Effect on Wall Shear Rates of Taylor Vortex Instabilities Induced by Progressive Variation of the Inner Cylinder

Taylor–Couette flows are frequently reencountered on engineering processes, such as for cooling the rotors of electric motors by admitting cold air and improving the mixing in the combustion area, for the injection of fuel into the cylinders of gasoline engines, for the injection of just-prepared emulsions on diesel engines, etc. On its simplest configurations, Taylor–Couette systems (TCS) are widely studied when one or both cylinders steadily rotate. Limited studies are available as concerned on the acceleration-deceleration of one or both cylinders. This paper deals with the effect of acceleration-deceleration imposed to the inner cylinder of the CTS on flow instabilities. Imposing progressive periodic transitional motions through the cylindrical CTS walls alters the stability of the flow. The objective of this work is to study the flow dynamics and interactions between wall and vortices while imposing progressive periodic motions. We used the electrodiffusion (ED) method, known also as the polarography technique. The apparition and disappearance of the regime transitions, due to the acceleration-deceleration of the inner cylinder speed, on the frequency response of the ED sensor were illustrated and discussed. The regimes’ instabilities and the corresponding Taylor numbers of the first and the second transitions of TC flow were determined. The results show coherence between the temporal wall velocity gradient (determined from the mass transfer rates) and the flow turbulent characteristics, particularly at the transitions between the Couette flow (CF) regime, the wavy vortex flow (WVF), and the modulated wavy vortex flow (MWVF). Instantaneous mass transfer rate and velocity gradient have been compared for the CF, the WVF, and the MWVF regimes. The apparition of small then large vortices, when the Taylor number increases, i.e., from laminar regime to turbulent one, affects the mass transfer and the wall shear rate. Sensor inertia effect and a slight hysteresis effect, due to sudden periodic flow velocity decrease, were detected. The ED results allowed testing the robustness of the inverse method, based on the inversion of the convection-diffusion equation, on mass transfer to determine the wall shear rate.