Abstract
AbstractPublications on underwater drag reduction by gas have been gathered in the present study. Experimental methods, results and conclusions from the publications have been discussed and analyzed. The stable existence of gas is a requirement for underwater drag reduction induced by slippage at the water-solid interface. A superhydrophobic surface can entrap gas in surface structures at the water-solid interface. However, many experimental results have exhibited that the entrapped gas can disappear, and the drag gradually increases until the loss of drag reduction with immersion time and underwater flow. Although some other surface structures were also experimented to hold the entrapped gas, from the analysis of thermodynamics and mechanics, it is difficult to prohibit the removal of entrapped gas in underwater surface structures. Therefore, it is essential to replenish a new gas supply for continued presence of gas at the interface for continued underwater drag reduction. Active gas supplement is an effective method for underwater drag reduction, however, that needs some specific equipment and additional energy to generate gas, which limits its practical application. Cavitation or supercavitation is a method for passive gas generation, but it is only adaptive to certain vehicles with high speed. Lately, even at low speed, the evaporation induced by liquid-gas-solid interface of a transverse microgrooved surface for continued gas supply has been discovered, which should be a promising method for practical application of underwater drag reduction by gas.
Highlights
Drag reduction is essential for vehicles on water or underwater to increase voyage and voyaging speed and decrease energy consumption, thermal damage, and noise
Due to the much smaller viscosity of gas compared to water, superhydrophobic surfaces show a promising nature for passive drag reduction by entrapped gas
Superhydrophobic and some other surfaces have the capability of holding air pockets in their surface microstructures and have demonstrated an effective slippage for underwater drag reduction, it is difficult to stably sustain air pockets for a long time, especially under conditions where liquid is flowing over the surface with high speed or a certain liquid pressure
Summary
Drag reduction is essential for vehicles on water or underwater to increase voyage and voyaging speed and decrease energy consumption, thermal damage, and noise. Except modifying the turbulent structure of boundary layers, a transverse microgrooved structure has been proposed to achieve non-zero velocity at the same surface height to reduce the velocity gradient in laminar sub-layer near the solid surface for drag reduction [5]. In this method, the scale of microgrooves is less than the thickness of laminar sub-layer. Film on the solid surface can achieve much more effective drag reduction aided by significantly small viscosity of gas compared to water. Major achievements of underwater drag reduction by gas for 20 years have been gathered and analyzed, and critical points to achieve viable underwater drag reduction are proposed
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