Bulletin of the American Physical Society
2005 58th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 20–22, 2005; Chicago, IL
Session AK: Acoustics I |
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Chair: Meng Wang, Stanford University Room: Hilton Chicago Joliet |
Sunday, November 20, 2005 8:00AM - 8:13AM |
AK.00001: Use of Potentials for Acoustic Diffraction in a Viscous Fluid Anthony Davis In inviscid acoustics, the dilatation and pressure satisfy a Helmholtz equation in the frequency domain. Viscosity generates vorticity which satisfies a Helmholtz equation that yields viscous decay on a scale assumed much smaller than the acoustic wavelength. Guided by the similar solution structure for an elastic wave displacement vector, it is common practice to write the velocity as a sum of gradient and curl fields and assume that each has only wavelike disturbances. However, when a sharp edge is present and a similarity solution in its vicinity is sought, it is found that previously rejected solutions of Laplace's equation are needed to achieve the known biharmonic behavior near the edge. Occurring as conjugate harmonic functions, they do not contribute to the total velocity field. Illustrative examples have a damped acoustic plane wave incident on a solid, or possibly elastic, sphere or on a half-plane. [Preview Abstract] |
Sunday, November 20, 2005 8:13AM - 8:26AM |
AK.00002: One-Dimensional Analysis Techniques for Pulsed Blowing Distribution Frank Chambers, Kalyanasundaram Krishnan Pulsed blowing offers reductions in bleed air requirements for aircraft flow control. Efficient pulsed blowing systems require careful design to minimize bleed air use while distributing blowing to multiple locations. Pulsed blowing systems start with a steady flow supply and process it to generate a pulsatile flow. The fluid-acoustic dynamics of the system play an important role in overall effectiveness. One-dimensional analysis techniques that in the past have been applied to ventilation systems and internal combustion engines have been adapted to pulsed blowing. Pressure wave superposition and reflection are used with the governing equations of continuity, momentum and energy to determine particle velocities and pressures through the flow field. Simulations have been performed to find changes in the amplitude and wave shape as pulses are transmitted through a simple pulsed blowing system. A general-purpose code is being developed to simulate wave transmission and allow the determination of blowing system dynamic parameters. [Preview Abstract] |
Sunday, November 20, 2005 8:26AM - 8:39AM |
AK.00003: Three-dimensional instabilities of compressible flow over open cavities Guillaume Bres, Tim Colonius We developed a three-dimensional algorithm for direct numerical simulations (DNS) of open cavity flow to extend a previous study of cavity oscillations (JFM 455:325-346, 2002). Complementary methodologies for extracting information about global instabilities (including their receptivity and optimal control) of two- and three-dimensional cavity flows have been implemented. For a low Mach number cavity with length-to-depth ratio of two, the two-dimensional steady flow was found unstable to three-dimensional (spanwise homogeneous) disturbances that consist of a spanwise modulation of the recirculating vortex interior to the cavity. The oscillations are unstable over a narrow band of spanwise wavelengths comparable to the cavity depth, and oscillatory in time, but with a very low frequency (about 10 times lower than the two-dimensional Rossiter mode). Results from simulations for different cavity aspect ratios and Mach numbers will also be presented. [Preview Abstract] |
Sunday, November 20, 2005 8:39AM - 8:52AM |
AK.00004: Contribution of vortical structures from a circular jet to far-field sound generation Jungwoo Kim, Haecheon Choi In the present study, large eddy simulation of a circular jet at $Re=10^4$ using a dynamic subgrid-scale model is performed to investigate far-field sound propagation from the Lighthill's acoustic analogy. The jet-exit velocity imposed is the top-hat velocity with laminar Blasius profile near the wall. A modal analysis is performed to investigate the contribution of vortical structures to far-field sound generation. The velocity components obtained from LES are decomposed into azimuthal modes and the acoustic sources are obtained from these modal components. Each acoustic source is used to obtain the corresponding far-field sound from the acoustic analogy. In the circular jet, vortical structures have the dominant azimuthal modes such as axisymmetric (mode-0) and helical (mode-1) modes. From the present modal analysis, it is shown that each modal structure has its own propagation direction. For example, the vortical structure having the axisymmetric mode mainly produces the downstream sound, whereas the direction of far-field sound from the helical mode is $\alpha=\pm 45^o$ and $\pm 135^o$. [Preview Abstract] |
Sunday, November 20, 2005 8:52AM - 9:05AM |
AK.00005: On the Reduction of High-speed Jet Noise with Heating Daniel Bodony, Sanjiva Lele For turbulent axially symmetric (in the mean) jets it is known that for jet velocities $U_j$ greater than $0.7$ times the ambient speed of sound $a_\infty$ heating reduces the jet's radiated acoustic output. The cause of the noise reduction, however, has remained unknown. Using large-eddy simulations (LES) we examine this issue by computing the radiated noise of two high-speed jets at velocity $U_j/a_\infty = 1.47$: one jet is heated to a static-to-ambient temperature ratio 2.3 while the other is unheated. The directly-computed radiated noise is found to reduce by 10 decibels in the peak radiation direction, consistent with experimental data. Using the LES databases we examine the near-field changes with jet heating and attempt to correlate them with changes in the sound field. The analysis suggests that the sound reduction is due primarily to two factors. Relative to the unheated jet: (i) the heated jet has a smaller region of sound generation; and (ii) heating induces temperature-velocity anticorrelation within the jet. Because of item (ii) the radiated sound experiences destructive interference with a corresponding reduction in the total radiated sound. [Preview Abstract] |
Sunday, November 20, 2005 9:05AM - 9:18AM |
AK.00006: Are surface shear stress fluctuations a true source of sound? Karim Shariff, Meng Wang The sound due to a localized flow over a large (compared to the acoustic wavelength) plane no-slip wall is considered. It is known that the sum of the pressure and normal viscous stress at the wall, while appearing to be a source of dipole sound in the formal solution to Lighthill's equation, is \textit{not} a true source but rather represents the surface reflection of volume quadrupoles. Whether a similar surface shear stress term constitutes a true source of dipole sound, has been controversial. Some have assumed it to be so and used it to calculate the noise in boundary-layer flows. Others have argued that, just like the surface pressure, surface shear stress is not a true source. Here, a numerical experiment is undertaken to investigate the issue. An acoustically compact portion of an otherwise static wall is oscillated tangentially to create shear stress fluctuations. The resulting sound field, computed directly from the compressible Navier-Stokes equations, is \textit{almost} everywhere dipolar and its amplitude agrees with an acoustic analogy prediction that regards the surface shear as acoustically compact and as a true source of sound. However, for observers near wall-grazing angles, there is a correction that increases as the computational domain size is increased. A consistency argument, validated by the simulations, shows that as the domain size $\to \infty$, \textit{and} for observers at angles sufficiently close to grazing, shear stress fluctuations cannot be regarded as a source independent of the sound field. [Preview Abstract] |
Sunday, November 20, 2005 9:18AM - 9:31AM |
AK.00007: Numerical simulation of formation of asymmetric acoustic streaming in resonators Takeru Yano Rayleigh type acoustic streaming induced by resonant gas oscillations in a closed tube is studied numerically, with particular emphasis on the flow patterns of large Reynolds number streaming motions before turbulent transition. The system of two-dimensional compressible Navier--Stokes equations is solved with a high-resolution TVD finite-difference scheme without the assumption of the symmetry of flow field. The streaming velocity field is evaluated from a time-averaged mass flux density vector. We shall demonstrate that (i) more than a thousand of acoustic cycles are required for the establishment of quasi-steady streaming; (ii) the symmetry in the flow pattern of acoustic streaming is lost when the streaming Reynolds number is moderately large, even before the transition to turbulence. [Preview Abstract] |
Sunday, November 20, 2005 9:31AM - 9:44AM |
AK.00008: Direct Numerical Simulation of Grazing Flow over an Acoustic Liner in a Sound Field Christopher Tam, Hongbin Ju The effect of grazing flow on the fluid mechanical and acoustic performance of an acoustic liner in a sound field is investigated by direct numerical simulation. The simulations are carried out using the Dispersion-Relation-Preserving (DRP) scheme and advanced computational aeroacoustic (CAA) boundary conditions. The computation algorithm is verified by comparing numerical results with an exact linear solution. At high incident sound pressure level, the flow at the mouths of the resonators of the liner is dominated by vortex shedding. Vortex shedding, which converts acoustic energy into rotational kinetic energy of the shed vortices that are subsequently dissipated by viscosity, is the dominant mechanism for damping incident acoustic waves. The grazing flow organizes the shed vortices into a single large vortex. This large vortex is convected downstream by the mean flow. This convected vortex has the potential of disrupting the flow of the downstream resonator. Past models of acoustic liners do not include this fluid mechanical interaction between resonators. Modification, therefore, becomes necessary. At low level of incident sound waves, the flows at the mouths of the resonators of the liner consist mainly of oscillatory shear layers. Viscous dissipation is the main damping mechanism. Computed streamline patterns of the flow are found to exhibit remarkable resemblance to those measured experimentally. [Preview Abstract] |
Sunday, November 20, 2005 9:44AM - 9:57AM |
AK.00009: Propagation of Pressure Waves, Caused by a Thermal Shock, in Liquid Metals Containing Gas Bubbles Kohei Okita, Shu Takagi, Yoichiro Matsumoto Propagation of pressure waves caused by a thermal shock in liquid metals containing gas bubbles was investigated numerically, to examine the influences of bubble radius and void fraction on the absorption of thermal expansion of liquid metals and attenuation of the pressure waves. The present approach is to solve the mass, momentum and energy conservation equations with the equation of state for liquid metals. To consider the thermal damping effect for bubble oscillation, the mass, momentum and energy conservation equations for gas inside each bubble are solved with the Keller equation of bubble dynamics. As the result of the calculation, since the large bubbles have a lower natural frequency than the small bubbles, the peak pressure at the heated region increases with increasing bubble radius. Especially, when the bubble radii are quite large, the calculation reproduces that the pressure wave propagates through the mixture not at the sound speed of the mixture but at that of liquid mercury. On the other hand, in the condition of low void fraction, in which bubbles oscillate nonlinearly and the collapse of bubble cloud causes the high pressure rise, the pressure waves are attenuated by the thermal damping effect of bubbles. [Preview Abstract] |
Sunday, November 20, 2005 9:57AM - 10:10AM |
AK.00010: Hybrid Adaptive Wavelet Collocation -- Brinkman Penalization -- Ffowcs Williams and Hawkings Method for Compressible Flow Simulation and Far-Field Acoustics Prediction Qianlong Liu, Oleg V. Vasilyev One of the most practically important problems of computational aero-acoustics is the efficient and accurate calculation of flows around solid obstacles of arbitrary surfaces. To simulate flows in complex domains, we combine two mathematical approaches, the Adaptive Wavelet Collocation Method, which tackles the problem of efficiently resolving localized flow structures in complicated geometries, and the Brinkman Penalization Method, which addresses the problems of efficiently implementing arbitrary complex solid boundaries. Through them, we can resolve and automatically track all the important flow structures on the computational grid that automatically adapts to the solution. To obtain accurate long-time flow simulation and accurately predict far-field acoustics using a relatively small computational domain, appropriate artificial boundary conditions are critical to minimize the contamination by the otherwise reflected spurious waves. Once the near-field accurate simulation is available, Ffowcs Williams and Hawkings (FWH) equations are used to predict the far-field acoustics. The method is applied to a number of acoustics benchmark problems and the results are compared with both the exact and the direct numerical simulation solutions. [Preview Abstract] |
Sunday, November 20, 2005 10:10AM - 10:23AM |
AK.00011: Single-bubble Acoustic Cavitation in Inorganic Liquids David Flannigan, Kenneth Suslick We have discovered that single-bubble sonoluminescence (SBSL) from concentrated aqueous solutions of the mineral acids, especially sulfuric acid (H$_{2}$SO$_{4})$, can be made to be over 10$^{3}$ times brighter than SBSL from pure water. In addition, we have observed intense and well-resolved line emission within the SBSL spectra arising from many different ions (e.g., Xe$^{+}$, Ar$^{+}$, O$_{2}^{+})$, atoms (e.g., Ar, Ne, H, O), and small molecules (e.g., N$_{2}$, SO, SO$_{2})$; the observation of monocationic emission lines provides the first definitive experimental evidence of plasma formation during SBSL. By studying the relative intensities of, for example, Ar atom emission lines observed in the SBSL spectra, we are able to measure observable emission temperatures in excess of 15,000 K and pressures approaching 1,000 bar. The temperatures determined from molecular emissions are lower, however, and do not exceed 5,000 K. This observation suggests the presence of a spatial temperature gradient within the bubble or a temporal dependence to the SBSL emissions. [Preview Abstract] |
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