1,720,983 research outputs found
Prediction of boundary layer sound radiation from wall shear stresses using DNS data
No full textSound radiation from a plane turbulent boundary layer is investigated using databases from direct numerical simulations (DNS) of plane turbulent Poiseuille flow up to Reynolds number Re? = 1440. Correlation areas for fluctuating wall shear stresses are found to collapse on viscous scaling over the current Reynolds number range. The power spectral density of radiated pressure and the spectrum of radiated sound power, per unit wall area, are predicted in the low Mach number limit by solving a Ffowcs Williams–Hawkings type wave equation using a half-space Green function. The same DNS data are used to predict the spectrum of turbulent boundary layer noise measured in a diffuser downstream of a fully-developed channel flow [Greshinov & Mironov, Soviet Physics Acoustics, 29(4):257-280,1983]. The measured spectrum is higher at low frequencies, but converges with the prediction at high frequencies
Modelling of turbulent jets and wall layers: extensions of Lighthill's acoustic analogy with application to computational aeroacoustics
No full textTwo extensions to Lighthill’s aeroacoustic analogy are presented. First, equivalent sources due to initial conditions are derived that supplement those due to boundary conditions, as given by Ffowcs Williams & Hawkings. The resulting exact inhomogeneous wave equation is then reformulated with pressure rather than density as the wave variable, and the right-hand side is rearranged using the energy equation with no additional assumptions. Applications to computational aeroacoustics are discussed, and illustrated with examples based on 2D and 3D simulations
Wall pressure and shear stress spectra from direct numerical simulations of channel flow
No full textWall pressure and shear stress spectra from direct numerical simulations of turbulent plane channel flow are presented in this paper. Simulations have been carried out at a series of Reynolds numbers up to Re? = 1440, which corresponds to Re = 6:92 x 10(4) based on channel width and centerline velocity. Single-point and two-point statistics for velocity, pressure, and their derivatives have been collected, including velocity moments up to fourth order.§ The results have been used to study the Reynolds number dependence of wall pressure and shear stress spectra. It is found that the point spectrum of wall pressure collapses for Re? ? 360 under a mixed scaling for frequencies lower than the peak frequency of the frequency-weighted spectrum, and under viscous scaling for frequencies higher than the peak. Point spectra of wall shear stress components are found to collapse for Re? ? 360 under viscous scaling. The normalized mean square wall pressure increases linearly with the logarithm of Reynolds number. The rms wall shear stresses also increase with Reynolds number over the present range, but suggest some leveling off at high Reynolds number
High resolution x-ray characterization of periodically domain-inverted nonlinear optical crystals
Full text linkA high-resolution triple-axis diffractometer has been used for the structural characterization of periodically domain-inverted nonlinear optical crystals of KTiOPO4 and LNbO3. Striations have been revealed in high strain-sensitivity multiple-crystal topographs of the domain-inverted regions of both these samples and these are dominated by orientation contrast. The combination of high-resolution reciprocal-space mapping and topography has shown that the extended diffraction streak in the q[210] direction for domain-inverted LiNbO3 originates from the "minutely misoriented structure" which is related to the original configuration of dislocations. The reason for the generation of the structural imperfections via the domain-inversion processing is interpreted in terms of the converse piezoelectric effect
Sound radiation from exponentially growing and decaying surface waves
No full textA simplified model problem is used to illustrate some of the parameters controlling the radiation of sound into an ambient medium due to the growth and subsequent decay of subsonic travelling waves, such as may occur via non-linear interactions in turbulent free shear flows. It is shown that substantial sound may be generated by apparently subsonic modes as a result of their growth and decay characteristics. Low frequency modes that saturate over a short timescale are the most effective radiators
The effect of a moving ground on the flow and aerodynamic noise behaviour of a simplified high-speed train bogie
No full textAeroacoustic sources around the train bogie area are particularly important but involve complex flow. In this study, a numerical investigation is presented of the effect of a moving ground on the flow and aeroacoustic noise behaviour of a train bogie. These make use of the delayed detached-eddy simulation combined with an acoustic analogy for noise prediction. First the flow around an isolated wheelset (1:5 scale) is calculated when it is in proximity to the ground and this is compared with wind-tunnel measurements to verify the simulation. Then the flow and aerodynamic noise behaviour of a simplified high-speed train bogie at scale 1:10 with and without the ground underneath are studied numerically. It is found that a highly turbulent flow is generated within the bogie cavity and the ground increases the noise levels by 6-8 dB due to a combination of acoustic reflection from the ground and modifications to the flo
Evaluation of acoustic sources in an excited unstable laminar shear layer
No full textThe radiation of sound from artificial sources in a developing shear layer is studied numerically, in order to address an issue that arises in acoustic analogy models of jet noise: namely whether the unstable response of the mean-flow shear layer has a significant effect on sound radiation. Direct numerical simulation of a forced two-dimensional compressible laminar mixing layer has been carried out at a Reynolds number of 250, based on the mixing layer initial vorticity thickness and the upper free-stream velocity. The free-stream Mach numbers of the mixing layer are 0.9 and 0.45. The flow is excited with a single-frequency body force field that is acoustically compact and is derived from an applied-stress distribution. Sound radiation from the mixing layer is calculated at the forcing frequency, and compared with radiation from a uniform flow under the same forcing. Comparisons are shown for the most-unstable forcing frequency over a wide amplitude range. The pressure radiated on either side of the mixing layer differs very little from that radiated into a uniform flow of the same Mach number under the same forcing, although the higher forcing amplitudes used are sufficient to trigger the non-linear process of vortex roll-up in the case of the mixing layer. The dominant source position for the radiated pressure at the forcing frequency is estimated via a wavenumber–frequency domain analysis. It is found to be close to the location of the applied forcing, with little contribution from mixing-layer vortical structures that develop downstream
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