Method and apparatus for determining flow resistance of fully assembled

Abstract

Apparatus and method for determining the steady state flow resistance of face sheets on fully assembled acoustic duct liners. Resistance is measured by subjecting the liner to a sinusoidal acoustic pressure field at the face sheet surface. The pressure field is applied through a cylindrical waveguide and is measured by a pressure transducer flush mounted on the inside of the waveguide near the liner face. A second pressure measurement, obtained by a similar transducer positioned further from the face, is combined with the first measurement to calculate the acoustic resistance for the excitation frequency. If the excitation frequency coincides with the resonant frequency of the liner configuration, then no significant pressure exists on the back surface of the liner face sheet. For this condition the root mean square (rms) pressure measured at the face sheet is equated to the static pressure drop used to obtain the steady state flow resistance. The rms velocity of the oscillating pressure wave is calculated from the ratio of the rms pressure to resistance. A repetition of the procedure for several magnitudes of dynamic pressure permits the development of a curve which describes the non-linear characteristics of resistance with changes in velocity. This curve may be equated to a similar curve obtained using steady state flow resistance measurements of the liner in its partially assembled state without a backing surface.