This paper presents a computational investigation on the effects of Gurney flaps on the aerodynamic performance of a horizontal axis wind turbine, which is part of the EU FP7 AVATAR project. The research investigates two configurations of Gurney flaps applied at the inboard part of the blade (r/R=0.30∼0.46) at 85% chord location on the pressure surface. The computational method applied in the investigation solves the Reynold-Averaged Navier-Stokes (RANS) equations with multiple reference frame (MRF) approach, which models the rotating turbulent flow over the wind turbine rotor. Numerical simulations are performed for the wind turbine rotor with and without Gurney flap at the tip speed ratios λ=4.59 and 6.35. Comparison of the numerical results with experimental measurements shows that the deployment of Gurney flaps effectively increases the power coefficients of the rotor by 21% at λ=6.35. Gurney flaps have a considerable 3D effect on spanwise thrust and torque coefficients distribution. The performance of two Gurney flaps configurations is compared. It is shown that the larger Gurney flap reduces the effect on the power generated due to protruding out of the local boundary layer of the flow. The numerical results are in good agreement with the experimental results in terms of total thrust and power within 14.1% difference, and complement the experimental database.

The research presented in this paper focuses on the effects of structural failures on the safe flight envelope of an aircraft, which is the set of all the states in which safe maneuver of the aircraft can be assured. Nonlinear reachability analysis basedonan optimal control formulation is performed to estimate the safe flight envelope using actual aircraft control surface inputs. This approach uses the physical model of an aircraft, where the aerodynamic stability and control derivatives are calculated using Digital Datcom. Symmetrical damages to a Cessna Citation II are considered with 25, 50, 75, and 100% spanwise vertical tail tip losses, leading to gradual shrinkage in the safe flight envelope. Based on the estimated safe flight envelopes, a discussion on the effects of structural damages and different flight conditions on the safe flight envelope is presented. In particular, the interpolatibility of the resulting safe flight envelopes is demonstrated. This property is essential for a novel database-driven flight envelope prediction method, where a database of safe flight envelopes is created offline to be accessed later in real time.

This paper reports the latest progress in the development of a database-driven safe flight envelope prediction system. By building up a database containing safe flight envelopes of different damage and fault scenarios, the challenges associated with online flight envelope prediction can be circumvented. The database is designed for different flight conditions at which the flight envelopes are computed. Both longitudinal and lateral envelopes are computed via the level set method, which shows obvious shrinkage between damaged and undamaged aircraft. It is found that by interpolating between two retrieved envelopes in the database, more accurate results can be achieved.

This paper aims to provide an explanation for the overprediction of aerodynamic loads by CFD compared to experiments for the MEXICO wind turbine rotor and improve the CFD prediction by considering laminar-turbulent transition modeling. Large deviations between CFD results and experimental measurements are observed in terms of sectional normal and tangential forces at the blade tip (r/R=0.82 and 0.92) of the MEXICO rotor operating in axial condition at the design tip speed ratio λ=6.7. The first part of this study identifies the effects of ZigZag tape, which is used in the experiment to trigger boundary layer transition, by analyzing the available experimental data of a single, non-rotating MEXICO rotor blade. The analysis indicates that ZigZag tape has a significant impact on sectional aerodynamic tip loads: it alters the boundary layer thickness and additionally reduces the effective airfoil camber besides the expected tripping. These additional effects most likely also occur in the rotating MEXICO experiment, reducing the sectional loads and hence lead to an overprediction by CFD. To eliminate the ZigZag tape interference, experimental data with an untripped blade is preferred to be used as validation case. In the second part of this study, a transitional flow simulation for the MEXICO rotor is performed by using RANS-based transition model k−k_L−ω within OpenFOAM-2.1.1. The numerical results are compared against experimental data obtained from the untripped, new MEXICO experiments. The comparison gives that transitional simulation present a very good tip loads prediction for the untripped blade. The measured data also confirms that the ZigZag tape indeed has a significant influence on the blade tip loads in rotating conditions. The transition onset over 3D MEXICO blade is visualized and transition locations are identified. The results shown in the present study can explain the causes of the large differences between CFD and experiment observed in the MEXICO blind comparisons.

This paper presents a numerical investigation of transitional flow on the wind turbine airfoil DU91-W2-250 with chord-based Reynolds number Re_c = 1.0 × 10⁶. The Reynolds-averaged Navier–Stokes based transition model using laminar kinetic energy concept, namely the k − k_L − ω model, is employed to resolve the boundary layer transition. Some ambiguities for this model are discussed and it is further implemented into OpenFOAM-2.1.1. The k − k_L − ω model is first validated through the chosen wind turbine airfoil at the angle of attack (AoA) of 6.24° against wind tunnel measurement, where lift and drag coefficients, surface pressure distribution and transition location are compared. In order to reveal the transitional flow on the airfoil, the mean boundary layer profiles in three zones, namely the laminar, transitional and fully turbulent regimes, are investigated. Observation of flow at the transition location identifies the laminar separation bubble. The AoA effect on boundary layer transition over wind turbine airfoil is also studied. Increasing the AoA from −3° to 10°, the laminar separation bubble moves upstream and reduces in size, which is in close agreement with wind tunnel measurement.