Abstract
In the present work two dimensional airfoil computations are used to investigate the effects of compressibility in the tip region of large scale wind turbines of 20 MW+ size. In the past application of incompressible CFD solvers have been wide spread for wind turbine aerodynamics, due to their efficiency and robustness at the near incompressible conditions experienced near the rotor center. With the increasing size of modern wind turbines and the desire to approach high tip speeds, the incompressible assumption might be violated in the tip region of the turbine.To investigate the effects of compressibility and the possibility of correcting incompressible flow solutions using explicit compressibility corrections, a CFD study of 2D airfoil aerodynamics at conditions of a large scale wind turbine is performed. The present study show that classical compressibility corrections can be successfully applied as a post-processing step to incompressible solutions, reducing the error in the predicted lift and drag to within a few percent for attached flow conditions where viscous effects are limited at Mach numbers upto 0.3.
Highlights
For large wind turbines with rotor radii of the order of 100 meters, the blade tip might reach high relative velocities approaching to thirty percent of the speed of sound
In the past application of incompressible CFD solvers have been wide spread for wind turbine aerodynamics, due to their efficiency and robustness at the near incompressible conditions experienced near the rotor center
The present study show that classical compressibility corrections can be successfully applied as a post-processing step to incompressible solutions, reducing the error in the predicted lift and drag to within a few percent for attached flow conditions where viscous effects are limited at Mach numbers upto 0.3
Summary
For large wind turbines with rotor radii of the order of 100 meters, the blade tip might reach high relative velocities approaching to thirty percent of the speed of sound. Most engineering codes are relying on airfoil data taken at Mach numbers below 0.2 intended for incompressible conditions, and several of the advanced CFD codes applied to wind turbines are based on the assumption of incompressibility. For Mach number substantially below 0.1 the numerical approach in many compressible CFD solvers might require small time-steps, a high number of sub-iterations or advanced pre-conditioning. The incompressible flow solvers are tailored for zero Mach number, and do not face these numerical issues, but are due to the incompressible assumption incapable of resolving the weak compressible effects observed at the tip connected to Mach numbers close to 0.3
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