Abstract
Helmholtz resonators are usually used in ducts or in engines inlet and exhaust systems in order to reduce radiated noise. Their efficiency may be strongly affected since they are submitted to a fully turbulent grazing flow. An experimental analysis of a branched Helmholtz resonator composed by a circular neck and a circular cavity is presented here. An attempt is made to understand the evolution of entry impedance modification with respect to mean flow and the shear layer characteristics at the vicinity of the resonator aperture. An inverse method is used to identify the impedance of 25 resonators. This method presents the advantage that it is completely nonintrusive and does not require source control. The grazing flow modifies the orifice resistance and the effective neck length. A parametric study of these corrections is performed using experimental results obtained for a set of 25 resonators and a flow Mach number varying from 0 up to 0.25. The mean flow structure is also analyzed using a numerical model. The dimensions of the tested resonators are similar to those usually used in the exhaust and inlet systems of automotive engines. That leads to a new parametric model of these corrections more suitable to engine applications.
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