Modal analysis of dynamic stall on a cross-flow turbine blade

A cross-flow turbine blade, that rotates on an axis perpendicular to the flow, undergoes dynamic stall due to the cyclical variation of relative flow speed and angle of attack it experiences. In this work we investigate the control of dynamic stall through modal analysis of highly resolved flow fields from large-eddy simulation with the goal of enhancing power generation. An optimized intracycle variation of angular velocity, by modifying the dynamic stall cycle, has been shown to enhance the power conversion efficiency of a two-blade straight-bladed turbine in confined flow by 54%. A generalized intracycle control strategy requires identifying the flow mechanisms causing this change. A proper orthogonal decomposition (POD) is able to identify the dominant flow modes at the cross-flow turbine blade such as the fully attached, fully separated, and vortex formation modes and their temporal activation functions. Comparing these for a constant and intracycle variation of angular velocity, while correlating the mode activation with the temporal variation of relative flow and power, highlights the modes which are prominently modified. Such a reduced-order approach for modeling the performance can lead to an efficient design process with the need for less experiments and computations.