Experimental Investigations on Oscillatory Couette Taylor Flow Wall Shear Stress Behavior using Electrochemical Technique: Low Modulation Effect

In the simplest and original case of study of the Taylor–Couette TC problems, the fluid is contained between a fixed outer cylinder and a concentric inner cylinder which rotates at constant angular velocity. Much of the works done has been concerned on steady rotating cylinder(s) i.e. rotating cylinders with constant velocity and the various transitions that take place as the cylinder(s) velocity (ies) is (are) steadily increased. On this work, we concentrated our attention in the case in which the inner cylinder velocity is not constant, but oscillates harmonically (in time) clockwise and counter-clockwise while the outer cylinder is maintained fixed. Our aim is to attempt to answer the question if the modulation makes the flow more or less stable with respect to the vortices apparition than in the steady case and if there are any reversing or non reversing flows apparition. If the modulation amplitude is large enough to destabilize the circular Couette flow, two classes of axisymmetric Taylor vortex flow are possible: reversing Taylor Vortex Flow (RTVF) and Non-Reversing Taylor Vortex Flow (NRTVF). Our work presents an experimental investigation of the effect of oscillatory Couette-Taylor flow on the instantaneous and local mass transfer and wall shear rates evolutions, i.e. the impact of vortices at wall; and the detection of any RTVF and/or NRTVF apparition. The vortices may manifest themselves by the presence of timeoscillations of mass transfer and wall shear rates; this generally corresponds to an instability apparition even for steady rotating cylinder. On laminar CT flow, the time-evolution of wall shear rate is linear. It can be presented as a linear function of the angular velocity. For a mean Taylor number corresponding to a laminar Couette flow, a modulation frequency F = 0.1 Hz and an amplitude respectively β = 0.53 andβ = 1.08 are sufficient to destabilize the laminar CT flow, Taylor vortices appear. Comparing to a steady rotational velocity case, oscillatory flow accelerates the instability apparition, i.e. the mean critical Taylor number corresponds to the transition is smaller than that of the steady rotational case. The vortices direction can be deduced from the sign of the instantaneous wall shear rate time evolution.