Local Correlation-based Transition Models for High-Reynolds-Number Wind Turbine Airfoils

Modern wind-turbine airfoil design requires robust performance predictions for varying thicknesses, shapes, and appropriate Reynolds numbers. The airfoils of current large offshore wind turbines operate with chord-based Reynolds numbers in the range of 3–15 million. Turbulence transition in the airfoil boundary layer is known to play an important role in the aerodynamics of these airfoils near the design operating point. While the lack of prediction of lift stall through Reynold-averaged Navier-Stokes (RANS) computational fluid dynamics (CFD) is well-known, airfoil design using CFD requires the accurate prediction of the glide ratio (L / D) in the linear portion of the lift polar. The prediction of the drag bucket and the glide ratio is greatly affected by the choice of the transition model in RANS-CFD of airfoils. We present the performance of two existing local correlation-based transition models – one-equation (γ) and two-equation model coupled with the Spalart-Allmaras (SA) RANS turbulence model – for offshore wind-turbine airfoils operating at a high Reynolds number. We compare the predictions of the two transition models with available experimental and CFD data in the literature in the Reynolds number range of 3–15 million including the AVATAR project measurements of the DU00-W-212 airfoil. Both transition models predict a larger L / D compared to fully turbulent results at all Reynolds numbers. The two models exhibit similar behavior at Reynolds numbers around 3 million. However, at higher Reynolds numbers, the one-equation model fails to predict the natural transition behavior due to early transition onset. The two-equation transition model predicts the aerodynamic coefficients for airfoils of various thickness at higher Reynolds numbers up to 15 million more accurately compared to the one-equation model. The two-equation model also predicts the correct trends with the variation of Reynolds number comparable to the eN transition model. However, a limitation of this model is observed at very high Reynolds numbers of around 12–15 million where the predictions are very sensitive to the inflow turbulent intensity. The combination of the transition model coupled with the Spalart-Allmaras (SA) RANS turbulence model is a robust method for performance prediction of modern wind-turbine airfoils using CFD.

Numerical and Experimental Investigation of the Reynolds Number and Reduced Frequency Effects on Low-Pressure Turbine Airfoils

This article compares experimental and numerical data for a low-speed high-lift low pressure turbine (LPT) cascade under unsteady flow conditions. Three Reynolds numbers representative of LPTs have been tested, namely, 5 × 104, 105, and 2 × 105; at two reduced frequencies, fr = 0.5 and 1, also representative of LPTs. The experimental data were obtained at the low-speed linear cascade wind tunnel at the Polytechnic University of Madrid using hot wire, Laser Doppler Velocimetry (LDV), and pressure tappings. The numerical solver employs a sixth-order compact scheme based on the flux reconstruction method for spatial discretization and a fourth-order Runge–Kutta method to march in time. The longest case ran 550 h on 40 GPUs to reach a statistically periodic state. Pressure coefficients around the profile, boundary layer profiles and exit cross section distributions of velocity, pressure loss defect, shear Reynolds stress, and angle are compared against high-quality experimental data. Cascade loss and exit angle have also been compared against the experimental data. Very good agreement between experimental and numerical data is seen. The results demonstrate the suitability of the present methodology to predict the aerodynamic properties of unsteady flows around LPT linear cascades accurately.

Numerical Investigation of Aerodynamic Performance of Wind Turbine Airfoils with Ice Accretion

One of the emerging problems in modern computational fluid dynamics is the simulation of flow over rough surfaces, wind turbine blades with ice on its surface for instance. An alternative method to numerically simulate rough surfaces instead of using a grid with y+

Numerical Framework for Aerodynamic Characterization of Wind Turbine Airfoils: Application to Miniature Wind Turbine WiRE-01

A numerical framework for the aerodynamic characterization of wind turbine airfoils is developed and applied to the miniature wind turbine WiRE-01. The framework is based on a coupling between wall-resolved large eddy simulation (LES) and application of the blade element momentum theory (BEM). It provides not only results for the airfoil aerodynamics but also for the wind turbine, and allows to cover a large range of turbine operating conditions with a minimized computational cost. In order to provide the accuracy and the flexibility needed, the unstructured finite volume method (FVM) and the wall-adapting local eddy viscosity (WALE) model are used within the OpenFOAM toolbox. With the purpose of representing the turbulence experienced by the blade sections of the turbine, a practical turbulent inflow is proposed and the effect of the inflow turbulence on the airfoil aerodynamic performance is studied. It is found that the consideration of the inflow turbulence has a strong effect on the airfoil aerodynamic performance. Through the application of the framework to WiRE-01 miniature wind turbine, a comprehensive characterization of the airfoil used in this turbine is provided, simplifying future studies. In the same time, the numerical results for the turbine are validated with experimental results and good consistency is found. Overall, the airfoil and turbine designs are found to be well optimized, even if the effective angle of attack of the blades should be reduced close to the hub.