Abstract
Engineering experience with axial-flow wind and water turbines suggests that when streamwise turbine spacing is less than ten diameters, the wake from upstream turbines significantly degrades the power output of downstream turbines and imposes higher fatigue loads. It has been hypothesized that cross-flow turbines may be able to achieve higher array efficiencies than axial-flow turbines, particularly in situations where dense arrays are desirable). Cross-flow turbines can also obviate the need for yaw control and can be arranged in high blockage configurations to enhance power output. In dense arrays, power output may be increased when the turbines are controlled in a coordinated manner, allowing downstream turbines to beneficially interact with the flow structures produced by upstream turbines. This type of coordinated control may benefit from reduced order models that describe the wake within the array from a limited set of online measurements (e.g., the angular position, velocity, and acceleration of each turbine). The construction of such models does, however, require a thorough understanding of cross-flow turbine wakes. There have been a number of recent studies in this area, including a study of a hydrokinetic turbine by Bachant and Wosnik, which characterized the time-average wake at a vertical plane one turbine downstream. Given the complicated, unsteady flow dynamics present in normal operation of cross-flow turbines, an improved understanding is best gained experimentally