WP4

This WP deals with the post-processing and reporting of the data obtained by the Wind Tunnel test campaign. The post-processing of the tests results included the evaluation of the wind tunnel effects.

The PIV measurements at 65% of the retreating blade have been post-processed with the new developed technique in order to obtain the pressure distribution in the field around the blade section. The blade section aerodynamic coefficients were calculated from PIV data using the control volume approach. The considered condition is characterized by not negligible compressibility effects (local Mach number equal to 0.35). Furthermore, the flow on the integration contour is outside the boundary layer (except for a very thin segment in the wake) and can be therefore considered irrotational. Thus, due to these considerations the Bernoulli equation for compressible flow was used for pressure calculation on the integration contour. Viscous stresses along the contour of the control surface were neglected as their contribution to the calculation of the aerodynamic loads were negligible, while turbulent stresses obtained from the measured velocity fluctuations were considered in the calculation. The velocity field around the blade section was reconstructed from the measurements carried out on the upper and lower surface of the airfoil.
The validation of the method was carried out by comparison of the vertical aerodynamic load calculated for the clean blade section configuration with the results of the blade element momentum theory in hovering. In particular, a classical blade element momentum theory (BEMT) approach was used to calculate the distribution of the aerodynamic loads along the clean blade span for the selected hovering test condition. With this aim, the performance of the clean blades was computed considering the measured rotor thrust as trim requirement. 
The discrepancy between the vertical force coefficient computed from PIV data using the control volume approach and the BEMT analysis results is in the order of few percents. The good level of accuracy obtained for the calculation of the vertical aerodynamic force validates the applicability of the method for the calculation of the blade section performance from PIV data. The method was therefore applied to the PIV measurements carried out around the blade section equipped with the Gurney flaps to evaluate the effects of the flaps with different height by comparison with the clean condition for the same commanded collective angle. An apparent increase of the vertical force coefficient was computed for the blade section configuration equipped with the different Gurney flaps with respect to the clean geometry for this hovering test condition. In particular, as it can be expected, the computed vertical force coefficient is higher increasing the Gurney flaps height with a maximum increase of 37% of the clean blade section value obtained for the flap with height of 2.5 mm. The good level of confidence of this computed trend was confirmed by the comparison of the rotor thrust coefficient measured in hovering for the different blade configurations.