Analysis of Wake Induced Dynamic Stall on Wind Turbine Blades

• Applicant: Professor Dr.-Ing. Wolfgang Schröder
• Subject Area: Fluid Mechanics
• Term: from 2012 to 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 218742980

• Final Report Year: 2019

Final Report Abstract

In this project, a synthetic turbulence generation has been developed to analyze the impact of upstream small-scale turbulence on the dynamic stall characteristics of the wind turbine blade. Different constraints were imposed to control and optimize the properties of the randomly distributed eddies such that a predefined turbulence state could be obtained. This synthetic turbulence formulation was applied to successfully generate the turbulent flow field obtained in wind tunnel facilities. The effect of freestream turbulence on the aerodynamic characteristics of the airfoil at fixed angle of attack was investigated. The simulations of the flow over the airfoil were conducted for two turbulence intensities of 0% and 5%. The numerical results showed that freestream small-scale turbulence delays stall and increases lift at high angle of attack. The effect of inflow turbulence on dynamic stall events was analyzed. The most significant impact of upstream turbulence was the increase of the lift coefficient with turbulent inflow during downstroke of the dynamic stall process compared to laminar inflow. This effect was caused by the accelerated flow attachment during downstroke. In the case of deep stall, the formation of the second dynamic vortex during downstroke was confined by the incoming turbulence and the negative damping was reduced. The simulations for a swept airfoil showed that sweep tends to delay the onset of dynamic stall. In addition, the simulations confirmed the effect that the mean lift decreases when the sweep angle increases. The aeroelastic model for the adaptive camber airfoil was implemented in the flow solver and validated by comparing simulation results for static configurations with experimental results. Large-eddy simulations for two configurations of adaptive camber blade sections were performed for dynamic stall. It was found that the adaptive camber airfoil reduces dynamic loads on the blade section during the angle of attack variation. The load fluctuation was reduced by 50% for the configuration (a) and 30% for the configuration (b). Additionally, the incoming turbulence has an impact on the aerodynamic performance of the adaptive camber airfoil. The flap deflection becomes larger for incoming turbulent flow, due to the delay of separation on the TE flap. In conclusion, the numerical analyses of this project have shown how dynamic stall of wind turbine blades is changed by freestream turbulence.

Publications

• Analysis of wind turbine blade flows based on LES, In Smart Blade Conference, Oldenburg, 2015
  X. Huang, M. Meinke, W. Schröder
  
• Large eddy simulation of the flow around a wind turbine blade, In DEWEK 2015 - 12th German Wind Energy Conference, 2015
  X. Huang, M. Meinke, W. Schröder
  
• Numerical and experimental investigation of wind turbine wakes, AIAA Paper 2015-2310, 2015
  X. Huang, S. Vey, M. Meinke, W. Schröder, G. Pechlivanoglou, C. Nayeri, and C. Paschereit
  
• Numerical analysis of the effect of flaps on the tip vortex of a wind turbine blade, Int. J. Heat and Fluid Flow, 77:336- 351, 2019
  X. Huang, S. M. Alavi Moghadam, P. S. Meysonnat, M. Meinke, W. Schröder
  
• Predicting wind turbine wake breakdown using a free vortex wake code, AIAA Paper 2019-2080, 2019
  D. Marten, C. O. Paschereit, X. Huang, M. Meinke, W. Schröder, J. Müller, K. Oberleithner