Session: 06-05-04 Marine Hydrodynamics IV

Paper Number: 80807

80807 - Leading-Edge Tubercles Applied Onto a Flapped Rudder 

Most modern surface ships and submarines rely on movable control surfaces such as rudders, fin stabilisers, or hydroplanes for their course-keeping and manoeuvring. The control surface performance is defined by its effectiveness, the ability to produce lift, and its efficiency, reflected in the lift-to-drag ratio. Researchers therefore constantly strive to find new rudder designs with high lift and reduced drag characteristics.

One such high-lift rudder concept is the flapped rudder, an all-movable rudder with a mechanically or hydraulically actuated trailing edge flap. The trailing edge flap makes up a percentage of the total rudder area and is deflected at a flap angle relative to the rudder angle. When the rudder is turned the deflection of the flap introduces camber to the rudder section profile, which increases the lift curve slope and can yield in maximum lift coefficients (C_L,max) up to 60%-70% higher compared to a similar conventional rudder without flap (Liu and Hekkenberg, 2017). As a result, the flapped rudder can provide larger manoeuvring forces without affecting straight-line performance, allowing for a smaller rudder to be used thus reducing resistance and fuel consumption. A wind tunnel study by Kerwin et al. (1974) analysing a rudder with a 20% flap supports the advantages of flapped rudders, showing an increase in C_L,max of over 40%. However, these benefits generally come at the cost of a drag penalty as well as a reduction of stall angle to lower angles of attack.

An alternative rudder concept is the Tubercle Leading Edge (TLE) rudder, inspired by Humpback whale flippers, which targets the delay of flow stall and abrupt separation of the boundary layer from the rudder surface that can lead to sudden loss of steering. The sinusoidal serrations defining the TLE compartmentalise the flow over the rudder suction side and limit flow separation to the troughs in between adjacent tubercles whilst maintaining attachment over the crests, smoothing the onset of stall (Johari et al., 2007). Weber et al. (2010) successfully applied TLE to a conventional marine rudder model, reporting that the tubercle rudders stalled less abruptly than the smooth rudder and had superior post-stall lifting performance.

This poses the question whether the adverse stall characteristics of a flapped rudder could be minimised or mitigated through the introduction of a TLE. This study numerically analyses the effects of TLE applied to a reference flapped rudder using Detached Eddy Simulations. The NACA 66₂­A015 section rudder with 20% flap presented in Kerwin et al. (1974) was replicated, subsequently modified with two different TLE geometries. The rudder models are simulated for a fully-turbulent Reynolds number of 1.15×10⁶. The three rudder geometries are compared at selected angles of attack in the pre-stall, stall-onset, and post-stall regime with flap angle deflections of 0°, 10°, 20°, and 35°. It is expected that the TLE modification will improve the post-stall lifting performance of the flapped rudder whilst maintaining most of the lift improvements, resulting in a more versatile high-lift rudder.

Johari, H., Henoch, C.W., Custodio, D., Levshin, A., 2007. Effects of Leading-Edge Protuberances on Airfoil Performance. AIAA J. 45, 2634–2642.

Kerwin, J.E., Lewis, S.D., Oppenheim, B.W., 1974. Experiments on rudders with small flaps in free-stream and behind a propeller. Boston, Massachusetts, U.S.

Liu, J., Hekkenberg, R., 2017. Sixty years of research on ship rudders: effects of design choices on rudder performance. Ships Offshore Struct. 12, 495–512. https://doi.org/10.1080/17445302.2016.1178205

Weber, P.W., Howle, L.E., Murray, M.M., 2010. Lift, Drag, and Cavitation Onset On Rudders With Leading-edge Tubercles. Mar. Technol. 47, 27–36. https://doi.org/10.5957/mtsn.2010.47.1.27

Presenting Author: Moritz Troll University of Strathclyde

Authors:

Moritz Troll University of Strathclyde
Weichao Shi University of Strathclyde
Callum Stark University of Strathclyde