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Turbulence and Shear Flow Phenomena -1 First International Symposium

ISBN Imprimer: 1-56700-135-1



This investigation is concerned with the (re)transition to turbulence of the flow in a straight pipe downstream of a long helical coil where the flow has previously laminarized. The pipe is tangent to the coil exit and they are both of constant diameter D = 2R0. The coil diameter to curvature ratio is D/Rc = 1/10.64 and the coil pitch is small. The Reynolds number of the flow, defined as Re = DUb, where Ub is the bulk average velocity of the fluid and ν its kinematic viscosity, ranges between 2500 and 12,200. A laser-Doppler velocimeter (LDV) is used to obtain time records, mean and rms values of the streamwise velocity component over the pipe cross-section at various axial locations 2 < x/D < 45 downstream of the coil-pipe juncture. Limited flow visualization is also performed. The coil exit flow is laminar up to Re ~ 5200. That in the straight pipe has transitioned by x/D = 45 when Re = 4600 and, with increasing Re, the transition location moves upstream while the flow rapidly evolves towards the turbulent state. Analysis of the results suggests that transition to turbulence in the straight pipe may be occurring in two ways, through the growth of disturbances induced at the pipe wall, and through an inflectional instability in the core flow.
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