On the convection velocity of source events related to supersonic jet crackle

2016 ◽  
Vol 793 ◽  
pp. 477-503 ◽  
Author(s):  
Nathan E. Murray ◽  
Gregory W. Lyons
Keyword(s):  
Shock Front ◽  
Shear Layer ◽  
Supersonic Jet ◽  
Extreme Value Distribution ◽  
Front Propagation ◽  
Convection Velocity ◽  
Analysis Method ◽  
Acoustic Measurements ◽  
Shock Fronts ◽  
Field Event

An image analysis method is developed and applied to shadowgraph images of supersonic jet flow to measure shock front propagation angles at numerous interrogation points distributed throughout the quiescent region outside of the jet shear layer. These shock fronts manifest in acoustic measurements of jet noise as steepened temporal waveforms that have been linked to the perception of crackle. The analysis method uses the Radon transform to quantitatively determine a local shock front propagation angle at each point. The dataset of angles is subsequently used to determine the locations and convection velocities of the sources inside the jet shear layer. The results indicate that the shock-like waves emerge immediately from the jet shear layer and are created by the supersonic convection of coherent structures. The statistical distribution of convection velocities follows an extreme value distribution, indicating that the shock front emitting sources are maxima of the underlying turbulence. A noise reduction method known to reduce the convection velocities in the jet shear layer is applied to the jet to investigate the effect on the shock front emission. The shock front angles change in concert with the reduction in convection velocity giving further evidence that the source of crackle is a flow field event.

Aero-Acoustic Coupling Inside Large Deep Cavities at Low-Subsonic Speeds

2008 ◽  
Vol 131 (1) ◽  
Author(s):  
Mouhammad El Hassan ◽  
Laurent Keirsbulck ◽  
Larbi Labraga
Keyword(s):  
Wind Tunnel ◽  
Shear Layer ◽  
Convection Velocity ◽  
Pressure Amplitude ◽  
Hydrodynamic Mode ◽  
Freestream Velocity ◽  
Acoustic Coupling ◽  
Deep Cavity ◽  
Cross Correlations ◽  
Deep Cavities

Aero-acoustic coupling inside a deep cavity is present in many industrial processes. This investigation focuses on the pressure amplitude response, within two deep cavities characterized by their length over depth ratios (L/H=0.2 and 0.41), as a function of freestream velocities of a 2×2m2 wind tunnel. Convection velocity of instabilities was measured along the shear layer, using velocity cross-correlations. Experiments have shown that in deep cavity for low Mach numbers, oscillations of discrete frequencies can be produced. These oscillations appear when the freestream velocity becomes higher than a minimum value. Oscillations start at L/θ0=10 and 21 for L/H=0.2 and 0.41, respectively. The highest sound pressure level inside a deep cavity is localized at the cavity floor. A quite different behavior of the convection velocity was observed between oscillating and nonoscillating shear-layer modes. The hydrodynamic mode of the cavity shear layer is well predicted by the Rossiter model (1964, “Wind Tunnel Experiments on the Flow Over Rectangular Cavities at Subsonic and Transonic Speeds,” Aeronautical Research Council Reports and Memo No. 3438) when measured convection velocity is used and the empirical time delay is neglected. For L/H=0.2, only the first Rossiter mode is present. For L/H=0.41, both the first and the second modes are detected with the second mode being the strongest.

The structure of a highly decelerated axisymmetric turbulent boundary layer

2021 ◽  
Vol 929 ◽  
Author(s):  
N. Agastya Balantrapu ◽  
Christopher Hickling ◽  
W. Nathan Alexander ◽  
William Devenport
Keyword(s):  
Boundary Layer ◽  
Pressure Gradient ◽  
Shear Layer ◽  
Layer Thickness ◽  
Boundary Layer Thickness ◽  
Large Scale ◽  
Convection Velocity ◽  
Mean Flow ◽  
Mean Velocity ◽  
The Mean

Experiments were performed over a body of revolution at a length-based Reynolds number of 1.9 million. While the lateral curvature parameters are moderate ( $\delta /r_s < 2, r_s^+>500$ , where $\delta$ is the boundary layer thickness and r s is the radius of curvature), the pressure gradient is increasingly adverse ( $\beta _{C} \in [5 \text {--} 18]$ where $\beta_{C}$ is Clauser’s pressure gradient parameter), representative of vehicle-relevant conditions. The mean flow in the outer regions of this fully attached boundary layer displays some properties of a free-shear layer, with the mean-velocity and turbulence intensity profiles attaining self-similarity with the ‘embedded shear layer’ scaling (Schatzman & Thomas, J. Fluid Mech., vol. 815, 2017, pp. 592–642). Spectral analysis of the streamwise turbulence revealed that, as the mean flow decelerates, the large-scale motions energize across the boundary layer, growing proportionally with the boundary layer thickness. When scaled with the shear layer parameters, the distribution of the energy in the low-frequency region is approximately self-similar, emphasizing the role of the embedded shear layer in the large-scale motions. The correlation structure of the boundary layer is discussed at length to supply information towards the development of turbulence and aeroacoustic models. One major finding is that the estimation of integral turbulence length scales from single-point measurements, via Taylor's hypothesis, requires significant corrections to the convection velocity in the inner 50 % of the boundary layer. The apparent convection velocity (estimated from the ratio of integral length scale to the time scale), is approximately 40 % greater than the local mean velocity, suggesting the turbulence is convected much faster than previously thought. Closer to the wall even higher corrections are required.

scholarly journals Application of the Gravity Flow Theory to the Percolation of Melt Water Through Firn

1981 ◽  
Vol 27 (95) ◽  
pp. 67-75 ◽  
Author(s):  
W. Ambach ◽  
M. Blumthaler ◽  
P. Kirchlechner
Keyword(s):  
Experimental Data ◽  
Shock Front ◽  
Water Flux ◽  
Flow Theory ◽  
Gravity Flow ◽  
Total Thickness ◽  
Total Input ◽  
Melt Water ◽  
Shock Fronts ◽  
Good Agreement

Abstract Application of the gravity flow theory to the percolation of melt water through the firn in the accumulation area of a temperate glacier explains the occurrence of shock fronts in the melt-water flux. The time of propagation of a shock front moving from the surface through the entire firn was calculated under various assumptions. Various time input functions of melt-water flux at the surface with constant total input volumes yield only slight differences in the time of propagation of the shock front at greater depths. The dependence of the time of propagation of a shock front on the input volume, on snow parameters, and on the total thickness of the firn was calculated. An approximately linear relation was found to exist between the time of propagation of a shock front moving through the firn and the total thickness of the firn. The drainage of melt water from the firn after the summer ablation period is also quantitatively explained by the gravity flow theory. All results are in good agreement with experimental data.

scholarly journals Application of the Gravity Flow Theory to the Percolation of Melt Water Through Firn

1981 ◽  
Vol 27 (95) ◽  
pp. 67-75 ◽  
Author(s):  
W. Ambach ◽  
M. Blumthaler ◽  
P. Kirchlechner
Keyword(s):  
Experimental Data ◽  
Shock Front ◽  
Water Flux ◽  
Flow Theory ◽  
Gravity Flow ◽  
Total Thickness ◽  
Total Input ◽  
Melt Water ◽  
Shock Fronts ◽  
Good Agreement

AbstractApplication of the gravity flow theory to the percolation of melt water through the firn in the accumulation area of a temperate glacier explains the occurrence of shock fronts in the melt-water flux. The time of propagation of a shock front moving from the surface through the entire firn was calculated under various assumptions. Various time input functions of melt-water flux at the surface with constant total input volumes yield only slight differences in the time of propagation of the shock front at greater depths. The dependence of the time of propagation of a shock front on the input volume, on snow parameters, and on the total thickness of the firn was calculated. An approximately linear relation was found to exist between the time of propagation of a shock front moving through the firn and the total thickness of the firn. The drainage of melt water from the firn after the summer ablation period is also quantitatively explained by the gravity flow theory. All results are in good agreement with experimental data.

Near- and Far-Field Acoustic Measurements for Stepped Nozzles at Over- and Perfectly-Expanded Supersonic Jet Flow Conditions

2020 ◽  
Vol 142 (11) ◽  
Author(s):  
X. F. Wei ◽  
L. P. Chua ◽  
Z. B. Lu ◽  
H. D. Lim ◽  
R. Mariani ◽  
...  
Keyword(s):  
Near Field ◽  
Source Region ◽  
Supersonic Jet ◽  
Jet Flow ◽  
Far Field ◽  
Acoustic Measurements ◽  
Flow Conditions ◽  
Spectra Analysis ◽  
Highly Sensitive ◽  
Nozzle Configuration

Abstract Detailed near- and far-field acoustic measurements were conducted for two circular stepped nozzles with 30 deg and 60 deg design inclinations at over- and perfectly-expanded supersonic jet flow conditions and compared to those for a circular nonstepped nozzle. Far-field acoustic results show that stepped nozzles play an insignificant role in altering noise emissions at perfectly expanded condition. At an over-expanded condition, however, the longer stepped nozzle produces significant noise reductions at the sideline and upstream quadrants, while the shorter stepped nozzle does not. Noise spectra analysis and Schlieren visualizations show that noise reduction can be primarily attributed to mitigations in the broadband shock-associated noise (BSAN), due to the ability of the longer stepped nozzle in suppressing shock strengths at downstream region. Near-field acoustic measurements reveal that the source region, as well as the intensity of turbulent and shock noises, are highly sensitive to the stepped nozzle configuration. Furthermore, BSAN seems to be eliminated by the longer stepped nozzle in near-field region due to the shock structure modifications.

scholarly journals Investigation into the turbulence statistics of installed jets using hot-wire anemometry

2020 ◽  
Vol 61 (10) ◽  
Author(s):  
Anderson Proença ◽  
Jack Lawrence ◽  
Rod Self
Keyword(s):  
Shear Layer ◽  
Turbulence Intensity ◽  
Single Point ◽  
Radial Distance ◽  
Convection Velocity ◽  
Turbulence Characteristic ◽  
Hot Wire ◽  
Mean Flow ◽  
Turbulence Statistics ◽  
Jet Mixing

Abstract This work presents a detailed study of the turbulence flow statistics of a jet mounted with its axis parallel to a rigid flat plate. Hot-wire constant temperature anemometry has been used to measure the single-point and two-point statistics of the axial velocity component at several locations within the jet flow field. Results show that the jet mean flow near the plate surface is subjected to a local acceleration and redirection due to a Coandă-type effect. The propagation of these effects downstream of the plate trailing edge is strongly dependent on the plate position. Regarding the velocity fluctuations, the mean turbulence intensity levels are seen to decrease as the radial distance between the jet and surface decreases. Analysis of the single-point power spectral density data on the shear layer close to the plate shows that the reduction in magnitude of the low-frequency content of the energy spectrum is responsible for the decrease in turbulence intensity. Additionally, the characteristic time and length scales computed from two-point measurements reduce as the plate is mounted closer to the jet centre-line. The axial eddy convection velocity is seen to increase in the region of high turbulent kinetic energy in the shear layer adjacent to the surface. Empirical models for turbulence characteristic scales and eddy convection velocity are presented. These findings suggest that both the amplitude and distribution of the jet mixing noise sources are affected when closely installed next to a surface. This paper is a continuation of a recent investigation on the turbulence statistics of isolated jets presented in Proença (Exp Fluids 60(4):63, 2019). Graphic abstract

Flow field and noise characteristics of a supersonic impinging jet

1999 ◽  
Vol 392 ◽  
pp. 155-181 ◽  
Author(s):  
A. KROTHAPALLI ◽  
E. RAJKUPERAN ◽  
F. ALVI ◽  
L. LOURENCO
Keyword(s):  
Large Scale ◽  
Ground Plane ◽  
Large Diameter ◽  
Supersonic Jet ◽  
Jet Impingement ◽  
Impinging Jet ◽  
Acoustic Measurements ◽  
Sonic Nozzle ◽  
Underexpanded Jets ◽  
Noise Characteristics

This paper describes the results of a study examining the flow and acoustic characteristics of an axisymmetric supersonic jet issuing from a sonic and a Mach 1.5 converging–diverging (C–D) nozzle and impinging on a ground plane. Emphasis is placed on the Mach 1.5 nozzle with the sonic nozzle used mainly for comparison. A large-diameter circular plate was attached at the nozzle exit to measure the forces generated on the plate owing to jet impingement. The experimental results described in this paper include lift loss, particle image velocimetry (PIV) and acoustic measurements. Suckdown forces as high as 60% of the primary jet thrust were measured when the ground plane was very close to the jet exit. The PIV measurements were used to explain the increase in suckdown forces due to high entrainment velocities. The self-sustained oscillatory frequencies of the impinging jet were predicted using a feedback loop that uses the measured convection velocities of the large-scale coherent vortical structures in the jet shear layer. Nearfield acoustic measurements indicate that the presence of the ground plane increases the overall sound pressure levels (OASPL) by approximately 8 dB relative to a corresponding free jet. For moderately underexpanded jets, the influence of the shock cells on the important flow features was found to be negligible except for close proximity of the ground plane.

Interferometrische Untersuchungen an elektromagnetisch beschleunigten Stoßwellen

1965 ◽  
Vol 20 (2) ◽  
pp. 196-202 ◽  
Author(s):  
H. Brinkschulte ◽  
H. Muntenbruch
Keyword(s):  
Shock Waves ◽  
Shock Front ◽  
Discharge Plasma ◽  
Shock Velocity ◽  
Computational Method ◽  
Density Jump ◽  
Precursor Effects ◽  
Shock Fronts ◽  
Selection Of ◽  
Made In

The phenomena of shock waves generated electromagnetically in T-tubes were studied with a MACH-ZEHNDER interferometer. The measurements were made in hydrogen at initial pressures from 2.5 to 10 mm Hg. Shock velocity varied between Mach 6 and Mach 20. It was found that there are two fronts: the luminousity front due to the discharge plasma and the non-luminous shock front in front of this. The distance between the shock front and the luminousity front decreases with increasing velocity. At vs ⍙ Mach 20 the luminousity front reaches the shock front. Shock fronts are always plane. The density decreases directly behind the shock front. The shock waves thus formed cannot be described with the RANKINE-HUGONIOT equations. At small velocities, the density jump is 6, at higher velocities the gas is dissociated. The refractive index of atomic hydrogen can be measured. Simultaneously the selection of the computational method used to describe the shock conditions in hydrogen can be justified. Precursor effects have no influence, relaxations could not be seen.

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