A Numerical Study on Compressible Flows over Thick Airfoils for Tilted Wind Turbines

Abstract

While the assumption of incompressible flow has been the prevailing standard for numerical simulations of wind turbine aerodynamics, the limit of this assumption is approached as the industry progresses towards increasingly higher operational tip speeds. This thesis explores the implications of compressibility and transonic flow phenomena, particularly in relation to a new rotor design that integrates the concept of rotor tilt.

The study is split into two phases. The first phase involves two-dimensional Computational Fluid Dynamics (CFD) simulations, employing an Unsteady Reynolds Averaged Navier-Stokes (URANS) framework, specifically for the thick NACA 63(4)-421 airfoil. Attention is directed at the effects of high subsonic and transonic Mach numbers, and the complex flow phenomena emerging from shock wave-boundary layer interactions (SBLI). The second phase extends the findings from the first phase by simulating a 3MW concept rotor, utilizing a lifting line method. Here, particular emphasis lies on runaway conditions; the operating point of an unloaded rotor where rotational velocity maximizes. Since airfoil drag significantly increases through the onset of “shock-stall”, it might provide a passive mechanism for overspeed protection.

The URANS study provides the first exploration of transonic buffet for a thick non-symmetrical airfoil; the phenomenon characterized by periodic shock movement, leading to large-scale load oscillations. The findings indicate that these fluctuations are significant and could pose a threat to the structural integrity of a full-scale rotor. Additionally, regarding the subcritical compressible regime, the study found that the von Karman-Tsien correction is the most appropriate method for the prediction of airfoil loads prior to reaching the critical boundary.

In the second phase, the rotor is simulated using the averaged values of the compressible aerodynamic coefficients, obtained in the first phase. Two flow scenarios are considered: Storm conditions typical for the North Sea, and subsequently, higher wind speeds often encountered during tropical typhoons in the Western Pacific. The results show that for the first scenario, shock-stall does not prevent the rotor from overspeeding. Instead, using rotor tilt is suggested as a more effective strategy. For the second scenario, a distinctive maximum rotational velocity has been found for an incoming wind speed of $57.5$ m/s, leading to the conclusion that shock-stall could effectively prevent the rotor from overspeeding.

Since the averaged values of the aerodynamic coefficients are used to assess the adequateness of shock-stall as an overspeed protection mechanism, the load fluctuations attributed to transonic buffet are initially neglected. A subsequent preliminary assessment of these fluctuations revealed that operating in the high transonic regime potentially poses severe risks regarding rotor safety.