numbers coherent flow instabilities produce a dominant portion of the noise. In the nominal Mach number 2.3 jet these instability waves convect downstream supersonically with respect to the ambient air. In the nominal Mach number 1.5 jet the instabilities convect downstream subsonically. In both cases however, sound pressure level amplitude contours show that the low Reynolds number jets radiate noise comparable to intermediate and high Reynolds number jets. These measurements constitute substantial evidence that a flow instability model of the dominant noise generators may be appropriate for conventional high Reynolds number supersonic jets. a0 C D d M m n r Re St u U Nomenclature speed of sound outside jet wavespeed in the downstream direction diameter of the jet effective diameter of the jet Mach number of the jet at the exit normalized mass velocity fluctuations = azimuthal mode number = radial distance from jet centerline = Reynolds number = p Ud/n. = Strouhal number =fd/ (/(/is frequency) = local velocity =mean centerline velocity of the jet at the nozzle exit