Air-Ventilated Hulls

The conservation of energy and reduction of environmental impacts are the major goals of any modern engineering developments. Most of the international trade is accomplished using marine vessels for cargo transport. The world fleet of ships is one of the major consumers of oil-based fuels and contributors to pollutant emission. It is estimated that marine vessels burn 280 million tons of fuels annually and that shipping emissions are responsible for about 60,000 premature deaths per year, with projected 40% increase in the annual mortalities due to this cause by 2012. Other negative consequences of marine shipping include wake wash eroding coastal lines and underwater noise radiation harmful for marine wildlife.

Reducing fuel consumption and environmental impact of marine vessels is very important for sustainability of our civilization. The progress in fluid dynamics can make a significant contribution to achieving this objective. A reduction in hydrodynamic resistance of ships will lead to lower fuel consumption and pollutant emission. Little practical progress has been made toward this goal in recent decades. Traditional ship hulls are already optimized to the point where small hull form modifications do not produce a significant benefit. Advanced marine vehicles with radically different hulls (such as hydrofoils, air cushion vehicles, small waterplane area ships, and multi-hulls) have found niche applications, but their limited payload capabilities and high cost have prevented their broader use in marine transportation. Thus, new ideas are in great demand for i ncreasing sustainability of marine transportation. In our opinion, the best drag-reducing solution is to employ air-ventilated cavities formed on ship hulls.

The main idea is to supply the air into a special recess on the hull surface to generate thin, large-area air cavities that separate a significant fraction of the wetted hull surface from the contact with water at a minimal air supply rate. This results in a substantial frictional drag reduction. The wave drag can also be reduced due to smaller pressure gradients in the flow around the ship hull. The demonstrated drag reduction on air cavity ships (ACS) is about 20% in comparison with similar-sized traditional ships (Matveev 2005).Examples of possible implementations of air-ventilated cavities on displacement and planing hulls are illustrated below. A multi-wave air cavity can be accommodated inside a bottom recess on a displacement hull (a), and a single-wave cavity can be arranged on a bottom of a fast planing hull (b). More complicated air-cavity systems can be applied on semi-displacement ships and small waterplane area hulls. 1 shows an air blower/compressor, 2 is the air cavity.



Dr. Matveev has been intensely involved in studies on air-ventilated flows and various types of advanced marine vehicles. He conducted experimental and modeling studies on the air-cavity phenomena, including a development of demonstrators of drag-reducing systems employing artificial cavitation. An example of experimental results showing drag reduction obtained on a tanker model is shown below. This project was carried out by DK Group (Matveev et al. 2006). 18% drag reduction is deemed to be achievable with this system on the full-scale vessel at the cruising speed of 14.5 knots in calm-water conditions. Alternatively, the speed increase of about 0.8 knot can be gained if the propulsive power is kept the same.



An insight into air-cavity flows can be given through a simple flow scheme shown below. There is a two-dimensional water flow under a horizontal wall with a rear-face step. The liquid weight is important in this problem. An air cavity is produced by injecting air at the rear side of the step. Different flow regimes are possible. A short cavity of shape 1 with a pronounced re-entrant jet at the tail and significant air leakage is similar to natural cavitation . The optimal cavity of shape 2 smoothly re-attaches to the wall with the minimal air leakage (down to zero in theory). This (so-called limiting) regime is preferable for air-lubricated ships. Longer cavities can also be achieved, but at the expense significantly larger air supply rates and loss of stability in the cavity shape and length. These cavities exhibit violent oscillations similar to over-ventilated flows without the wall restriction. A linearized steady potential-flow theory in this case produces an unrealistic solution with the cavity boundary piercing the horizontal wall (shape 3).