Experimental Investigations on Oscillatory Couette-Taylor Flow Wall Shear Stress Behaviour Using Electrochemical Technique: High Modulation Effect

Abstract

This work is an experimental investigation on oscillatory rotational flows with relatively high modulation frequencies and amplitudes. Two principal flow protocols have been defined and studied. The first one corresponds to advancing, stopped inner cylinder of a Couette-Taylor system with non-zero mean velocity, while the second protocol corresponds to the case where the inner cylinder oscillates harmonically (in time) clockwise and counterclockwise with non-zero mean velocity, i.e. it advances then moves back then advances again. According to the protocol, reversing or non-reversing Taylor vortex flows (RTVF or NRTVF) have been detected. Corresponding mass transfer and wall shear rate time evolutions strongly depend on modulation frequency and amplitude. If the oscillation amplitude is large enough, it can destabilize the laminar Couette flow; Taylor vortices appear even if the maximum oscillatory Taylor number is lesser than the critical steady rotational one (Taoscillatory maximum < Tacritical steady). Mass transfer evolution has a sinusoidal evolution and still positive at high frequencies and high amplitudes bigger than the unit. The vortices direction can be deduced from the sign of the instantaneous wall shear rate time evolution. The presence of RTVF or NRTVF is dependent on oscillation amplitude. High modulation amplitude gives to vortices sufficient time to follow the direction of the inner cylinder (RTVF) despite the relatively high frequency which seems to be not enough to have NRTVF.