Time-resolved volumetric measurements of the interaction between energetic coherent motions and tip vortices in the wake of an axial-flow marine turbine

Instantaneous and time-averaged volumetric velocity fields were obtained in the wake of a single, three-bladed hydrokinetic turbine model placed in a water flume at St. Anthony Falls Laboratory at the University of Minnesota, under subcritical conditions. A cylinder was placed upstream of the turbine in order to introduce large coherent motions into the incoming flow. Six unique configurations were tested, including a base flow characterization, the wake with the turbine-only, the wake of the turbine with a cylinder placed 5 diameters upstream, the wake of the turbine with a cylinder placed 8 diameters upstream, the wake of only the cylinder at the 5 diameters upstream position, and the wake of only the cylinder at the 8 diameters upstream position. The Q-criterion and vorticity as well as the volumetric velocity fields were used to study the interaction between energetic coherent motions generated by the cylinder and tip vortices generated by the turbine. Vortex shedding from the cylinder produced specific coherent vortical structures that affected the turbine wake by increasing the disorder of the otherwise relatively orderly helical downstream wake. Other relevant insights on the tip vortex dynamics, and flow characteristics in the turbine wake are discussed.