THE SOUND FIELD IN FULLY-DEVELOPED TURBULENT PIPE-FLOW DUE TO INTERNAL FLOW SEPARATION .1. WALL-PRESSURE FLUCTUATIONS

A detailed investigation of the wall-pressure fluctuations in fully developed turbulent pipe flow, disturbed due to an orifice plate, is presented. Measurements were made in the initially undisturbed flow, the separated and re-attachment regions and in the re-established fully developed flow. In nominally undisturbed regions (i.e., just before separation upstream of the orifice plate and following disturbance, where the mean velocity and turbulence profiles revert to their undisturbed states at some distance downstream (X ≥ 48)), the local hydrodynamic pressure fluctuations should also be those characteristic of undisturbed fully developed pipe flow; but the values of root mean square (r.m.s.) pressure and its power spectral density are, in fact, much higher than those for the undisturbed flow. The difference is attributed to propagating acoustic waves generated by the flow disturbance produced by the orifice plate. Wall-pressure spectra have a basic broadband character, due to turbulence, with superimposed narrow-band peaks. The peaks are attributable to acoustic modes, and the frequencies at which these sudden rises in spectral level occur correspond to the modal cut-off frequencies. In separation and re-attachment regions, peaks are smaller; at larger X, the acoustic contributions becomes more prominent, as the turbulence levels relax towards their undisturbed flow values. Scaling in these undisturbed regions, separation and re-attachment regions, for total r.m.s. pressure and wall-pressure spectra, is introduced. © 1994 Academic Press Limited.