The stream-function formulation of the potential flow solver has been modified to include new boundary conditions of an airfoil with arbitrary rotational velocity and center of rotation. As now, the reference frame is rotating, a change in the computation of pressure coefficient has to be made. The method of computing the surface velocities of the airfoil is also modified, as the original code was incompatible with the new modifications. A result is shown in Figure 2, where the pressure distribution of a NACA0015 airfoil computed by XFOIL is compared to the U2DIVA benchmark, for a chord-to-radius ratio of 0.2 and a mounting location at half the chord. The inviscid solution showed that no cumulative differences between the computed pressure distribution and the benchmark larger than 10% have been found (i.e. be- tween the lift coefficients). It is assumed that with the modified potential flow solver, the viscous calculations can be left untouched and should perform as before. As there are no viscous benchmark solutions for rotating airfoils, this cannot be verified.

A final investigation into airfoil optimization and turbine performance was performed with the newly modified version of XFOIL. Using optimized software based on a genetic algorithm, numerous different airfoils were generated and analyzed for their aerodynamic and structural properties. The optimiser scores each airfoil on two objectives, one to optimize the power output of the turbine and the other to maximize the area moment of inertia of the airfoil so as to obtain a blade which is as stiff as possible (Simão Ferreira, 2015). The result is a whole range of optimised airfoils, varying from very aerodynamic and not so stiff, to structurally optimal and aerodynamically infeasible. Only the former are used in further investigations.