Numerical optimization and experimental investigations on the principle of the vortex diffuser

The present work deals with numerical optimization of and experimental studies on aft mounted wing tip devices, so called vortex diffusers. The purpose of most kinds of wing tip devices is to reduce the induced drag by converting vortex energy into thrust. In order to obtain an optimal design for the vortex diffuser, the parameterized diffuser blade shape is optimized with respect to axial thrust within the framework of lifting line theory. Finite airfoil performance is accounted for by applying lift and drag coefficients from wind tunnel data. The so gained optimal design for the vortex diffuser blades is realized on a small scale model, and wind tunnel tests are carried out to investigate drag reduction of the vortex diffuser for comparison with numerical results. Furthermore, the goal is to gain some experience and data for preparation of real flight testing on a UAV. Experiments clearly show that the vortex diffuser is able to reduce drag in high lift / high angle of attack configurations. Wind tunnel tests, however, also demonstrate some problems for practical design of the vortex diffuser.