An experimental study of the fluid mechanics of turbulent pipe flow when subjected to forced oscillation at high frequencies.

Abstract

The effects of high frequency forced unsteadiness on fully developed turbulent pipe flow have been studied experimentally at higher frequencies than have been achieved in previous experiments. A fully developed turbulent pipe flow at an average Reynolds number of 11,500 was subjected to a range of frequencies and amplitudes of forced sinusoidal unsteadiness. The amplitudes were as high as 27% of the centerline velocity, and the frequencies reached three times the mean-flow burst frequency. The response of the pipe flow to these organized unsteady effects was studied through laser-Doppler anemometer measurements of the steamwise velocity. In addition, hydrogen-bubble flow visualization was used to investigate the dynamical behavior of near-wall streak patterns in unsteady flow. The results of this study confirm that the time-averaged behavior of the flow was invariant to changes in amplitude or frequency of forced unsteadiness and almost indistinguishable from its counterpart in steady flow--even at frequencies comparable to and greater than the mean flow burst frequency. When the velocity field was decomposed into mean, oscillatory and turbulent components, the oscillatory components of velocity and turbulence intensity were shown to be linearly proportional to the amplitude of forced unsteadiness. At high oscillation frequencies ($r\sb0\sqrt{\omega/\nu}\ge 32,\ \omega\sp+=\omega\nu/u\sbsp{\tau}{2}\ge 0.004$ and the turbulent Stokes layer does not reach the centerline), these responses were characterized by the dimensionless frequency, $\omega\sp+$, and scaled on the laminar Stokes thickness, $l\sb{s}=3.23\sqrt{\nu/\omega}$. Profiles of the oscillatory component velocity matched the quasi-laminar Stokes solution close to the wall for $\omega\sp+\ge 0.06$. For frequencies of oscillation in this range, the turbulence intensity was frozen everywhere except within the Stokes layer. The peak amplitude of oscillatory turbulence intensity decreased and moved closer to the wall monotonically with increasing frequency. While time-mean streak spacings, from hydrogen-bubble visualization, were not affected by the forced unsteadiness, phase-conditioned streak spacings were strongly modulated by the oscillation (though not sinusoidally)--the greatest streak separations corresponded to the largest values of turbulence intensity, while the smallest separations were found at the cycle phase of minimum turbulence intensity.