Engineering of surfaces for drag reduction in water with validation using computational and experimental methods

The concept of using air coated hulls to reduce drag has previously been suggested in the maritime literature. Reducing the skin friction component of drag by injecting microbubbles was first reported in 1973 by the US Naval Academy using a cylinder coated with small bubbles of hydrogen generated by electrolysis to study reduction in friction. More recently, the US Defense Advanced Research Projects Agency (DARPA) funded a programme to research reduction in friction drag focusing on developing numerical models and computer simulations for air/bubble injection and supported by scale model experiments. In Japan, the National Maritime Research Institute (NMRI) and the Shipbuilding Research Association has carried out microbubble experiment using ships and scale models in addition to plate experiments in test tanks. It has been reported that both an effect due to the reduced viscosity of air and the shearing of bubbles in the boundary layer occur. Skin friction reductions of up to 5% were reported for ships and up to 10% drag reduction for flat plates. In these experiments, and the US ones, the microbubbles were active injections and had a power penalty; they were only effective near the point of injection because they did not remain within the boundary layer close to the hull. In the NMRI full-scale tests they also degraded the efficiency of propellers. Another approach pioneered in Russia has been to pump air behind wedge and stepped shaped features to create an air-film along the body of the object (e.g. torpedo) or via supercavitation to create the same effect. Researchers in the Netherlands have also reported a net 10% reduction in drag for a barge using active air lubrication and DARPA has funded an air cavity drag reduction (AirCat) programme with a target of reducing by 80% the hull wetted area. It is clear that air films retained at a submerged solid surface should be able to reduce drag, but current approaches require an active input of energy to do so. The materials approach in our proposal seeks to provide the equivalent of a bubble layer or an air film in a manner that does not require active power input and which has a strong chance of being retained at the surface where it is needed for maximum effect. In this work, we provide comprehensive research including materials developments, computational fluid dynamics modelling of the surface region, and drag and tow tank testing. Our approach is based on the creation of super-water repellent surfaces using a combination of small-scale (micro- and/or nano-scale) topography and surface chemistry that also have the ability to retain a film of air when submerged in water. This involves collaboration between three research groups (Materials/Physical Sciences, Aerodynamics & Flight Mechanics and the Wolfson Marine Unit).