Bulletin of the American Physical Society
60th Annual Meeting of the Divison of Fluid Dynamics
Volume 52, Number 12
Sunday–Tuesday, November 18–20, 2007; Salt Lake City, Utah
Session JH: Acoustics III |
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Chair: Scott Morris, University of Notre Dame Room: Salt Palace Convention Center 250 B |
Monday, November 19, 2007 3:35PM - 3:48PM |
JH.00001: Finite-wavelength scattering of incident vorticity waves at a shrouded-jet exit Arnab Samanta, Jonathan B. Freund We consider a round jet shrouded for a finite downstream distance by a sharply-terminated concentric cylinder. The scattering of waves supported by a vortex-sheet model of the jet into acoustic modes in the neighborhood of the sharp shroud exit is studied using the Wiener--Hopf method. Scattering into radiating acoustic modes is quantified. Of greater interest in the present study, however, is the scattering of the outgoing vortical disturbance into acoustic modes, that travel back into the shroud surrounding the jet. These are hypothesized to close a resonance ``loop'' that sustains the high-amplitude resonances commonly observed in similar configurations. These upstream-moving acoustic waves are thought to seed vortical disturbances at the jet-nozzle lip. Finite-wavelength (finite-frequency) analysis is essential for investigating this mechanism: the long-wavelength limit degenerates to a fully reflective fixed-pressure condition at the shroud exit, while the short-wavelength limit produces no reflection since the vortex sheet in this case does not interact with the shroud. Resonances are observed for wavelengths comparable to the shroud exit diameter, upon which we focus. [Preview Abstract] |
Monday, November 19, 2007 3:48PM - 4:01PM |
JH.00002: Active noise control in an ideally expanded Mach 1.3 jet* Jin-Hwa Kim, Igor Adamovich, Mo Samimy Localized arc filament plasma actuators developed at OSU were used for jet noise control in an ideally expanded Mach 1.3 jet with an exit diameter of 2.54 cm and a Reynolds number of 1x10$^{6}$. The azimuthal modes of forcing were m = 0-3. The forcing Strouhal number was varied from 0.13 to 5.0. The far-field noise was measured at 30\r{ } and 90\r{ } relative to the jet axis. Laser sheet flow visualization was also carried out. When the jet was forced at a low frequency, the noise level was increased at both measuring locations, but not as much as that in a Mach 0.9 jet in our previous work. At 30\r{ } location, the noise was decreased at forcing Strouhal numbers higher than 0.5 for all azimuthal modes. At higher azimuthal modes, the noise level decreased at lower forcing Strouhal numbers in general. At this measuring location, the maximum reduction of 2.2 dB was at a forcing Strouhal number of about 1.0 for m = 3 mode excitation. At 90\r{ } location, the noise level reduction started at higher forcing Strouhal numbers than at 30\r{ } location. The maximum noise reduction was 1.0 dB at a forcing Strouhal number of 2.0 for m = 3 mode excitation. The flow visualization showed well organized large-scale structures at low forcing Strouhal numbers, which are probably responsible for the increased noise at low Strouhal numbers. $^{\ast }$Supported by NASA Glenn Research Center [Preview Abstract] |
Monday, November 19, 2007 4:01PM - 4:14PM |
JH.00003: Control and Scaling of Radius-Vectored Turbulent Boundary Layers using Plasma Actuators Thomas Corke, Ryan Hewitt An experiment was performed to examine the performance and scaling of single dielectric barrier discharge plasma actuators for vectoring an approaching turbulent boundary layer around a circular radius. The experiment consisted of a suspended flat plate on which a turbulent boundary layer developed. The trailing edge of the plate had removable sections forming 90 degree turns with different radii ranging from 6.4 to 12.7 cm. The flow around the circular radii was documented with particle tracers introduced upstream in the boundary layer, surface flow visualization, and by measured velocity profiles. The surface visualization was a mixture of an evaporating oil and white china clay. This was used to quantitatively measure the turning angle of separation. Turning angles of the flow were documented for a range of free-stream speeds and boundary layer thicknesses with and without plasma actuator control. The results indicated a minimum sensitivity to free-stream speed and boundary layer thickness. There was a strong sensitivity to the turning radius. For flow control, the sensitivity of the flow vectoring to actuator amplitude increased with decreasing turning radius. This suggests a strategy for flow turning using compound shapes of different radii. [Preview Abstract] |
Monday, November 19, 2007 4:14PM - 4:27PM |
JH.00004: Flowfield and Acoustic radiation from imperfectly expanded supersonic jets; Experimental Investigation David Munday, Ephraim Gutmark This project involves the study of sound generated by supersonic jets like those emanating from the exhausts of high-performance military aircraft. Four different nozzles are employed: one convergent nozzle and three convergent-divergent nozzles with design mach numbers of 1.3, 1.5 and 1.65. All four nozzles have the same throat area. They are all tested in a perfectly expanded condition and in an under-expanded condition. The convergent-divergent nozzles are also tested in an over-expanded condition. Flowfield and Acoustic data is presented for all cases. Far-field acoustic measurements are taken in an anechoic chamber with microphones arranged in an arc at a distance of 56 throat diameters and at a range of inlet angles representing forward, sideline and aft-angles in order to capture both mixing noise and shock-associated noise. This is a joint experimental/numerical project with the Naval Research Lab. Comparison to the numerical predictions is covered in an accompanying presentation. [Preview Abstract] |
Monday, November 19, 2007 4:27PM - 4:40PM |
JH.00005: Flowfield and Acoustic radiation from imperfectly expanded supersonic jets: Computational Studies Junhui Liu, Ravi Ramamurti, Kazhikathra Kailasanath, Rainald Lohner This project involves the study of sound generated by supersonic jets like those emanating from the exhausts of high-performance military aircraft. This is a joint experimental/computational project with the University of Cincinnati. The flowfield and near-field noise from both convergent and convergent-divergent nozzles have been simulated. The emphasis is on imperfectly expanded or off-design conditions. The impact of grid resolution, initial and boundary conditions on the computed solutions have been assessed. Comparisons with analytical predictions on shock-cell spacing show very good agreement. Comparison to the experimental observations are underway and will be presented at the meeting. [Preview Abstract] |
Monday, November 19, 2007 4:40PM - 4:53PM |
JH.00006: An Experimental Study of Airfoil Self-Noise Daniel W. Shannon, Scott C. Morris, Mark Ross The acoustic signature of airfoils at low Mach number is typically dominated by noise generated at the trailing edge. This ``self-noise'' is dipole-like in nature, and results from the scattering of the turbulent pressure fluctuations in proximity to a sharp edge. Predicting the radiated sound requires a detailed understanding of the stochastic properties of the turbulence, as well as knowledge of the acoustic Green's function for a particular geometry. The present research has focused on the direct measurement of edge scattering acoustics for a number of geometries. The experiments were conducted in an Anechoic Wind Tunnel using a pair of 40 microphone phased arrays. The test specimens include compact and non-compact airfoils, circular cylinders, blunt and sharp trailing edges, and a blown slot edge used for circulation control. The discussion will focus on the comparison of the various results in order to derive an improved understanding of the sound generation physics. [Preview Abstract] |
Monday, November 19, 2007 4:53PM - 5:06PM |
JH.00007: Acoustics of Jet Impingement on a Porous Plane Nathan Murray, John Seiner It is well known that a high speed impinging jet has a resonance-like behavior that results from acoustic feedback from the impingement of coherent structures on the impingement plane. In this work, the acoustics produced by a sonic axisymmetric impinging jet are compared to that produced by the same axisymmetric jet impinging on a porous screen. The desire is to study the effect of the screen porosity on the feedback mechanism. In the case of air flowing over a rectangular cavity, it has been observed that the over-all noise levels are noticeably reduced as a result of the placement of a porous screen on the aft wall and trailing deck such that a small void downstream of the cavity is enclosed by the screen. However, it appears that the flow resistance provided by the screen is a driving factor in the effectiveness of this mechanism for cavity flow noise reduction. The impinging jet offers a well documented control for studying the effect of the flow resistance on a feedback mechanism that is similar in many respects to the cavity flow. [Preview Abstract] |
Monday, November 19, 2007 5:06PM - 5:19PM |
JH.00008: The sound of boundary-layer flow over a roughness element Qin Yang, Meng Wang As a first step toward predicting rough-wall boundary layer noise, the sound radiation from a single hemispherical roughness element in a turbulent boundary layer at $Re_{\theta}=7500$ is investigated numerically. The roughness height is $3.6\%$ of the boundary layer thickness, or 95 wall units. The flow field is computed by large-eddy simulation using an unstructured-mesh code developed at the Center for Turbulence Research. The velocity statistics show reasonable agreement with the experimental values measured at Virginia Tech. Acoustic calculations are based on the Curle-Powell integral solution to the Lighthill equation for an acoustically compact roughness element. The sound radiation is dominated by unsteady drag dipoles and their images in the wall. It is found that the spanwise dipole, which has often been overlooked in previous studies of roughness noise, is of similar magnitude or stronger compared to the streamwise dipole. The viscous contribution to the drag dipole is negligible relative to the pressure contribution. The effects of roughness height and upstream wake on noise generation are examined, and source mechanisms are discussed in terms of diffraction and turbulence generation by the roughness element. [Preview Abstract] |
Monday, November 19, 2007 5:19PM - 5:32PM |
JH.00009: Investigating Aeroacoustic Sources in a Subsonic Jet Adam J. Wachtor, Peter Jordan, William K. George George, W\"{a}nstr\"{o}m, and Jordan (2007) suggested an alternative approach to identifying aeroacoustic sources. Through this method, contributions to the pressure field are effectively separated into three separate terms. One term is unique in that it present only in compressible flows. This compressible term has been argued to be the only term that can radiate acoustically. An investigation into this approach is presented in the specific case of a subsonic jet. Particular attention is paid to the compressible term and its interaction with the mechanism that is responsible for the hydrodynamic pressure in an incompressible flow. We extend our thanks to Jonathan B. Freund for access to data from his DNS jet simulation. [Preview Abstract] |
Monday, November 19, 2007 5:32PM - 5:45PM |
JH.00010: Numerical simulation of flow generated noise of a hydrofoil at Re = 1.9M Yaser Khalighi, Parviz Moin, Frank Ham The goal of present work is to study the nature of broadband noise generated by flow of practical interest. With this objective, flow over trailing of a hydrofoil at chord Reynolds number of 1.9 Million is considered. This flow configuration includes practical complexities such as turbulent boundary layers with pressure gradients, turbulent wake and irregular geometry. An incompressible formulation with 2nd order, non-dissipative numerical scheme is applied to calculate the hydrodynamic noise sources. Flow scales are fully resolved in the boundary layers and in the vicinity of the trailing edge resulting in a 100M grid point unstructured computational domain. Lighthill's acoustics analogy with a boundary element formulation is applied to accurately predict the propagation and scattering of acoustics waves. The large unstructured grid is generated by the recursive refinement of a coarser grid, facilitating solution verification of both the flow and noise calculation by a grid convergence study. The results will be validated by comparison to hot-wire flow measurements from Michigan State University as well as acoustics measurements from an anechoic chamber at Notre Dame University. [Preview Abstract] |
Monday, November 19, 2007 5:45PM - 5:58PM |
JH.00011: Interaction between the Flow and the Shock Structure in a Supersonic Jet Catalina Stern, Cesar Aguilar, Manuel Alvarado, Alejandro Carre\~no A 534nm laser is used to study density fluctuations in the near field of a small supersonic jet through Rayleigh scattering. Besides the expected peaks due to acoustic and entropic fluctuations, a third low frequency fluctuation has been detected always close to the shocks along the centerline. In this work, this low frequency motion is studied in all points of the jet. At each point, the direction for which the amplitude of the peak is higher is considered the direction of propagation of the fluctuation. With our method, the local speed of the flow and of the fluctuation can be determined. Results are compared to the shock pattern visualized also through Rayleigh scattering. We obtain not only the acoustic radiation pattern inside the flow but also some insight on the flow-shock interaction. [Preview Abstract] |
Monday, November 19, 2007 5:58PM - 6:11PM |
JH.00012: Investigation of Instability Wave Dynamics in High-Speed Turbulent Jets Using LES Jaiyoung Ryu, Sanjiva K. Lele Instability waves have been frequently invoked to explain the dominant noise from high-speed jets. Current methods for predicting jet noise do not, as of yet, use the instability wave formalism. We decompose the results of the large-eddy simulation of high-speed jets (Bodony and Lele, 2005) by Fourier, adjoint (Ryu, Lele and Viswanathan, 2007) and POD methods (Suzuki, 2007) to extract the instability wave contribution to the fluctuations. Three operating conditions are analyzed. Jet instability modes at different frequencies and azimuthal mode numbers as a function of downstream position are traced. The deduced instability wave amplitude and phase dynamics are compared with the predictions of the parabolized stability equations (Cheung, 2007). The least square method is used to provide the amplitude estimate for the linear PSE results. The decomposed LES database shows ``the physics of instability waves'' to a limited extent. The agreement is best for the lowest frequency considered (St=0.1) and for the first azimuthal mode (n=1). For higher St and other modes larger discrepancies are observed. [Preview Abstract] |
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