Vertical-axis wind-turbine computations using a 2D hybrid wake actuator-cylinder model

The actuator-cylinder model was implemented in OpenFOAM by virtue of source terms in the Navier-Stokes equations. Since the stand-alone actuator cylinder is not able to properly model the wake of a vertical-axis wind turbine, the steady incompressible flow solver simpleFoam provided by OpenFOAM was used to resolve the entire flow and wakes of the turbines. The source terms are only applied inside a certain region of the computational domain, namely a finite-thickness cylinder which represents the flight path of the blades. One of the major advantages of this approach is its implicitness - that is, the velocities inside the hollow cylinder region feed the stand-alone actuator-cylinder model (AC); this in turn computes the volumetric forces and passes them to the OpenFOAM solver in order to be applied inside the hollow cylinder region. The process is repeated in each iteration of the solver until convergence is achieved. The model was compared against experimental works; wake deficits and power coefficients are used in order to assess the validity of the model. Overall, there is a good agreement of the pattern of the power coefficients according to the positions of the turbines in the array. The actual accuracy of the power coefficient depends strongly on the solidity of the turbine (actuator cylinder related) and both the inlet boundary turbulence intensity and turbulence length scale (RANS simulation related).