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

使用空气涂层船体来减少阻力的概念以前曾在海洋文献中提出过。1973年，美国海军学院首次报道了通过注入微气泡来减少阻力的表面摩擦分量，该报告使用一个涂有电解产生的氢气小气泡的圆柱体来研究减摩。最近，美国国防高级研究计划局(DARPA)资助了一项研究减少摩擦阻力的计划，重点是开发空气/气泡喷射的数值模型和计算机模拟，并以比例模型实验为支持。在日本，国家海事研究所(NMRI)和造船研究协会除了在测试水箱中进行平板实验外，还利用船舶和比例尺模型进行了微气泡实验。据报道，由于空气粘度的降低和边界层中气泡的剪切，两者都会产生影响。据报道，船舶的表面摩擦减少了高达5%，平板的阻力减少了高达10%。在这些实验和美国的实验中，微泡是活跃的注入，具有功率损失；它们只在喷射点附近有效，因为它们不留在靠近船体的边界层内。在NMRI的全尺寸测试中，它们还降低了螺旋桨的效率。俄罗斯首创的另一种方法是在楔形和阶梯形特征后面泵入空气，沿着物体(例如鱼雷)的身体形成气膜，或者通过超空泡产生同样的效果。荷兰的研究人员还报告称，使用主动空气润滑的驳船阻力净减少10%，DARPA资助了一项气腔减阻(AIRCAT)计划，目标是将船体湿面积减少80%。显然，保留在水下固体表面的空气膜应该能够减少阻力，但目前的方法需要主动输入能量才能做到这一点。我们建议中的材料方法寻求以一种不需要有源功率输入的方式提供相当于气泡层或气膜的材料，并且很有可能将其保留在需要达到最大效果的表面。在这项工作中，我们提供了全面的研究，包括材料开发，表面区域的计算流体力学模拟，以及拖曳和拖曳水箱试验。我们的方法是基于使用小规模(微米和/或纳米级)地形和表面化学的组合来创建超防水表面，这些表面化学在浸入水中时也具有保留一层空气的能力。这涉及三个研究小组(材料/物理科学、空气动力学和飞行力学以及沃尔夫森海军陆战队)之间的合作。

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).