Evaluating the accuracy of the actuator line model against blade element momentum theory in uniform inflow

We evaluate the accuracy of the actuator line model (ALM) approach by performing simulations for the NREL 5-MW wind turbine in uniform inflow using three large eddy simulation codes. The power and thrust coefficients obtained using the three codes agree within 1% when the grid spacing Δgrid≤5.25Δgrid≤5.25 m and are cross-validated against blade element momentum (BEM) theory. We find that the results of ALM converge towards BEM theory without the need for tip correction when the numerical resolution is increased. For Δgrid=0.98Δgrid=0.98 m, the difference between the power and thrust coefficient obtained using ALM and BEM is 4.5% and 2.1%, respectively, although we note that no absolute convergence between ALM and BEM can be obtained as both models use different assumptions, such as the use of a force projection method in the ALM. The difference in the local axial and tangential forces along the blades obtained from ALM simulations using Δgrid=1.97Δgrid=1.97 m and Δgrid=0.98Δgrid=0.98 m can be as large as 10%. The effect of the number of actuator points on the obtained turbine power and thrust coefficients is limited as the results converge when the spacing between the actuator points is about three times the grid spacing. This insight on the required number of blade points can be used to improve the efficiency of actuator line simulations.