Unsteady Turbulent Flows in Channels with Parallel or Diverging Walls
Final technical rept. 26 Jun 87-26 Aug 91,
GRENOBLE UNIV (FRANCE) INST DE MECANIQUE
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Turbulent flows subjected to imposed velocity oscillations have been studied experimentally in a water channel. A wide range of forcing conditions have been covered amplitudes 10 do 70, frequencies omega 0,0005 to 0. 25. Velocities u have been measured with LDA and hot-film anemometer. The wall shear-stress tau has been measured with flash mounted hot-films gages calibrated in situ with the LDA. Near wall dye-streak visualisations have also been performed. The phase average of any quantity q e.g. qu, quu, q tau, q tau tau, etc. is decomposed into a time-mean q-bar and a modulation q-wiggle qq-bar q-wiggle. It has been confirmed that the time-mean statistical characteristics of the channel flow between parallel walls unless otherwise specified are essentially not affected by the imposed oscillations even when these have amplitudes of 64. It has been shown that the appropriate similarity parameter of the oscillating quantities u and tau is the non-dimensional Stokes length. In the regime of high frequency forcing the oscillating flow is governed by purely viscous shear forces although the time-mean flow is fully turbulent. At lower frequencies, the oscillating flow is influenced by the turbulence, in particular the amplitude of tau increases with respect to the Stokes value and becomes proportional to it. This behavior is explained by the fact that the oscillating flow in confined in the near wall layer where viscous forces are dominant.
- Fluid Mechanics