Transitional flow of a yield stress fluid in a pipe: Evidence of a robust coherent structure
Abstract
In two independent articles, Escudier and Presti [1] and Peixinho et al. [2] studied experimentally the flow structure of a yield stress fluid in a cylindrical pipe. It was observed that the mean velocity profiles were axisymmetric in the laminar and turbulent regimes, and presented an increasing asymmetry with increasing Reynolds number in the transitional regime. The present paper provides a three-dimensional description of this asymmetry from axial velocity profiles measurements at three axial positions and different azimuthal positions. The observed transitional flow suggests the existence of a robust non-linear coherent state characterized by two weakly modulated counterrotating longitudinal vortices. This new state mediates the transition between laminar and turbulent flow and is qualitatively similar to the edge-states recently computed in the newtonian case [3-5].
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