Airfoils in Turbulent Inflow

Wind turbines operate in inflow turbulence whether it originates from the shear in the atmospheric boundary layer or from the wake of other wind turbines. Consequently, the airfoils of the wings experience turbulence in the inflow. The main topic of this thesis is to investigate the effect of resolved inflow turbulence on airfoil simulations in CFD.

The detached-eddy simulation technique is used because it can resolve the inflow turbulence without becoming too computationally expensive due to its limited requirements for mesh resolution in the boundary layer. It cannot resolve the turbulence that is formed in attached boundary layers, but the freestream turbulence can penetrate the boundary layer. The idea is that the resolved turbulence from the freestream should mix high momentum flow into the boundary layer and thereby increase the resistance against separation and increase the maximum lift. However, it turns out that the velocities in the inner part of the boundary layer only increase slightly, and there is no effect on the obtained surface pressures or lift coefficients. It appears that the resolved turbulence has a too large length scale to cause the effect as seen in experiments. This indicates that it might be necessary to run large-eddy simulations or direct numerical simulations to capture the effect of inflow turbulence.

A method for imposing resolved turbulence inside the domain in a CFD simulation is also presented. The idea is that by imposing the resolved turbulence immediately upstream of the airfoil the total number of computational cells can be reduced, which in turn reduces the total computational cost of the simulations. The method can be used in any simulation where inflow turbulence is included and is therefore not limited to the application investigated in this thesis. It is shown that synthetic turbulence can be imposed successfully since the obtained turbulence accurately resembles the synthetic field. The synthetic turbulence should be run through a very short precursor simulation and it is best imposed by adding a term to the source terms of the discrete Navier-Stokes equations.

Finally, two methods for generating synthetic turbulence with different characteristics are given. One is a semi-analytical solution to the crossspectral densities for turbulence in the atmospheric shear. The method is based on the same theory as the Mann method. The second method generates synthetic turbulence in arbitrary domains. The purpose is to generate a synthetic turbulence field corresponding to the field encountered by a rotating blade.