An experimental and computational assessment of blockage effects on wind turbine wake development

Abstract

An experiment was conducted to evaluate the initial wake expansion in scaled wind turbine tests as a means to guide future wake interference studies. Five scaled wind turbine rotors with different diameters were designed for testing in a closedloop water channel to evaluate the effects of blockage on the initial wake expansion behind a wind turbine. The initial wake expansion was assessed by using quantitative dye visualization to identify the propagation of tip vortices downstream of the rotor. The thrust coefficient developed by the scaled models was recorded using a six-component balance and was correlated to the downstream wake expansion. The rotors used in the experiment were operated at a tip speed ratio of 6, a Reynolds number based on the tip speed and tip chord of approximately 23,000 and resulted in blockage values that ranged from 6% to 25%. Dye visualization indicated that the initial wake expansion downstream of a rotor was narrowed and that tip vortex pairing behaviour was modified because of increasing blockage. Blockage effects were significant and resulted in a wake that was more than 50% narrower when blockage was 25% compared with the observed expansion with 10% blockage. A computational simulation was conducted with the Generalized Unsteady Vortex Particle (GENUVP) discrete vortex method code using the rotor in freestream conditions and was compared with the experiments. The magnitude of the wake expansion in the freestream computations was similar to the wake expansion in the experiment when blockage was less than 10%.

An investigation of in‐field blockage effects in closely spaced lateral wind farm configurations

“…The computations were therefore conducted in unbounded freestream conditions with uniform inflow, and the resulting turbine behaviour is representative of the wind farm's performance in the absence of walls or blockage. The ability of GENUVP to predict an appropriate level of wake expansion in unbounded freestream conditions was previously validated by McTavish et al , . A higher Reynolds number was selected for the computations than for the experiments because of the similarity of the E387 airfoil's lift curve with those of other airfoils at higher Reynolds numbers.…”

“…The chord and twist distributions of the rotor blades were developed using blade element momentum theory from ref., and the normalized chord of the scaled blades was increased relative to full‐scale designs in order to increase the Reynolds number of the blades. Additional details regarding the design of the MAAE wind turbine can be found in ref …”

“…Wind tunnel blockage based on a single rotor in the ABL tunnel was 1.67%, and blockage based on the combined swept area of three laterally aligned rotors was 5%. Blockage corrections were therefore not applied in this experiment as previous work by the current authors demonstrated that the initial wake expansion of this particular rotor in isolation should not be greatly affected when blockage is less than 10% and that the effect of such low blockage on recorded parameters will be minimal …”

Tidal energy “Round Robin” tests comparisons between towing tank and circulating tank results

“…This correction attempts to reflect the increased thrust and power generated by a turbine in an enclosed tunnel due to the constriction of the wake behind the turbine. The actuator disc approach fails to capture some key aspects of the blockage on the flow physics, including 255 the impact of blockage on details of the near-rotor vortex wake (see for example [21]) and neglects many aspects of the detailed geometry of the turbine in the tunnel such as the cross-section shape of the tunnel and the proximity of the turbine to flow boundaries. However, there is some experimental evidence to suggest that an actuator disc approach can represent the wake reasonably well far downstream (e.g.…”

Rotor Scaling Methodologies for Small Scale Testing of Floating Wind Turbine Systems

“…This acts to reduce the Reynolds number as compared to the TUDelft turbine, however an increase on the result which would be achieved if the smaller turbine were to be operated in a wind tunnel. Also defined by McTavish et al and studied by McTavish and Feszty [10] is a 3 bladed, 0.25m diameter rotor for operation in a 0.6m x 0.8m water channel at Carleton University. The rotor loads and wake expansion are measured to determine the Reynolds number and blockage effects on the model.…”