Actuator surface hybrid model

A hybrid model has been developed in order to represent the flow downstream a wind turbine rotor. The blades are replaced by their mean surfaces and a "pressure jump" boundary condition is applied here. The hybrid model combines CFD solver with a blade element method (BEM). The solving method is iterative: at the beginning of iteration a BEM determines the pressure discontinuities along the blade span, using the rotor inflow and the aerodynamic properties of blade sections. Then the CFD solver applies this pressure discontinuity in order to model the blade forces and the wake downstream the rotor. At the end of iteration the obtained rotor inflow, the wake development and the residuals are compared with those obtained from previous iteration. If the required precision is attained, the calculation stops. The solution is obtained after thousands of iterations, depending on the number of nodes, residuals etc. The approach of replacing blades by pressure discontinuity surfaces is validated by comparison the wakes in the cases of flat plate and the S809 airfoil. Finally, the proposed hybrid model is used to calculate the performance of NREL Phase VI wind turbine and the results obtained are satisfactory.