Underwater drag reduction by gas

Abstract

Publications 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.

One-Dimensional Analysis of Gas Diffusion-Induced Cassie to Wenzel State Transition

“…In the context of microgap cooling, structured surfaces that have high longevity at high operating pressure are desirable because they enable high channel pressure drops which result in high volumetric flow rate and, thus, low caloric resistance in comparison to planar Poiseuille flow [8]. Similarly, in underwater applications, structured surfaces that have high longevity under high hydrostatic pressure may enable vehicles to operate at high speeds with low energy consumption [9,10]. Among the methods of enhancing longevity suggested in the literature are gas injection and electrolysis of water to produce inert gas [7,10,11].…”

Drag reduction methods at solid-liquid interfaces

“…Supercavitation drag reduction refers to the creation of an air clad on the surface of a moving object, which transfers the solid-liquid interface into a solidair interface, so as to fully reduce the skin-friction drag and achieve an extremely high speed, as shown in Fig. 14 [19,20,124,125].…”

“…Consequently, solid-liquid drag reduction has been extensively studied, and a range of methods have been developed, including surface microstructures [9][10][11][12][13], superhydrophobic surfaces [14][15][16], supercavitation [17][18][19][20], microbubbles [21][22][23][24][25][26][27], polymer additives [28][29][30][31], oscillating wall [32][33][34][35][36][37][38], and compliant wall [39][40][41][42][43][44][45]. Moreover, a new technique named superlubricity, defined as the state of friction coefficient in the order of 0.001 or lower, has made the solidliquid interface drag reduction usher in a new dawn.…”

Drag reduction of stable biomimetic superhydrophobic steel surface by acid etching under an oxygen-sufficient environment

“…The air bubble layer within the grooves that resides between the no-shear solid-liquid interface generates the boundary slippage. Grooves spacing distribution in superhydrophobic surfaces can enhance slippage to the significant benefit of the increasing velocity of falling steel ball in the water compared with smooth steel ball surface [29][30][31][32]. Water velocity vector is concentrated in the middle of each groove and peak flow velocity appears near the midpoint of the groove.…”