Bulletin of the American Physical Society
64th Annual Meeting of the APS Division of Fluid Dynamics
Volume 56, Number 18
Sunday–Tuesday, November 20–22, 2011; Baltimore, Maryland
Session R18: Acoustics II: Internal and External Flows |
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Chair: Edward White, Texas A\&M University Room: 321 |
Tuesday, November 22, 2011 12:50PM - 1:03PM |
R18.00001: A simple sound source for temporally-developing mixing layers Lin Zhou, Mingjun Wei, Dejun Sun Applying Lighthill's acoustic analogy equation to temporally-developing mixing layers, we derived a direct relation between the near-field work by pressure fluctuation and the far-field sound. The sound radiation predicted by the new model was compared to the direct numerical simulation, and the results agreed well for the sound radiation from different vortex dynamics in mixing layers: roll-up, pairing (or tearing), merging, saturation, and viscous damping. Using the new formulation, we explained the mechanism for sound generation from the considered flow, and depicted general topological structures for the distribution of sound sources. [Preview Abstract] |
Tuesday, November 22, 2011 1:03PM - 1:16PM |
R18.00002: Large-amplitude acoustic streaming Greg Chini A mechanism for the generation of large amplitude acoustically-driven streaming flows is proposed. Motivated by streaming observed in high-intensity discharge (HID) lamps, two-dimensional flow of an ideal gas in a channel geometry is analyzed in the asymptotic limit of high frequency forcing. Predictions of streaming flow magnitudes based on classical arguments invoking Reynolds stress divergences originating in viscous boundary layers are orders of magnitude too small to account for the observed mean flows. Moreover, classical ``Rayleigh streaming" theory cannot account for the direction of the cellular mean flows often observed in HID lamps. In contrast, the inviscid mechanism proposed here, which invokes fluctuating baroclinic torques away from viscous and thermal boundary layers, can account both for the magnitude and the orientation of the observed streaming flows. [Preview Abstract] |
Tuesday, November 22, 2011 1:16PM - 1:29PM |
R18.00003: Control of Acoustic Reflections in a Closed-Circuit Wind Tunnel Matthew Kuester, Edward White Closed return wind tunnels, such as the Klebanoff--Saric Wind Tunnel (KSWT) at Texas A\&M University, can provide low-disturbance flows that are required to study boundary layer receptivity. Receptivity is the process through which environmental disturbances become the initial condition for boundary layer instabilities. One instability mechanism, Tollmien--Schlicting waves, is especially receptive to freestream sound. The receptivity of these waves is studied by introducing downstream-traveling planar sound waves that interact with the leading edge of a flat plate; however, changes in wind tunnel area create reflected waves that complicate the experiment. Reflections are mitigated using a secondary speaker located downstream of the test section that eliminates upstream- traveling reflections. The secondary speaker is controlled using a finite impulse response (FIR) filter. Microphone measurements are used to document the wave cancellation at multiple locations in the test section. [Preview Abstract] |
Tuesday, November 22, 2011 1:29PM - 1:42PM |
R18.00004: Non-Linear High Amplitude Oscillations in Wave-shaped Resonators Dion Antao, Bakhtier Farouk A numerical and experimental study of non-linear, high amplitude standing waves in ``wave-shaped'' resonators is reported here. These waves are shock-less and can generate peak acoustic overpressures that can exceed the ambient pressure by three/four times its nominal value. A high fidelity compressible axisymmetric computational fluid dynamic model is used to simulate the phenomena in cylindrical and arbitrarily shaped axisymmetric resonators. Working fluids (Helium, Nitrogen and R-134a) at various operating pressures are studied. The experiments are performed in a constant cross-section cylindrical resonator in atmospheric pressure nitrogen and helium to provide model validation. The high amplitude non-linear oscillations demonstrated can be used as a prime mover in a variety of applications including thermoacoustic cryocooling. [Preview Abstract] |
Tuesday, November 22, 2011 1:42PM - 1:55PM |
R18.00005: Rijke tube with flexible walls Shreyas Mandre, Bao-Nhat Nguyen, Marvin Li Sound is excited spontaneously in a Rijke tube because the small temperature perturbations in an acoustic field interact with heat transfer from a heat source in the tube. The air particles near the heat source undergo a thermodynamic cyle converting heat to mechanical energy, which is heard as the sound emanating from the Rijke tube. This principle of energy conversion is used in thermoacoustic engines, and the main objective of this study is to improve their performance. The acoustic oscillations in the Rijke tube regulate the thermodynamic cycle, just as in conventional engines the cycle is controlled by the motion of a piston and the action of inlet and exit valves. The acoustic regulation in the Rijke tube, however, does not allow arbitrary control of the cycle in thermodynamic phase space. In this presentation, we introduce a new way of overcoming this limitation, one by using Rijke tubes with flexible walls. We will discuss how this modification allows for more general thermodynamic cycles to be executed by the air particles in the tube. This possibility, when used in thermoacoustic engines, opens a channel for further improving the engine performance. [Preview Abstract] |
Tuesday, November 22, 2011 1:55PM - 2:08PM |
R18.00006: Computational study of flow noise from small gaps in turbulent boundary layers Jin Hao, Minsuk Ji, Meng Wang The noise induced by small gaps underneath low-Mach-number turbulent boundary layers is studied using large-eddy simulation and Lighthill's equation. The latter is solved by employing an acoustically compact Green's function for the gap and by a boundary-element method. The gap leading-edge height is $13\%$ of the boundary-layer thickness, and the gap width and trailing-edge height are varied to investigate their effect on sound generation. The radiated acoustic field is dominated by the forward-facing step in the gap and resembles forward-step noise for wide gaps and/or asymmetric gaps with the trailing edge higher than the leading edge. For narrow and symmetric gaps, destructive interference of the sound from leading and trailing edges causes a significant decline in the low-frequency spectral content and thereby creates a broad spectral peak in the mid-frequency range. The effect of acoustic noncompactness of gaps is investigated by comparing solutions based on a compact Green's function and those from a boundary-element calculation. Excellent agreement is observed at low frequencies and away from the wall-normal direction. At higher frequencies, the sound field deviates from that of a compact streamwise dipole. The elevated level of surface pressure fluctuations induced by gaps and their recovery to equilibrium conditions are also examined. [Preview Abstract] |
Tuesday, November 22, 2011 2:08PM - 2:21PM |
R18.00007: Investigation of a tone in flap tip noise Rafael S. Gioria, Stergios P. Tsiloufas, Francisco K. Klug, Julio R. Meneghini In this paper, we investigate numerically the noise generated by the flow around a high-lift configuration. The case studied is the noise generated by the flap tip of the MDA-30P30N high-lift geometry. We propose a cavity model as a simplification for the flap cove flow near the flap tip. The idea of representing the flap cove as a cavity is due to a tone in the spectrum from the numerical simulations of the full high lift geometry MDA-30P30N without spanwise gap between extended flap and the stowed flap. A high peak around 900Hz to 1000Hz was not expected and its source is not clear. The effort here is to investigate the source of frequencies around this value. There is a possibility of noise tones generated from the flap cove with frequencies around 900Hz according to the Rossiter model for the cavity. This mechanism of noise generation seems to be associated to the peak in the spectrum. Snapshot of instantaneous vorticity field near the flap tip supports that a mechanism of noise generation resembles the cavity mechanism. Further corroboration is presented through the Koopman modes (dynamic mode decomposition) of the pressure fields. [Preview Abstract] |
Tuesday, November 22, 2011 2:21PM - 2:34PM |
R18.00008: 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. [Preview Abstract] |
Tuesday, November 22, 2011 2:34PM - 2:47PM |
R18.00009: Investigation of noise production from a turbulent cylinder Zane Nitzkorski, Krishnan Mahesh We investigate noise production by a cylinder in turbulent flow (Re=10,000 and M=0.2), using the Ffowcs-Williams and Hawkings acoustic analogy, where the sound sources are computed from a compressible direct numerical simulation. It has been shown that vortices passing through a data surface generate spurious noise if the quadrupole term is neglected. Our implementation of the acoustic analogy, hence, uses porous data surfaces as well as the volume term. We compare our solution to available results, examine the effect of the placement of the data surface on the noise calculation by using multiple surfaces, and the effect of different proposed correction schemes that try to compensate for neglecting the quadrupole noise. [Preview Abstract] |
Tuesday, November 22, 2011 2:47PM - 3:00PM |
R18.00010: Acoustic Scattering from Interaction of Dual Frequency Incident Fields Max Denis, Chrisna Nguon, Kavitha Chandra, Charles Thompson The pressure field produced by the spatial interaction of two high frequency incident plane waves in a three-dimensional scattering object is investigated. Of particular interest is the field produced in response to the difference-frequency component generated from the non-linear interaction of the two harmonically time varying acoustic beams in a high contrast medium. The scattered pressure at the difference frequency field has been shown to enhance the identification of abnormal biological tissue in recent medical ultrasound experiments. This work presents a computational study of the scattered pressure that results from the Reynolds stress in a fluid scatterer. Using Pade approximants, it is shown that the stress tensor can be computed using a uniform expansion in the contrast gauge for the scattered pressure. This allows one to investigate scattering volumes characterized by high compressibility contrast. [Preview Abstract] |
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