Experimental and computational fluid dynamic analysis of laboratory-scaled counter-rotating cross-flow turbines in marine environment

Power generation of laboratory-scaled marine hydrokinetic (MHK) cross-flow (vertical axis) turbines in counter-rotating configurations was scrutinized both experimentally and numerically. A tabletop experiment, designed around a magnetic hysteresis brake as the speed controller and a Hall-effect sensor as the speed transducer was built to measure the rotor rotational speed and the hydrodynamic torque generated by the turbine blades. A couple of counter-rotating straight-three-bladed vertical-axis turbines were linked through a transmission of spur gears and timing pulleys/belt and coupled to the electronic instrumentation via flexible shaft couplers. A total of 6 experiments in 3 configurations, with various relative distances and phase angles, were conducted in the water channel facility (3.5 m long, 0.30 m wide, and 0.15 m deep) at rotor diameter base Reynolds number of 20,000. The power curve of the counter-rotating turbines (0.068-m rotor diameter) was measured and compared with that of a single turbine of the same size. Experimental results show the tendency of power production enhancement of different counter-rotating configurations. Additionally, the two-dimensional (2D) turbine wakes and blade hydrodynamic interactions were simulated by the shear stress transport k-omega (SST k-omega) model using OpenFOAM. The computational domain included a stationary region and two rotating regions (for the case of counter-rotating turbines) set at constant angular velocities. The interface between the rotating and stationary region was modeled as separated surface boundaries sliding on each other. Velocity, pressure, turbulent kinetic energy, eddy viscosity, and specific dissipation rate field were interpolated between these boundaries.