Bulletin of the American Physical Society
65th Annual Meeting of the APS Division of Fluid Dynamics
Volume 57, Number 17
Sunday–Tuesday, November 18–20, 2012; San Diego, California
Session A24: Acoustics I: Turbulence and Aerodynamics |
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Chair: Daniel Bodony, University of Illinois at Urbana-Champaign Room: 30E |
Sunday, November 18, 2012 8:00AM - 8:13AM |
A24.00001: Discrete tones around airfoils: a global stability analysis. Miguel Fosas de Pando, Peter J. Schmid, Denis Sipp Airfoil self-noise stems from an interaction between the airfoil surface, the boundary layers and the wake. At moderate Reynolds number and small angles of attack, the acoustic spectrum is dominated by discrete tones correlated to the ringing of coherent structures localized in the vicinity of the trailing edge. Local stability analyses show strong amplification of hydrodynamic instabilities in the frequency range of acoustic tones, suggesting an interplay between sound waves and instabilities. However, owing to the intrinsic limitations of local approaches, a satisfactory explanation of the tonal-noise phenomenon is still missing. We present a global stability analysis of the mean-flow linearized dynamics. Features of the global modes spectrum and of the resolvent norm will be discussed. The least-stable direct modes show a link between the suction-side boundary layer, the near wake dynamics, and acoustic radiation; conversely, the corresponding adjoint modes pinpoint at the pressure side as the location of maximum sensitivity. Although the linearized operator is stable, the resolvent norm shows substantial energy amplification. Finally, the pressure-side, suction-side and wake dynamics will be analyzed in isolation to assess their respective contribution to the overall process. [Preview Abstract] |
Sunday, November 18, 2012 8:13AM - 8:26AM |
A24.00002: Noise prediction from external flows using Ffowcs-Williams and Hawkings techniques Zane Nitzkorski, Krishnan Mahesh We investigate noise production from turbulent flow using the Ffowcs-Williams and Hawkings (FWH) acoustic analogy for general hydrodynamic flow configurations. We describe our methodology of using porous implementations of the FWH equations to calculate far-field sound from sources that are computed by either incompressible or compressible LES/DNS. Details of the development including arbitrary surface extraction techniques on unstructured grids and a novel end-cap correction for the quadrupole sound will be presented. The methodology allows for estimation of volumetric noise computed over a small volume as opposed to the common approach of ignoring the entire volume term. We have used these techniques to compute the noise from high Reynolds number cylinder flows and compare our results against available computations and experiments; base flow results as well as acoustic data will be compared. [Preview Abstract] |
Sunday, November 18, 2012 8:26AM - 8:39AM |
A24.00003: Numerical investigation of acoustic radiation from vortex-airfoil interaction Anne Legault, Minsuk Ji, Meng Wang Numerical simulations of vortices interacting with a NACA 0012 airfoil and a flat-plate airfoil at zero angle of attack are carried out to assess the applicability and accuracy of classical theories. Unsteady lift and sound are computed and compared with the predictions by theories of Sears and Amiet, which assume a thin-plate airfoil in an inviscid flow. A Navier-Stokes solver is used in the simulations, and therefore viscous effects are taken into consideration. For the thin-plate airfoil, the effect of viscosity is negligible. For a NACA 0012 airfoil, the viscous contribution to the unsteady lift and sound mainly comes from coherent vortex shedding in the wake of the airfoil and the interaction of the incoming vortices with the airfoil wake, which become stronger at higher Reynolds numbers for a 2-D laminar flow. When the flow is turbulent at chord Reynolds number of $4.8 \times 10^5$, however, the viscous contribution becomes negligible as coherent vortex shedding is not present. Sound radiation from vortex-airfoil interaction at turbulent Reynolds numbers is computed numerically via Lighthill's theory and the result is compared with the predictions of Amiet and Curle. The effect of the airfoil thickness is also examined. [Preview Abstract] |
Sunday, November 18, 2012 8:39AM - 8:52AM |
A24.00004: Effects of step non-compactness and free-stream convection on step noise Jin Hao, Ahmed Eltaweel, Meng Wang The effects of acoustic non-compactness of steps and free-stream convection on sound generation in a turbulent boundary layer over small steps are investigated in the Lighthill framework by comparing solutions obtained using a boundary-element method with those based on a compact-step Green's function. When the ratio of acoustic wavelength to the step height, $\lambda/h$, is large, good agreement between the two is found in terms of sound spectra and directivity. Discrepancies become significant with decreasing $\lambda/h$. For mildly non-compact step heights, the sound directivity for forward steps exhibits asymmetry, with the upstream side maintaining an approximate dipole lobe while the rest significantly distorted. Dips in sound spectra and multiple lobes in directivity are observed along with significantly enhanced sound power when $\lambda/h$ is sufficiently small, indicating that edge-scattering becomes the dominant source mechanism. The effect of convection on the far-field sound is found to be significant at free-stream Mach number of $0.1$ or higher. These results indicate that both step non-compactness and convection effects must be taken into consideration when performing wind-tunnel experiments for hydroacoustic applications. [Preview Abstract] |
Sunday, November 18, 2012 8:52AM - 9:05AM |
A24.00005: Low Mach number prediction of the acoustic signature of fractal-generated turbulence Sylvain Laizet, V\'eronique Fortun\'e, Eric Lamballais, Christos Vassilicos We compare the acoustic properties of a fractal square grid with those of a regular grid by means of a hybrid approach based on Lighthill's analogy and Direct Numerical Simulation (DNS). Our results show that the sound levels corresponding to our fractal square grid of three fractal iterations are significantly reduced by comparison to a regular grid of same porosity and mesh-based Reynolds number. We also find a well-defined peak at a Strouhal number between 0.2 and 0.3 in the acoustic spectrum of our fractal square grid which is absent in the case of our regular grid. We explain this effect in terms of a new criterion for quasi-periodic vortex shedding from a regular or fractal grid. [Preview Abstract] |
Sunday, November 18, 2012 9:05AM - 9:18AM |
A24.00006: Mechanisms of ``crackle'' acoustic radiation from high speed turbulence Aaron Anderson, Jonathan Freund ``Crackle'' describes the perception of a particular type of jet noise produced by high specific thrust engines, which is both annoying and potentially damaging to hearing. Direct numerical simulations of free shear flows at Mach numbers ranging from $M = 1.5$ to $3.5$ are used to investigate its source mechanisms. Shear layers with $M > 2$ are seen to produced radiation with the accepted character of crackling jets: weak shocks, asymmetric and apparently steepened pressure waves, sound pressure levels exceeding 160 dB, and a distinctive Mach angle in the near acoustic field. Space--time correlations suggest that eddy advection is indeed responsible for the Mach-wave radiation. However, the length and time scales of the space--time statistics are significantly smaller than would be expected based upon the observed Mach waves, which suggests that near-field agglomeration of the waves is an essential feature. The pressure skewness, a metric that correlates with perception of crackle, is near zero for $M = 1.5$ and increases above 0.6 for $M = 3.5$ in the near acoustic field. Within the simulation domain, skewness is shown to increase with increasing distance from the turbulent shear layer indicative of multi-dimensional wave interaction effects. [Preview Abstract] |
Sunday, November 18, 2012 9:18AM - 9:31AM |
A24.00007: Modeling Unsteady Lift and Radiated Sound Generated by a 2-D Airfoil in an Intermittent Flow Mark Ross, Scott Morris The spanwise correlation length scale of lateral velocity and the gust response function are the quantities of interest in predicting the sound production from an airfoil. Typically, these quantities are taken to be a correlation length scale model based on isotropic turbulence and Sears' gust response function, respectively. The present study is an experimental investigation of the accuracy of these selections when the turbulent approach flow is intermittently irrotational. Acoustic measurements of a flat-plate airfoil placed at three lateral locations in a single stream shear layer are presented. The acoustic measurements are compared to radiated sound predictions based on detailed velocity field measurements. A potential model which accounts for the effect of approach flow intermittency on the radiated sound will also be presented. [Preview Abstract] |
Sunday, November 18, 2012 9:31AM - 9:44AM |
A24.00008: Poroelastic Trailing Edge Noise and the Silent Flight of the Owl Justin Jaworski, Nigel Peake Many species of owl rely on specialised plummage to reduce their self-noise levels and enable hunting in acoustic stealth. One such plummage arrangement, a compliant array of feathers at the wing trailing edge, is believed to mitigate the scattering of boundary layer turbulence which is the predominant source of airframe noise. The owl noise problem is modelled analytically by the diffraction of a quadrupole source by a semi-infinite porous and elastic edge, and the resulting set of equations is solved exactly using the Wiener-Hopf technique to identify important dimensionless parameters and their scaling behaviour with respect to the aerodynamic noise produced. Special attention is paid to the limiting cases of elastic-impermeable as well as rigid-porous plate conditions, the latter of which is compared against available experimental measurements in the literature. Results from this analysis and comparison seek to validate the weaker sixth-power dependence of far-field acoustic power on flow velocity for porous trailing edges, develop a rigorous basis for the aeroacoustic tailoring of poroelastic edges to reduce airframe noise, and help explain one of the mechanisms of aerodynamic noise suppression by owls. [Preview Abstract] |
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