Effects of acoustic disturbances on measured flow characteristics through a contraction

Abstract

An experiment involving the addition of randomly phased broadband acoustic disturbance to the flow through an axisymmetric contraction was conducted. Measurements of flow characteristics such as turbulence intensities, velocity spectra, and coherence functions were made at various locations through the contraction using two flow conditions. Acoustic excitation was shown to substantially increase the measured streamwise component of turbulence while leaving the radial component virtually unchanged. Although the radial component of turbulence energy remains relatively unaffected (less than 3%) inside the contracting region, a sudden increase was observed at the downstream end of the contraction. The coherence at x/L = 0.75 for the acoustically excited flow contains a small band of frequencies with low coherence. The location of the low-coherence frequency band was sensitive to the incoming flow condition, suggesting some type of turbulence/ac oustic interaction. Nomenclature : area c = contraction area ratio / = frequency, Hz or kHz L = contraction length M = turbulence-generating grid mesh size r = radial coordinate, measured from centerline R, R(x) = radius of contraction contour at x s = lateral distance from the axis of symmetry SPL = sound pressure level (dB Re 2 X 105 N/m2) u' = root-mean-square of velocity fluctuations in the streamwise direction U, U(x) = streamwise mean velocity v' = root-mean-square of velocity fluctuations in the radial direction V = radial (or lateral) component of mean velocity x = direction parallel to test section and contraction's axis of symmetry