Rotating Discs Actuators

Abstract

In the aviation, maritime and oil&gas sector, the potential benefit in yearly savings deriving from the reduction of turbulent skin-friction drag through active flow control are estimated in the millions of U.S. Dollars and million tonnes of CO2 emissions. Research in this topic has brought forward numerous techniques, offering enticing drag reduction performance. However, no technique has yet proven to be suitable for large scale application in the aviation sector. A small research stream has been initiated in the field of rotating disc actuators for skin-friction drag reduction. Currently, research on this topic is limited to low Reynolds number numerical simulations, that are not representative of flight conditions.This thesis project aims at investigating the disc actuator performance in terms of drag reduction and power balance at friction Reynolds numbers higher than current literature on disc actuators. This has been achieved through direct numerical simulations, using an incompressible channel flow solver adapted for the implementation of the disc actuators. In total two disc configurations have been tested at friction Reynolds number Reτ=180, 550 and Reτ =1000. The disc diameter and tip velocity scaled in viscous units are kept constant across the Reτ range. The simulation results confirm the positive drag reduction performance observed in previous literature at Reτ =180. With increasing Reτ, a decrease in drag reduction performance is observed, with skin-friction drag reduction going from 21.6% at Reτ =180 to 16.01% Reτ =1000. The net power saving of the disc actuators show small variation with increasing Reτ, with an optimal net power saving at Reτ =1000 at -3.2%. Visualizations of local velocity profiles and local time averaged velocity have highlighted the disc influence to be limited in the region below y+< 400. Thus, the performance of disc actuators at friction Reynolds number higher than Reτ =1000 should be less influenced by the Reynolds number. Comparison with experimental testing of disc actuators from a parallel thesis study have shown good qualitative agreement between experimental and computational results. Lastly, the consistency in the drag reduction, net power balance result and disc velocity distribution within the boundary layer seem to reinforce the hypothesis that the disc performance scales with the disc diameter and tip velocity expressed in wall units.The outlook of this research shows that the performance of disc actuators at friction Reynolds numbers more representative for flight applications puts disc actuators on par with other passive skin-friction drag reduction techniques such as riblets in terms of net power saving. However, the reinforcement of the hypothesis that disc performance scales in wall units may result in too small actuators for flight conditions, resulting in more difficult full scale implementation.