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 .

Predicting Turbulent Boundary Layer Wall Pressure Fluctuations in Adverse Pressure Gradients

2005 ◽  
Author(s):  
Frank J. Aldrich
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Pressure Gradient ◽  
Pressure Fluctuations ◽  
Adverse Pressure Gradient ◽  
Wall Pressure ◽  
Prediction Tool ◽  
Pressure Gradients ◽  
Wall Pressure Fluctuations ◽  
The Difference

A physics-based approach is employed and a new prediction tool is developed to predict the wavevector-frequency spectrum of the turbulent boundary layer wall pressure fluctuations for subsonic airfoils under the influence of adverse pressure gradients. The prediction tool uses an explicit relationship developed by D. M. Chase, which is based on a fit to zero pressure gradient data. The tool takes into account the boundary layer edge velocity distribution and geometry of the airfoil, including the blade chord and thickness. Comparison to experimental adverse pressure gradient data shows a need for an update to the modeling constants of the Chase model. To optimize the correlation between the predicted turbulent boundary layer wall pressure spectrum and the experimental data, an optimization code (iSIGHT) is employed. This optimization module is used to minimize the absolute value of the difference (in dB) between the predicted values and those measured across the analysis frequency range. An optimized set of modeling constants is derived that provides reasonable agreement with the measurements.

Characterization of the Response of a Curved Elastic Shell to Turbulent Boundary Layer

2010 ◽  
Author(s):  
Francesca Magionesi ◽  
Elena Ciappi
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Pressure Gradient ◽  
Pressure Fluctuations ◽  
Wall Pressure ◽  
Scale Model ◽  
Spectral Densities ◽  
Flow Parameters ◽  
Wall Pressure Fluctuations

For the effective operation of sonar system mounted inside the bulbous of a fast ship, it is important to reduce all the possible noise and vibration sources that cause the dome to vibrate thus radiating noise and interfering with sonar sensor response. In particular, pressure fluctuations induced by the turbulent boundary layer on the surface of the sonar dome represent one of the major sources of self-noise for the on board sensors. Calculation of the structural vibrations and of the noise radiated inside the dome requires as a first step the characterization of the frequency spectra of turbulent boundary layer excitation. Most of the literature related to wall pressure fluctuations is devoted to the study of equilibrium turbulent boundary layers on flat plates in zero pressure gradient (ZPG) flow, for which scaling laws for the power spectral densities and empirical models for the cross spectral densities are well established. The turbulent boundary layer on the bulbous can present several differences with respect to the canonical case because of the presence of hull surface curvatures and of the free water surface that produce pressure gradient variation along the bulbous surface. Moreover, hydrodynamic coincidence effects play a markedly different role in the underwater problem than in the aerodynamic problem. Therefore, an experimental campaign was performed in a towing tank to measure wall pressure fluctuations at different locations along a large scale model of a bulbous and to investigate their spectral characteristics in terms of auto and cross spectral densities. Boundary layer mean flow parameters were obtained with a finite volume code solving the Reynolds Averaged Navier Stokes Equations. The auto spectral densities (ASD) of the measured wall pressure fluctuations were scaled using different combinations of inner and outer flow parameters in order to make ASD independent of the tested conditions i.e. of Reynolds number. The modelled load was used as input for a numerical procedure aimed at evaluating the dynamical response of a section of the bulbous under analysis. The validation of this procedure was experimentally obtained through the measurements of the vibrational response of an elastic section inserted into the bulbous model. Moreover, this comparison indirectly provides additional insights on the physics of wall pressure fluctuations for complex flows.

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.

Hydroacoustics of Transitional Boundary-Layer Flow

1991 ◽  
Vol 44 (12) ◽  
pp. 517-531 ◽  
Author(s):  
Gerald C. Lauchle
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Turbulent Boundary Layer ◽  
Pressure Gradient ◽  
Boundary Layers ◽  
Boundary Layer Flow ◽  
Pressure Fluctuations ◽  
Local Pressure ◽  
Turbulent Spots ◽  
Layer Flow

Transitional boundary layers exist on surfaces and bodies operating in viscous fluids at speeds such that the critical Reynolds number based on the distance from the leading edge is exceeded. The transition region is composed of a simultaneous mixture of both laminar and turbulent regimes occurring randomly in space and time. The turbulent regimes are known as turbulent spots, they grow rapidly with downstream distance, and they ultimately coalesce to form the beginning of fully-developed turbulent boundary-layer flow. It has been long suspected that such a region of unsteadiness may give rise to local pressure fluctuations and radiated sound that are different from those created by the fully-developed turbulent boundary layer at equivalent Reynolds number. This article reviews the available literature on this subject. The emphasis of this literature is on natural and artificially created transitional boundary layers under mostly incompressible conditions; hence, the word hydroacoustics in the title. The topics covered include the dynamics and local wall pressure fluctuations due to the passage of turbulent spots created in a deterministic way, the pressure fluctuations under transitioning boundary layers where the formation and location of spots are random, and the acoustic radiation from transition and its pre-cursor, the Tollmien-Schlichting waves. The majority of this review is for zero-pressure gradient flat plate flows, but the limited literature on axisymmetric body and plate flows with pressure gradient is included.

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.

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.

Wall Pressure Fluctuations in an Axisymmetric Turbulent Boundary Layer under Strong Adverse Pressure Gradient

2020 ◽  
Author(s):  
N. Agastya Balantrapu ◽  
Daniel J. Fritsch ◽  
Anthony J. Millican ◽  
Christopher Hickling ◽  
Aldo Gargiulo ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Pressure Gradient ◽  
Pressure Fluctuations ◽  
Adverse Pressure Gradient ◽  
Wall Pressure ◽  
Wall Pressure Fluctuations

scholarly journals Analytical models of the wall-pressure spectrum under a turbulent boundary layer with adverse pressure gradient

2019 ◽  
Vol 877 ◽  
pp. 1007-1062 ◽  
Author(s):  
G. Grasso ◽  
P. Jaiswal ◽  
H. Wu ◽  
S. Moreau ◽  
M. Roger
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Pressure Gradient ◽  
Pressure Fluctuations ◽  
Adverse Pressure Gradient ◽  
Wall Pressure ◽  
Analytical Models ◽  
Function Technique ◽  
Fast Evaluation ◽  
Wall Pressure Fluctuations

This paper presents a comprehensive analytical approach to the modelling of wall-pressure fluctuations under a turbulent boundary layer, unifying and expanding the analytical models that have been proposed over many decades. The Poisson equation governing pressure fluctuations is Fourier transformed in the wavenumber domain to obtain a modified Helmholtz equation, which is solved with a Green’s function technique. The source term of the differential equations is composed of turbulence–mean shear and turbulence–turbulence interaction terms, which are modelled separately within the hypothesis of a joint normal probability distribution of the turbulent field. The functional expression of the turbulence statistics is shown to be the most critical point for a correct representation of the wall-pressure spectrum. The effect of various assumptions on the shape of the longitudinal correlation function of turbulence is assessed in the first place with purely analytical considerations using an idealised flow model. Then, the effect of the hypothesis on the spectral distribution of boundary-layer turbulence on the resulting wall-pressure spectrum is compared with the results of direct numerical simulation computations and pressure measurements on a controlled-diffusion aerofoil. The boundary layer developing over the suction side of this aerofoil in test conditions is characterised by an adverse pressure gradient. The final part of the paper discusses the numerical aspect of wall-pressure spectrum computation. A Monte Carlo technique is used for a fast evaluation of the multi-dimensional integral formulation developed in the theoretical part.

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