Abstract
When the inflow direction is perpendicular to the axis of rotation, the performance for cross-flow turbines is independent of direction, a significant appeal of these devices. However, their performance is still affected by inflow off-perpendicular, or inclined, to the axis of rotation. This can occur for seabed-mounted turbines on uneven foundations, for moored turbines that become tilted during operation, or when local bathymetry introduces significant three-dimensionality into the mean flow structure. For axial-flow turbines, misaligned inflow has been studied extensively with empirical corrections developed for power and thrust coefficient, as well as modifications to momentum models that describe changes in power output as a function of alignment angle. However, a similar understanding for cross-flow turbines has not yet been developed, and studies to date have only begun to explore these effects. With limited experimental data and associated gaps in the parametric space, further experimental work characterizing performance is necessary to reach more robust conclusions. To address this, we experimentally investigate mechanical efficiency of a cross-flow turbine over a range of incline angles for multiple orientations.
To quantify turbine performance, we tested a single-bladed turbine in a recirculating water flume. During these experiments, water depth, inflow speed, and temperature were varied with each incline angle to hold the blockage ratio, Reynolds number, and depth-based Froude number constant. The primary incline orientation was in the streamwise direction, with the upper end of the rotor advanced in the same direction as the inflow at incline angles up to 45 degrees. We also evaluated a more limited set of angles for other orientations, including the opposing streamwise direction and both cross-stream directions. For streamwise inclinations, we accounted for the change in the rotor projected area when setting the inflow conditions. The power coefficient, a key performance metric, was assessed across a range of tip-speed ratios for each incline angle. At each angle, we account for changes in the rotor projected area and velocity perpendicular to the blade.
Preliminary results show successive decreases in the power coefficient with increasing incline angle in the streamwise direction, similar to what has been observed for axial-flow turbines. Additionally, the optimal tip-speed ratio decreases with increasing incline angle. Within a given rotational cycle, the observed decrease in power is primarily seen in the upstream portion of the blade sweep, with no significant changes in the downstream sweep. When accounting for changes to the blade-normal velocity component and projected area, we see a collapse in power coefficient at incline angles up to 25 degrees and the optimal tip-speed ratio increases with increasing incline angle. Performance is shown to depend on orientation for the streamwise direction but not in the cross-stream direction. Through these experiments, we aim to build a comprehensive understanding of how cross-flow turbine power performance varies with incline angle, leading to more informed turbine designs.