An experimental study of the impedance of perforated plates with grazing flow

An experimental investigation of the linear impedance of perforated interfaces exposed to grazing fluid flow is presented, including a description of the branch-type setup used in the study. The experimental setup employs a movable microphone in the branch duct that improves the error characteristics of the system. The impedance parameters of three perforated samples have been measured including one circular, square-edged orifice and two production muffler louver geometries. The data are presented in terms of the equivalent length and resistance of the samples, with the friction velocity being used to characterize the grazing flow. At higher grazing flow velocities, the equivalent length of each sample decreased by an amount representative of the Rayleigh end correction for one side of an orifice. Resistance for the louvers appears to be somewhat less sensitive to grazing flow than the circular orifice, though all of the samples showed significant increases. While the experimental results for all three samples showed similar basic trends, distinct and substantial differences were observed between each perforate geometry.