Blockage corrections for the experimental results obtained for a small-scale wind turbine in a wind tunnel are required in order to estimate how the same turbine would perform in real conditions. The tunnel blockage is defined as the ratio of the wind turbine swept area to the wind tunnel cross-section area. Experimental measurements of the power coefficient were performed on a horizontal-axis wind turbine with two rotors of diameter equal to 2 m and different numbers of blades, namely three and five. Measurements were carried out for different tip speed ratios in the closed circuit open test section wind tunnel of the University of Perugia (Italy). The obtained experimental results were compared with the numerical ones carried out in free conditions by using a CFD approach based on the steady-RANS method with the SST k-$\omega$ turbulence model, adopting the multiple reference frame (MRF) strategy to reduce the computational effort. The comparison showed that the maximum value of blockage, which is reached in the asymptotic limit at very large tip speed ratio (TSR) values, does not depend appreciably on the number of blades. A higher number of blades, however, makes the occurrence of the maximum blockage come earlier at lower TSRs.

The present study computationally investigates the steady incompressible flow behavior through Langston turbine cascade. A three dimensional numerical model was developed by ANSYS-Fluent software. The model solves the RANS governing equations along with the transition-SST (4eq.) model. The results of the model have been validated using experimental data obtained by Langston. A proposed configuration to improve the turbine performance is adopted using fence mounted close to the leading edge. The function of the fence is to reduce the pressure losses and accordingly improve the turbine performance. The results showed that the total pressure loss coefficient decreased by 5.244 % according to fence addition. The fence effectively disrupted the horseshoe vortex and modified the pressure flow field, leading to a weakened passage vortex and suppressed secondary flow. Adding a fence resulted in an enhancement of the flow uniformity and reduction of the vortex strength with improvements in the aerodynamic efficiency of the turbine cascade.