Index of content:
Volume 128, Issue 5, November 2010
- AEROACOUSTICS, ATMOSPHERIC SOUND 
128(2010); http://dx.doi.org/10.1121/1.3495941View Description Hide Description
The polar coordinate method in You [Ph.D. thesis, The University of Mississippi (1993)] is rederived in differential form and is generalized by defining a local, continuously varying, radius of curvature. The generalization makes it possible to compute sound propagation over arbitrary large-scale terrain where the local radii of curvature are much larger than an acoustic wavelength. For a simple hill, both the original and generalized method are in good agreement with measured low-frequency propagation loss data. The generalized method is applied to more complex terrain in a numerical example. The example demonstrates the utility of the method and also shows that complex terrain can have a significant effect on low-frequency sound propagation, even when the slope angles are small.
128(2010); http://dx.doi.org/10.1121/1.3365240View Description Hide Description
An iterative technique, taken from the field of optics, is used to obtain tailored Green’s functions suitable for the evaluation, in the nearfield, of pressurefluctuations generated by turbulent flow in the vicinity of solid boundaries. Comparisons are made with the analytical solution for the solid sphere, and with results obtained using conventional boundary element method(BEM) for the case of a thick semi-infinite plate. A divergence issue in the case of the solid sphere is resolved by the introduction of a relaxation factor. The performance of the iterative approach is found to be comparable to that of conventional BEM, except at irregular frequencies, where the bandwidth of the error is slightly larger than that of the conventional BEM. The main advantage of the iterative approach is a significantly reduced computational cost, which allows for higher surface mesh densities and a broader useful frequency range.
128(2010); http://dx.doi.org/10.1121/1.3483735View Description Hide Description
A method is presented for measuring the aeroacoustic source strength in ducts with flow at frequencies at which the wave motion can be considered substantially one-dimensional. The method is based on coherent power flux measurements using pairs of microphones positioned both upstream and downstream of the source region. The method is applied to a flow excited expansion chamber with aeroacoustic source measurements presented for chambers with a range of flow velocities and chamber lengths. The results indicate locked-on flow tones are generated in the chamber. The frequency of these locked-on flow tones is compared with that predicted using describing function theory applied to resonators with a grazing flow as well as that of other literature.