Numerical study of acoustic radiation due to a supersonic turbulent boundary layer

2014 ◽  
Vol 746 ◽  
pp. 165-192 ◽  
Author(s):  
Lian Duan ◽  
Meelan M. Choudhari ◽  
Minwei Wu
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Numerical Simulations ◽  
Surface Pressure ◽  
Free Stream ◽  
Acoustic Radiation ◽  
Numerical Study ◽  
Near Field ◽  
Pressure Fluctuations ◽  
Spatial Coherence

AbstractDirect numerical simulations are used to examine the pressure fluctuations generated by fully developed turbulence in a Mach 2.5 turbulent boundary layer, with an emphasis on the acoustic fluctuations radiated into the free stream. Single- and multi-point statistics of computed surface pressure fluctuations show good agreement with measurements and numerical simulations at similar flow conditions. Consistent with spark shadowgraphs obtained in free flight, the quasi-homogeneous acoustic near field in the free-stream region consists of randomly spaced wavepackets with a finite spatial coherence. The free-stream pressure fluctuations exhibit important differences from the surface pressure fluctuations in amplitude, frequency content and convection speeds. Such information can be applied towards improved modelling of boundary layer receptivity in conventional supersonic facilities and, hence, enable a better utilization of transition data acquired in such wind tunnels. The predicted acoustic characteristics are compared with the limited available measurements. Finally, the numerical database is used to understand the acoustic source mechanisms, with the finding that the supersonically convecting eddies that can directly radiate to the free stream are confined to the buffer zone within the boundary layer.

scholarly journals Pressure fluctuations induced by a hypersonic turbulent boundary layer

2016 ◽  
Vol 804 ◽  
pp. 578-607 ◽  
Author(s):  
Lian Duan ◽  
Meelan M. Choudhari ◽  
Chao Zhang
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Surface Pressure ◽  
Free Stream ◽  
Spatial Scales ◽  
Pressure Fluctuations ◽  
Propagation Speed ◽  
Dominant Frequency ◽  
Mean Velocity ◽  
Taylor’S Hypothesis

Direct numerical simulations (DNS) are used to examine the pressure fluctuations generated by a spatially developed Mach 5.86 turbulent boundary layer. The unsteady pressure field is analysed at multiple wall-normal locations, including those at the wall, within the boundary layer (including inner layer, the log layer, and the outer layer), and in the free stream. The statistical and structural variations of pressure fluctuations as a function of wall-normal distance are highlighted. Computational predictions for mean-velocity profiles and surface pressure spectrum are in good agreement with experimental measurements, providing a first ever comparison of this type at hypersonic Mach numbers. The simulation shows that the dominant frequency of boundary-layer-induced pressure fluctuations shifts to lower frequencies as the location of interest moves away from the wall. The pressure wave propagates with a speed nearly equal to the local mean velocity within the boundary layer (except in the immediate vicinity of the wall) while the propagation speed deviates from Taylor’s hypothesis in the free stream. Compared with the surface pressure fluctuations, which are primarily vortical, the acoustic pressure fluctuations in the free stream exhibit a significantly lower dominant frequency, a greater spatial extent, and a smaller bulk propagation speed. The free-stream pressure structures are found to have similar Lagrangian time and spatial scales as the acoustic sources near the wall. As the Mach number increases, the free-stream acoustic fluctuations exhibit increased radiation intensity, enhanced energy content at high frequencies, shallower orientation of wave fronts with respect to the flow direction, and larger propagation velocity.

Acoustic Radiation From Separated Boundary-Layer Flow Over a Rearward-Facing Step on a Flat Panel

2003 ◽  
Author(s):  
Walter A. Kargus ◽  
Gerald C. Lauchle
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Acoustic Radiation ◽  
Fluid Dynamic ◽  
Pressure Fluctuations ◽  
Wall Pressure ◽  
Wall Jet ◽  
Measured Velocity ◽  
Wall Pressure Fluctuations ◽  
Layer Flow

The acoustic radiation from a turbulent boundary layer that occurs downstream of a rearward facing step discontinuity and reattaches to a flat plat is considred experimentally. The step is exposed ot a zero incidence, uniform subsonic flow. a quiet wall jet facility situated in an anechoic chamber is used for the studies. The “point” wall pressure spectra are measured by small, “pinhole” microphones located at various locations under the layer, including a point directly in the 90° corner of the step. The wall pressure fluctuations measured at the various locations are correlated with the signal detected by a far-field microphone. The measured cross-spectral densities are thus used to identify the relative contributions of the various flow regimes to the direct radiation. It is shown that the separation of the flow over the corner of the step is a dominant acoustic source, which is supported not only by the measured cross spectra, but also by the favorable comparison of the measured velocity power law to the theoretical value. Measurements made where the flow reattaches and at the turbulent boundary layer are less conclusive. This is because the pinhole tube attached to the microphone produced a sound due to a fluid-dynamic oscillation, which contaminated the measurement of the aeroacoustic sources.

scholarly journals Selection of a Suitable Wall Pressure Spectrum Model for Estimating Flow-Induced Noise in Sonar Applications

1995 ◽  
Vol 2 (5) ◽  
pp. 403-412 ◽  
Author(s):  
V. Bhujanga Rao
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
High Speed ◽  
Acoustic Radiation ◽  
Pressure Fluctuations ◽  
Theoretical Data ◽  
Wall Pressure ◽  
Wave Number ◽  
Wall Structure ◽  
Structural Noise

Flow-induced structural noise of a sonar dome in which the sonar transducer is housed, constitutes a major source of self-noise above a certain speed of the vessel. Excitation of the sonar dome structure by random pressure fluctuations in turbulent boundary layer flow leads to acoustic radiation into the interior of the dome. This acoustic radiation is termed flow-induced structural noise. Such noise contributes significantly to sonar self-noise of submerged vessels cruising at high speed and plays an important role in surface ships, torpedos, and towed sonars as well. Various turbulent boundary layer wall pressure models published were analyzed and the most suitable analytical model for the sonar dome application selected while taking into account high frequency, fluid loading, low wave number contribution, and pressure gradient effects. These investigations included type of coupling that exists between turbulent boundary layer pressure fluctuations and dome wall structure of a typical sonar dome. Comparison of theoretical data with measured data onboard a ship are also reported.

Acoustic radiation from a turbulent fluid containing foreign bodies

1960 ◽  
Vol 254 (1276) ◽  
pp. 129-146 ◽  
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Pressure Gradient ◽  
Density Gradient ◽  
Foreign Bodies ◽  
Acoustic Radiation ◽  
Pressure Fluctuations ◽  
Green Functions ◽  
Normal Density ◽  
Dipole Radiation

The problem of the acoustic radiation from a turbulent fluid containing foreign bodies of arbitrary shape and arbitrary composition is solved formally by the method of Green functions. Particular attention is given to the radiation from the surface of these bodies. A practical advantage of the method is that provided an appropriate Green function can be found, either exactly or approximately, then knowledge of the values on the surface of the fluctuations in only one scalar variable is needed to permit estimation of the radiation from the surface. This variable may be either the pressure, the normal pressure gradient, the density, or the normal density gradient. The pressure fluctuations at the surface, in particular, are relatively easy to measure. It is shown that if fluctuations in the fluid are locally isentropic the volume source distribution of the pressure fluctuations is quadrupole. A proof is given of the proposition that when arbitrary obstacles are immersed in a fluid all dipole radiation must come from surface source distributions on these obstacles. For rigid bodies these distributions represent physically the reaction by the obstacles to the stresses imposed upon them by the fluid. It is pro ved that if the density fluctuations or the normal density gradient fluctuations on these surfaces vanish then there is no dipole radiation. The same result is true for pressure and pressure gradient fluctuations within the limits of validity of the assumption of local isentropy. A brief description is given, together with references to more detailed accounts, of some of the principal features of the behaviour of Green functions which may be useful in practical estimates of aerodynamic surface sound. As a representative example of acoustic radiation from a turbulent boundary layer, the total acoustic power radiated by a turbulent boundary layer on an infinite rigid plane is estimated, using the limited available experimental data on wall pressure fluctuations. For low and moderate subsonic speeds the power radiated per unit wall area covered by the turbulent boundary layer is Kρ 0 a 3 0 M 6 0 , where M 0 is the free stream Mach number, ρ 0 is the density and a 0 the speed of sound in the undisturbed fluid, and the dimensionless parameter K is approximately a constant of order of magnitude 10 -5 .

Surface pressure fluctuations in a separating turbulent boundary layer

1987 ◽  
Vol 177 ◽  
pp. 167-186 ◽  
Author(s):  
Roger L. Simpson ◽  
M. Ghodbane ◽  
B. E. Mcgrath
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Turbulent Flow ◽  
Surface Pressure ◽  
Pressure Fluctuation ◽  
Wave Speed ◽  
Pressure Fluctuations ◽  
Shearing Stress ◽  
Flow Surface ◽  
Attached Flow

Measurements of surface pressure-fluctuation spectra and wave speeds are reported for a well-documented separating turbulent boundary layer. Two sensitive instrumentation microphones were used in a new technique to measure pressure fluctuations through pinhole apertures in the flow surface. Because a portion of the acoustic pressure fluctuations is the same across the nominally two-dimensional turbulent flow, it is possible to decompose the two microphone signals and obtain the turbulent flow contributions to the surface pressure spectra. In addition, data from several earlier attached-flow surface-pressure-fluctuation studies are re-examined and compared with the present measurements.The r.m.s. of the surface pressure fluctuation p′ increases monotonically through the adverse-pressure-gradient attached-flow region and the detached-flow zone. Apparently p′ is proportional to the ratio α of streamwise lengthscale to lengthscales in other directions. For non-equilibrium separating turbulent boundary layers, α is as much as 2.5, causing p′ to be higher than equilibrium layers with lower values of α.The maximum turbulent shearing stress τM appears to be the proper stress on which to scale p′; p′/τM from available data shows much less variation than when p′ is scaled on the wall shear stress. In the present measurements p′/τM increases to the detachment location and decreases downstream. This decrease is apparently due to the rapid movement of the pressure-fluctuation-producing motions away from the wall after the beginning of intermittent backflow. A correlation of the detached-flow data is given that is derived from velocity- and lengthscales of the separated flow.Spectra Φ (ω) for ωδ*/U∞ > 0.001 are presented and correlate well when normalized on the maximum shearing stress τM. At lower frequencies, for the attached flow Φ (ω) ∼ ω−0.7 while Φ(ω) ∼ (ω)−3 at higher frequencies in the strong adverse-pressuregradient region. After the beginning of intermittent backflow, Φ(ω) varies with ω at low frequencies and ω−3 at high frequencies; farther downstream the lower-frequency range varies with ω1.4.The celerity of the surface pressure fluctuations for the attached flow increases with frequency to a maximum; at higher frequencies it decreases and agrees with the semi-logarithmic overlap equation of Panton & Linebarger. After the beginning of the separation process, the wave speed decreases because of the oscillation of the instantaneous wave speed direction. The streamwise coherence decreases drastically after the beginning of flow reversal.

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