Investigation of the wake energy recovery of cross-flow turbines in paired configuration by means of 3d-CFD and analysis of the streamwise momentum budget

Dipartimento di Ingegneria dell’Energia, dei Sistemi, del Territorio e delle Costruzioni (DESTEC), Università di Pisa, largo L. Lazzarino 1, 56122 Pisa, Italy

^* Corresponding author: stefania.zanforlin@unipi.it

Abstract

Literature suggests that cross-flow turbines (CFTs) could be suitable for off-shore wind farms because of the high power-density achievable by shortening the distance between arrays as allowed by a fast wake energy recovery experimentally detectable. By means of 3d-CFD, we analysed the effect of the rotation verse of CFTs in counter-rotating paired configuration on the wake behaviour. Then, we applied the momentum-budget approach to identify the fluid dynamic mechanisms which are more effective in supporting the streamwise momentum recovery. The following results are found. (A) The counter-rotating vortices occurring in the near-wake as a consequence of the vortex shedding at the blade tip are responsible for the vertical advection that enters high momentum flow inside the wake. (B) The turbulent transport contribution is less important, yet it becomes significant starting from the medium wake. (C) For the inner-downwind layout the wake shape appears similar to that of a single turbine, whereas for the inner-upwind layout it is greatly contracted in horizontal direction and enlarged in vertical direction (D). The momentum recovery appears slightly more delayed than in case of a single turbine, yet the velocity deficit appears less extensive for the inner upwind layout that, especially thanks to the wake lateral contraction, could be the preferable in farms consisting in staggered arrays, allowing to shorten the lateral distance between adjacent pairs.