Turbulence structure in a relatively low frequency pulsatile pipe flow.

Abstract

Hot wire measurements of the fluctuating velocity components u', v' and w' have been carried out in a pulsatile turbulent pipe flow of a pulsation frequency which is lower about one order of magnitude than the mean burst frequency. The root mean square values of fluctuating components u'rms, v'rms, w'rms, and the Reynolds shear stress ^- were determined. Their instantaneous profiles differ appreciably from those of steady flow. Furthermore, the ordered motions were classified into four distinct categories, i. e. sweep, outward interaction, ejection and wallward interaction, and the contributions of these classes of motions to the Reynolds shear stress ^- and turbulence energies u'2^- and v'2^- were obtained. Also given is the relative frequency of occurrence of these motions. The inspection of the data disclosed the following fact. The turbulence is caused by the same ordered motions as those in a steady turbulent pipe flow and it propagates monotonically from the vicinity of the wall toward the pipe center while decaying. The rate of decaying is not affected by pulsation. But the propagation time is not negligible compared to the present pulsation period. Therefore, the propagation time is the main cause of the difference in the turbulence intensity and Reynolds shear stress profiles between pulsatile and steady pipe flows.