Verification, calibration, and validation of stall delay models using NREL phase VI and MEXICO data

Abstract

An accurate estimation of rotational effects is critical during the preliminary design of wind turbines. For this purpose, different stall delay models were developed based on the centrifugal pumping mechanism. However, their generality is not yet thoroughly evaluated. In this work, we investigated the causal relationship between the radial flow, the pressure reduction, and forces augmentation. Three stall delay models, which represent different solutions of the centrifugal pumping mechanism, were verified and modified to accurately predict the radial flow, the pressure coefficient, and normal and tangential force coefficients. Then, the three modified stall delay models were calibrated using the radial flow data available from the literature. Finally, they were validated against the experimental data of the NREL phase VI and MEXICO rotors. The results showed that the centrifugal pumping produces a small chordwise pressure gradient in the separated boundary layer, which produces a small augmentation in the normal and tangential force coefficients. In contrast, the measured pressure coefficient and the normal and tangential force coefficients showed a large augmentation compared to the three modified stall delay models. Consequently, the lack of generality of current stall delay models is mainly due to the centrifugal pumping assumption. Furthermore, the verification and calibration of these stall delay models allowed us to isolate large errors in the model's output due to the model's assumptions. Thus, the importance of a rigorous verification and calibration before performing the validation.

On the influence of twist and taper of HAWT blades on the rotational augmentation phenomenon: the NREL Phase VI – Phase II comparison

“…3 (bottom) the sectional distribution of the chord to radius ratio (c/r) for the two blades shows that the single sections have quite similar c/r values. The chord to radius ratio is widely recognized as an important parameter in the rotational augmentation models [2,8,15,16,17,18,20]. Indeed, most of 3D correction models are based on the chord to radius ratio.…”

“…In this case it was decided to simulate a one third periodic domain in order to reduce the computational effort while keeping the same level of accuracy of a full computational domain. This approach was widely validated in the literature [8,9,10,11,12,16,25,26]. The computational domain along with the boundary types are shown in Fig.…”

“…They demonstrated that the BEM codes, even if corrected with the existing stall delay models, could not predict the correct distribution of the airfoil characteristics along the blade. Ouakki and Arbaoui [16] investigated three different stall delay models with further corrections to improve the prediction capabilities on NREL Phase VI and MEXICO rotors. They concluded that the lack of generality of stall delay models is mainly due to the assumptions about the physics of the centrifugal pumping.…”