Bulletin of the American Physical Society
61st Annual Meeting of the APS Division of Fluid Dynamics
Volume 53, Number 15
Sunday–Tuesday, November 23–25, 2008; San Antonio, Texas
Session MQ: Acoustics |
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Chair: Bakhtier Farouk, Drexel University Room: 202B |
Tuesday, November 25, 2008 8:00AM - 8:13AM |
MQ.00001: Sound generation by boundary-layer flow over steps Minsuk Ji, Meng Wang Large-eddy simulations of turbulent boundary layer flows over backward and forward facing steps are performed to study flow-induced noise at low Mach number. The Reynolds number is 21000 based on the step height and free-stream velocity. The boundary layer thickness is approximately twice the step height near the step. Statistics of wall pressure fluctuations such as the root-mean-square values and frequency spectra yield favorable comparisons with previous experimental measurements. Sound generated by flow over backward and forward steps is examined in the framework of Lighthill's acoustic analogy. Green's function for the step geometry that is valid for an acoustically compact step height is employed to evaluate the volume integral in the solution to Lighthill's equation. For a far-field observer, the steps act primarily as a dipole source aligned in the streamwise direction. In line with experimental results, the flow over a forward step emits sound that is significantly stronger than that from a backward step. The underlying reason is analyzed in terms of source strength and distribution relative to the Green's function distribution. It is found that the forward step generates stronger source in regions closer to the upper step corner, which is acoustically most important. [Preview Abstract] |
Tuesday, November 25, 2008 8:13AM - 8:26AM |
MQ.00002: Turbulence scales and free-shear-flow noise Randall Kleinman, Jonathan Freund The role of turbulence scales in the generation of far-field sound is studied by direct numerical simulation of temporally-developing mixing layers. The range of scales is adjusted by varying the layer thickness Reynolds numbers by a factor of twelve in the different cases simulated. Turbulence kinetic energy and pressure spectra in the near field show the expected Reynolds number dependence, but far-field pressure spectra all decay rapidly with wavenumber and show less sensitivity to Reynolds number. Far-field streamwise wavenumber pressure spectra scale well with the layer momentum thickness, consistent with the insensitivity to Reynolds number of the largest turbulence structures. At higher wavenumbers the streamwise spectra scale best with the Taylor microscale. Above a momentum thickness Reynolds number of around 300, all the mixing layers radiate over 85 percent of the acoustic energy of the apparently asymptotically high-Reynolds-number value we are able to compute. Low wavenumbers account for nearly all of the acoustic energy in the far field. Implications of these results for large-eddy simulation of jet noise are discussed. [Preview Abstract] |
Tuesday, November 25, 2008 8:26AM - 8:39AM |
MQ.00003: Generation of sound by the scattering of entropy disturbances Daniel Bodony The interaction of a convecting entropy disturbance, such as generated by a gas turbine combustor, with a solid object is known to generate sound. The sound generation is due to (i) the acceleration of the convected disturbance by the mean flow and (ii) satisfaction of the wall boundary condition on the object. This process, often called indirect combustion noise, is known to be present in modern gas turbine engines but its specific details are not known, including its overall contribution to the acoustic signature of the engine and its influence on the combustor. In this presentation computational and analytical results are presented to examine the sound field created by a localized entropy disturbance convecting in the vicinity of a symmetric thin body. Unsteady calculations of the compressible Euler equations are used to directly compute the radiated sound and are compared to analytical predictions based on rapid distortion theory. RDT is also used to develop the relevant scalings of the radiated sound. [Preview Abstract] |
Tuesday, November 25, 2008 8:39AM - 8:52AM |
MQ.00004: High-frequency Analysis of Acoustic Refraction Effects in Turbulent Flames Ricky Reusser, Matthias Ihme The consideration of refraction effects in turbulent reacting flows is crucial for the accurate characterization of sound emission and directivity, particularly at high frequencies. The localized heat release and the associated large temperature gradients in flames can significantly affect the sound transmission to the far field. In this work, refraction of acoustic waves in a series of turbulent flames is considered using Gaussian beam analysis. This model is extended to accurately account for sound speed inhomogeneities. Acoustic results and sound directivity from a point source distribution are compared with solutions obtained from the classical ray acoustic analysis and continuous wave equation. Based on the extended Gaussian beam model, a simplified model for the prediction of the acoustic radiation in turbulent flames is presented, for which the acoustic source term distribution is extracted from a large-eddy simulation. Computational results are discussed and compared with detailed simulation data. [Preview Abstract] |
Tuesday, November 25, 2008 8:52AM - 9:05AM |
MQ.00005: Generation and Propagation of Thermally Induced Acoustic Waves in Supercritical Carbon Dioxide Bakhtier Farouk, Zhiheng Lei The generation, propagation and dissipation of thermally induced acoustic waves in supercritical nitrogen and supercritical carbon dioxide are investigated numerically. The NIST Database 12 is used to obtain the property relations for the supercritical fluids... One- and two-dimensional problems are considered where the supercritical fluid is contained either within two infinite parallel plates or within a square enclosure. The left vertical wall is heated rapidly to initiate acoustic waves within the supercritical fluid. The thermally induced acoustic waves are generated along the heated surface due to the high compressibility of the supercritical fluid. For a given rate of increase of wall temperature, the intensity of the waves is found to get higher as the critical point is reached. The acoustic waves reflect from the opposing sidewall and continue to reverberate between the opposing walls. Near-critical fluids have low thermal diffusivity values. However, the temperature of the bulk supercritical fluid is found to increase due to the dissipation of the acoustic energy, known as the so called \textit{piston effect}. For the two-dimensional cases, the effect of the acoustic waves on the buoyancy induced flows in supercritical fluid is investigated. [Preview Abstract] |
Tuesday, November 25, 2008 9:05AM - 9:18AM |
MQ.00006: Standing waves in a partitioned tube with passive membrane David E. Amundsen, Edward A. Cox, Michael P. Mortell The propagation of waves within a tube containing disparate gases separated by a passive membrane is modeled and analyzed in the limit of weak dissipation and applied forcing. This provides a simple setting in which to study the nonlinear interactions within and between each gas and provides a paradigm for other similar physical systems such as laminated elastic materials. The associated resonant frequencies are found in terms of a linear functional equation involving a non-trivial combination of the separate natural frequencies. As expected, in the limit that the gases have the same material properties, the modes become commensurate and the model reduces to that of the classical shock tube. However sufficiently away from this limit it is seen that this structure is lost and smooth single mode resonant solutions arise. Using a perturbative approach these solutions are approximated and compared to numerical solutions of the full system. The transition between smooth and discontinuous solutions is also studied both numerically and analytically, based on a dimensionless parameter associated with the relative material difference. [Preview Abstract] |
Tuesday, November 25, 2008 9:18AM - 9:31AM |
MQ.00007: Prediction of noise generated by complex flows at low Mach number Yaser Khalighi, Ali Mani, Parviz Moin We present a computational aero-acoustics method to evaluate noise generated by low Mach number flow over complex configurations. This method is a hybrid approach which uses Lighthill's acoustic analogy in conjunction with source-data from an incompressible calculation. Scattering of sound waves are computed using a Boundary Element Method. This approach can be applied to flow configurations with practical complexities where turbulence interacts with arbitrary shaped solid objects. We present a validation study for sound generated by flow over a circular cylinder at $Re=100$ and $Re=10000$. The hybrid method is validated against directly computed noise using a high order compressible flow solver as well as solution of the Ffowcs Williams-Hawkings equation in conjunction with compressible noise sources. We concluded that the noise predicted by a $2^{nd}$ order hybrid approach is as accurate as directly computed noise by a $6^{th}$ order compressible flow solver in the low frequency range where the low order numerics can accurately resolve the flow structures. [Preview Abstract] |
Tuesday, November 25, 2008 9:31AM - 9:44AM |
MQ.00008: Computational study of roughness-noise mechanisms Qin Yang, Meng Wang Boundary-layer flows over rough surfaces are known to be much noisier than smooth-wall boundary layers. To understand the source mechanisms for roughness noise, large-eddy simulations are performed for turbulent boundary-layer flows over a single hemispherical roughness element and a pair of roughness elements. The noise calculation based on Lighthill's theory highlights the role of unsteady drag dipoles, particularly the spanwise component, as the primary noise source. Noise-generation mechanisms are investigated through an analysis of the correlation and coherence between the dipole source functions and hydrodynamic pressure fluctuations on the surrounding surface, and through a numerical experiment with stress-free boundary conditions on the hemispheric surface, which eliminates shear-layer separation and vortex shedding and the associated noise. The results suggest that diffraction and distortion of incoming turbulence from upstream have an important effect on streamwise dipole radiation, while spanwise dipole radiation is more closely associated with turbulence structures generated by the hemisphere. The interaction of a hemisphere with the wake of an upstream hemisphere is shown to significantly enhanced noise radiation, particularly in the streamwise direction and at high frequencies. [Preview Abstract] |
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