Intracycle Control Sensitivity of Cross-Flow Turbines

Abstract

Intracycle control for cross-flow turbines employs sinusoidal perturbations in rotational speed to affect turbine power output and loads. This technique is explored for one- and two-bladed turbines with experiments that combine performance measurements with in-rotor flow-field measurements using non-intrusive particle image velocimetry. Performance enhancement is demonstrated for a wide range of control parameters but is found to be more sensitive to the sinusoidal phase offset than amplitude. Optimal performance is observed under conditions which maintain constant nominal blade angle of attack and consistently high blade-relative velocities once the static stall angle is exceeded during the power stroke. This maintains high lift and delays vortex separation. Perhaps most significantly, kinematics are also identified which decrease peak turbine loading by 12% while still producing marginally higher power output, highlighting the utility of intracycle control for load mitigation. The range of control parameters that produce these beneficial loading properties is relatively narrow and flow-fields are similar to operation at a constant rotation rate. We hypothesize that reductions in maximum loading are achieved by smoothing the changes in angle of attack and blade relative velocity profiles throughout the cycle.