The absorption of sound by helmholtz resonators with and without flow

Helmholtz resonators are commonly used as absorbers of incident acoustic power. Theoretical and experimental investigations have been performed in the four cases of no mean flow, grazing mean flow, bias mean flow and a combination of grazing and bias mean flows. In the absence of a mean flow, the absorption coefficient (deflned as the proportion of incident energy absorbed) is a non-linear function of the acoustic pressure and high incident acoustic pressures are required before the absorption becomes signiflcant. In contrast, when there is a mean flow present, either grazing or bias, the absorption is linear and thus absorption coefficient is independent of the magnitude of the acoustic pressure, and absorption is obtained over a wider range of frequencies. Non-linear effects are only discernible very close to resonance and at very-high amplitude. With grazing mean flow, there is the undesirable effect that sound can be generated over a range of frequencies due to the interaction between the unsteadily shed vorticity waves and the downstream edge of the aperture. This production is not observed when there is a bias flow because here the vorticity is shed all around the rim of the aperture and swept away by the mean flow. When there is both a grazing mean flow and a mean bias flow, we flnd that only a small amount of bias mean flow, compared with grazing mean flow, is required to destroy the production of acoustic energy. © 2002 by the author(s). Published by the American Institute of Aeronautics and Astronautics, Inc.

Corner effect and asymmetry in transonic channel flows

An experimental and numerical investigation into transonic shock/boundary-layer interactions in rectangular ducts has been performed. Experiments have shown that flow development in the corners of transonic shock/boundary-layer interactions in confined channels can have a significant impact on the entire flowfield. As shock strength is increased from M∞ = 1:3 to 1.5, the flowfield becomes very slightly asymmetrical. The interaction of corner flows with one another is thought to be a potential cause of this asymmetry. Thus, factors that govern the size of corner interactions (such as interaction strength) and their proximity to one another (such as tunnel aspect ratio) can affect flow symmetry. The results of the computational study show reasonable agreement with experiments, although simulations with particular turbulence models predict highly asymmetrical solutions for flows that were predominantly symmetrical in experiments. These discrepancies are attributed to the tendency of numerical schemes to overprediction corner-interaction size, and this also accounts for why computational fluid dynamics predicts the onset of asymmetry at lower shock strengths than in experiments. The findings of this study highlight the importance of making informed decisions about imposing artificial constraints on symmetry and boundary conditions for internal transonic flows. Future effort into modeling corner flows accurately is required. Copyright © 2011 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.