Coherent structures in Taylor-Couette flow: Experimental investigation

Taylor-Couette flow is defined as the flow confined between two coaxial cylinders which can rotate independently. Several different flow states can be observed in the gap between the cylinders by changing the rotation speeds and the rotating directions of the cylinders. The aim of this thesis is to investigate the previously reported change of torque values with the rotation speeds of the cylinders and to study its relation to coherent turbulent flow structures. The flow structures are investigated using tomographic PIV, which is a fully volumetric measurement method that resolves all three velocity components. Different flow states, and their contribution to the Reynolds stresses are revealed. Initially, the Taylor-Couette setup was used to investigate the spatial resolution of tomographic PIV, by exploiting the fact that the power input to the system, as determined from the torque measurements and the cylinder rotation speed, is balanced by the viscous dissipation rate, which can be computed using the measured velocities. Next, time-resolved tomographic PIV measurements were performed at fully turbulent flow to demonstrate the capability of the measurement system and the flow geometry to study dynamic events in turbulence. Turbulent flow with an approximately zero mean velocity was created by rotating the cylinders in opposite directions with the same wall velocities. Using this idea, the observation times of the flow structures could be increased by an order of magnitude as compared to similar studies in turbulent boundary layers. Finally, the connection between turbulent flow structures and the change of the torque was made by using tomographic PIV. In order to compute the contribution of the large and the smaller-scale structures to the torque, the instantaneous flow was decomposed into the large and the smaller-scale motions by filtering. The Reynolds stresses associated with these structures indicate that the orientational change of the structures may be the mechanism responsible for the reported change of the torque scaling.