Strouhal number dependency of the aero-acoustic response of wall perforations under combined grazing-bias flow

\u3cp\u3eThe influence of low Mach number grazing-bias flow on the linear acoustic response of slit shaped wall perforations is determined in terms of a dimensionless acoustical impedance for Strouhal numbers based on the perforation width of order unity. The influence of edge geometries is studied by experiments. In particular, slanted slits under an angle of 30° with respect to the grazing flow direction are considered. Sound production, i.e. whistling potentiality corresponding to a negative real part of the impedance, is observed for various geometries and flow conditions. Sound production restricts the largest perforation size which can be used in practice for acoustical liners. Whistling in the limit cases of purely bias and purely grazing flows can be explained qualitatively in terms of Vortex Sound Theory. For combined bias/grazing flow, most of the oscillations in the impedance as a function of the Strouhal number are related to these limit behaviours. A configuration with thin sharp edges both upstream and downstream corresponds to commonly used theoretical models assuming an infinite thin wall. This configuration displays a behaviour drastically different from a more realistic perforation geometry with sharp square edges.\u3c/p\u3

Accuracy assesment of planar nea-field acoustical holography applied on non-resonant vibration imaging

A numerical study is presented on the accuracy of Planar Near-field Acoustical Holography (PNAH) for measuring non-resonant vibration of plate-like structures. Lately the PNAH tech- nique is applied to detect vibrations, but in most cases the measurement are limited to the in- vestigation at resonance. However, in many practical situations the non-resonant behaviour of the structure is also important. Simulations are performed on a thin simply-supported baffled plate for which sufficient modes contribute to the vibration by applying modal superposition. The distribution in modal contribution as well as the radiation frequency is varied. The acous- tic pressure is calculated by means of the first Rayleigh integral, given the normal vibration velocity over the surface of the plate. The reconstruction of the velocity is performed by the PNAH algorithm. From the numerical study, it is found that the accuracy of the reconstruction depends on the radiation efficiency of the modes and the distribution in the contribution of the single modes to the total vibration