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Moving surface boundary-layer control with application to autonomous underwater vehicles Den Hertog, Vincent R.

Abstract

Moving Surface Boundary-layer Control (MSBC) is a technique to delay separation and control circulation over bodies in fluid flow through momentum inj ection. In the past, rotating cylinders have been successfully used as the moving surface elements on both bluff and streamlined bodies. On tractor-trailer trucks, for example, rotating cylinders have been shown to lower the drag by reducing the size of the separated wake. For aerofoil profiles, momentum injection has lead to dramatic improvements in lift and drag. With this as background, the present study explores fluid dynamics of two different configurations in the presence of MSBC and assesses their potential in improving the performance of the control surfaces (hydroplanes) used on Autonomous Underwater Vehicles (AUVs). To that end, a carefully planned experimental program is conducted using two-dimensional models in a closedcircuit wind tunnel. Surface pressure distribution results are used to characterize circulation and separation effects and explain trends exhibited by the force, moment and centre of pressure data. In the first configuration, a wedge-shaped profile is integrated with a rotating cylinder at the leading edge. The momentum injection moves the boundary-layer separation point downstream resulting in high levels of lift with a profile that is simpler and less expensive to construct than a regular aerofoil. The lift, drag and moment characteristics are measured for a family of wedge profiles with three different thickness-to-chord ratios. The results are compared with those for a symmetrical aerofoil with the MSBC. Results suggest that, among the configurations tested, the wedge-aerofoil with a thickness ratio of 16% is particularly suited to an application where a high level of constant lift is required in one direction, e.g. a depressor wing on a towed vehicle. The second configuration considers a symmetrical aerofoil equipped with a rotating cylinder at its trailing edge. Such a system is capable of generating a significant amount of lift, even at zero angle of attack through a circulation control mechanism related to the Magnus Effect. The lift is controlled by the speed and direction of rotation of the cylinder. This is shown to be an attractive replacement for the bi-directionally deflecting hydrofoils found on many AUVs. The study also shows that the power required for the momentum injection is rather modest, and cavitation is not a problem in its application to most AUVs.

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