The Experts below are selected from a list of 288 Experts worldwide ranked by ideXlab platform

Shiu Keung Tang - One of the best experts on this subject based on the ideXlab platform.

  • Basic Sound Generation mechanisms in inviscid vortex interactions at low Mach number
    Journal of Sound and Vibration, 2003
    Co-Authors: Shiu Keung Tang
    Abstract:

    The Sound Generation mechanisms during finite core vortex interactions at low Mach number are investigated in the present study. The theoretical deductions show clearly that the basic Sound Generation mechanisms are associated with the vortex core deformation and the vorticity centroid dynamics, independent of the vortex system. Such deductions are substantiated by numerical experiments with the interactions of two-dimensional vortices, vortex pairs and vortex rings. Detailed discussions on the similarities and differences between the Sound Generation processes of the two-dimensional and axisymmetric vortex systems are given. The relative importance of the two Sound Generation mechanisms in these vortex systems, their characteristics and interactions, which are hardly found in existing literature, are also examined. The present findings have also generalized and substantiated the previous results of the authors on the topic.

  • Vortex Sound Generation due to a flow impedance discontinuity on a flat surface.
    The Journal of the Acoustical Society of America, 2001
    Co-Authors: Shiu Keung Tang
    Abstract:

    The Sound generated by the unsteady motion of a vortex filament moving over a flat boundary with a sharp flow impedance discontinuity is studied theoretically. Theoretical results show that the vortex filament undergoes significant accelerating or decelerating motions and radiates Sound at the instant when it moves across the plane of impedance discontinuity. The accelerations and decelerations of the vortex filament are shown to be the major mechanisms of Sound Generation. The Sound so produced has a large low-frequency content such that the change in the flow impedance affects only the Sound Generation process but not the subsequent Sound propagation to the far field.

  • MECHANISMS FOR Sound Generation IN INVISCID TWO-DIMENSIONAL VORTEX INTERACTIONS
    Journal of Sound and Vibration, 2001
    Co-Authors: Shiu Keung Tang
    Abstract:

    Theoretical and numerical results obtained in the present investigation illustrate that the motions of the vorticity centroids and the unsteady deformation of the vortex cores are the sources of Sound in the inviscid two-dimensional vortex interactions at low Mach number. A theory for the Sound Generation, based on the low Mach number vortex Sound theory, is developed and the relationship between the core deformation and Sound radiation for slightly deformed vortices is explicitly found. The unsteady vortex core deformation is also found to be important in all the interaction cases discussed.

  • Sound Generation by interaction of two inviscid two-dimensional vortices
    The Journal of the Acoustical Society of America, 1997
    Co-Authors: Shiu Keung Tang
    Abstract:

    Sound generated by the pairing of two inviscid finite-core-size two-dimensional vortices is studied numerically using the contour dynamics method and vortex Sound theory. Results indicate that the resulting Sound field is a lateral quadrupole type and its axis rotates with nonuniform speed about the origin of the source region. Results also show that the accelerating and decelerating motions of the vortices are important in the Sound Generation.

  • Sound Generation by a vortex ring collision
    The Journal of the Acoustical Society of America, 1995
    Co-Authors: Shiu Keung Tang
    Abstract:

    A collision between two vortex rings and its Sound Generation is studied numerically using the contour dynamics method. Results suggest that the radial acceleration and the rate of change of the axial acceleration of the vortex rings are the more important dynamic parameters for Sound Generation during the interaction. The former is important at or near the pass‐through instant when the vortex rings are coplanar and are of unequal strength. The latter, being important in the Sound Generation during a head‐on collision before the vortex ring cores are very close to each other, especially when the rings are thin, is also important in Sound Generation by an unequal strength vortex ring collision after the pass through when the vortex cores are separated by a distance so that the mutual induction strength does not result in significant change of the impulse of the stronger ring.

Andrea Harrer - One of the best experts on this subject based on the ideXlab platform.

  • Physics of thermo-acoustic Sound Generation
    Journal of Applied Physics, 2013
    Co-Authors: Maxim Daschewski, Rainer Boehm, Jens Prager, Marc Kreutzbruck, Andrea Harrer
    Abstract:

    We present a generalized analytical model of thermo-acoustic Sound Generation based on the analysis of thermally induced energy density fluctuations and their propagation into the adjacent matter. The model provides exact analytical prediction of the Sound pressure generated in fluids and solids; consequently, it can be applied to arbitrary thermal power sources such as thermophones, plasma firings, laser beams, and chemical reactions. Unlike existing approaches, our description also includes acoustic near-field effects and Sound-field attenuation. Analytical results are compared with measurements of Sound pressures generated by thermo-acoustic transducers in air for frequencies up to 1 MHz. The tested transducers consist of titanium and indium tin oxide coatings on quartz glass and polycarbonate substrates. The model reveals that thermo-acoustic efficiency increases linearly with the supplied thermal power and quadratically with thermal excitation frequency. Comparison of the efficiency of our thermo-acoustic transducers with those of piezoelectric-based airborne ultraSound transducers using impulse excitation showed comparable Sound pressure values. The present results show that thermo-acoustic transducers can be applied as broadband, non-resonant, high-performance ultraSound sources.

Sanjiva K. Lele - One of the best experts on this subject based on the ideXlab platform.

  • Sound Generation due to unsteady motion of a cylinder
    Physics of Fluids, 2011
    Co-Authors: Arjun Sharma, Sanjiva K. Lele
    Abstract:

    A two-dimensional model problem of Sound Generation due to prescribed unsteady starting and stopping motion of a circular cylinder is studied by the numerical solution of two-dimensional compressible Navier–Stokes equations. The unsteady flow near the cylinder surface and the Sound Generation and propagation are analyzed with respect to three parameters: the time scale of the startup and stopping motion, the peak Mach number (based on peak velocity and ambient speed of Sound), and the Reynolds number (based on peak velocity, cylinder diameter, ambient density, and dynamic viscosity). The flow behavior is studied for fast (time scale of unsteady motion similar to acoustic time scale) or very slow startup motion. The Mach number is varied between 0.1 and 0.4 and the Reynolds number between 150 and 9500. The accuracy of unsteady flow solution is demonstrated by comparison to the experimental data. For fast startup motion, a sharp peak is observed in the drag curve during the acceleration phase of motion. We ...

  • Computational aeroacoustics: progress on nonlinear problems of Sound Generation
    Progress in Aerospace Sciences, 2004
    Co-Authors: Tim Colonius, Sanjiva K. Lele
    Abstract:

    Abstract Computational approaches are being developed to study a range of problems in aeroacoustics. These aeroacoustic problems may be classified based on the physical processes responsible for the Sound radiation, and range from linear problems of radiation, refraction, and scattering in known base flows or by solid bodies, to Sound Generation by turbulence. In this article, we focus mainly on the challenges and successes associated with numerically simulating Sound Generation by turbulent flows. We discuss a hierarchy of computational approaches that range from semi-empirical schemes that estimate the noise sources using mean-flow and turbulence statistics, to high-fidelity unsteady flow simulations that resolve the Sound Generation process by direct application of the fundamental conservation principles. We stress that high-fidelity methods such as Direct Numerical Simulation (DNS) and Large Eddy Simulation (LES) have their merits in helping to unravel the flow physics and the mechanisms of Sound Generation. They also provide rich databases for modeling activities that will ultimately be needed to improve existing predictive capabilities. Spatial and temporal discretization schemes that are well-suited for aeroacoustic calculations are analyzed, including the effects of artificial dispersion and dissipation on uniform and nonuniform grids. We stress the importance of the resolving power of the discretization as well as computational efficiency of the overall scheme. Boundary conditions to treat the flow of disturbances in and out of the computational domain, as well as methods to mimic anechoic domain extension are discussed. Test cases on some benchmark problems are included to provide a realistic assessment of several boundary condition treatments. Finally, highlights of recent progress are given using selected model problems. These include subsonic cavity noise and jet noise. In the end, the current challenges in aeroacoustic modeling and in simulation algorithms are revisited with a look toward the future developments.

Maxim Daschewski - One of the best experts on this subject based on the ideXlab platform.

  • Physics of thermo-acoustic Sound Generation
    Journal of Applied Physics, 2013
    Co-Authors: Maxim Daschewski, Rainer Boehm, Jens Prager, Marc Kreutzbruck, Andrea Harrer
    Abstract:

    We present a generalized analytical model of thermo-acoustic Sound Generation based on the analysis of thermally induced energy density fluctuations and their propagation into the adjacent matter. The model provides exact analytical prediction of the Sound pressure generated in fluids and solids; consequently, it can be applied to arbitrary thermal power sources such as thermophones, plasma firings, laser beams, and chemical reactions. Unlike existing approaches, our description also includes acoustic near-field effects and Sound-field attenuation. Analytical results are compared with measurements of Sound pressures generated by thermo-acoustic transducers in air for frequencies up to 1 MHz. The tested transducers consist of titanium and indium tin oxide coatings on quartz glass and polycarbonate substrates. The model reveals that thermo-acoustic efficiency increases linearly with the supplied thermal power and quadratically with thermal excitation frequency. Comparison of the efficiency of our thermo-acoustic transducers with those of piezoelectric-based airborne ultraSound transducers using impulse excitation showed comparable Sound pressure values. The present results show that thermo-acoustic transducers can be applied as broadband, non-resonant, high-performance ultraSound sources.

S. Schwemmer - One of the best experts on this subject based on the ideXlab platform.

  • Thermo-acoustic Sound Generation in the interaction of pulsed proton and laser beams with a water target
    Astroparticle Physics, 2015
    Co-Authors: R. Lahmann, J. Hößl, Andreas Kappes, U. Katz, K Graf, G. Anton, Klaus Mecke, S. Schwemmer
    Abstract:

    The Generation of hydrodynamic radiation in interactions of pulsed proton and laser beams with matter is explored. The beams were directed into a water target and the resulting acoustic signals were recorded with pressure sensitive sensors. Measurements were performed with varying pulse energies, sensor positions, beam diameters and temperatures. The obtained data are matched by simulation results based on the thermo-acoustic model with uncertainties at a level of 10%. The results imply that the primary mechanism for Sound Generation by the energy deposition of particles propagating in water is the local heating of the medium. The heating results in a fast expansion or contraction and a pressure pulse of bipolar shape is emitted into the surrounding medium. An interesting, widely discussed application of this effect could be the detection of ultra-high energetic cosmic neutrinos in future large-scale acoustic neutrino detectors. For this application a validation of the Sound Generation mechanism to high accuracy, as achieved with the experiments discussed in this article, is of high importance.