Dependence of the wake recovery downstream of a Vertical Axis Wind Turbine on its dynamic solidity

The momentum recovery downstream of a Vertical Axis Wind Turbine (VAWT) is analyzed as a function of its dynamic solidity, modified changing both the number of blades and the rotational speed of the turbine. Computations were carried out using Large-Eddy Simulations (LES), coupled with an Immersed-Boundary (IB) methodology. The streamwise increase of the momentum deficit in the near wake was found dominated by negative cross-stream advection, due to wake expansion, and especially negative pressure transport. Both of them were found larger at higher values of dynamic solidity. Downstream of the near wake region, the instability of the shear layers at both edges of the wake produced a substantial change of the wake topology and balance across momentum transport terms. The collapse of the shear layers into the wake core promoted wake recovery, via both positive cross-stream advection and turbulent transport. The destabilization of the wake system occurred earlier at larger values of dynamic solidity, producing this way a faster wake recovery, in spite of larger values of momentum deficit within the near wake. The switch to the wake of a bluff body was characterized by an increase of the energy at frequencies lower than that of the blades passage.