Underwater drag reduction by gas

Friction 2(4): 295–309 (2014) DOI 10.1007/s40544-014-0070-2 ISSN 2223-7690 CN 10-1237/TH REVIEW ARTICLE Underwater drag reduction by gas Jiadao WANG*, Bao WANG, Darong CHEN State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, China Received: 26 October 2014 / Revised: 28 November 2014 / Accepted: 02 December 2014 © The author(s) 2014. This article is published with open access at Springerlink.com 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. Keywords: drag reduction; entrapped gas; skin friction; underwater 1 Introduction Drag reduction is essential for vehicles on water or underwater to increase voyage and voyaging speed and decrease energy consumption, thermal damage, and noise. In general, a vehicle drag is composed of a pressure or form drag, wave-making resistance, and skin drag. The previous two mainly depend on body’s shape, and the latter one on the fluid–solid interface. The pressure drag and the wave making resistance mainly accounts for the total drag of blunt-nosed bodies and high-speed surface ships respectively. However, for streamlined bodies, skin drag represents the largest percentage, even over 60% or 80% in air or underwater and 100% for pipe transportation [1, 2]. Therefore, skin drag is the key for the drag reduction * Corresponding author: Jiadao WANG. E-mail: of a streamlined body. Studies on skin drag reduction have attracted attentions due to their practical value in engineering applications [3, 4]. The skin drag is caused by viscous drag in a boundary layer of fluid around a body. The boundary layer is in the immediate vicinity of a bounding surface, and can be divided into three types of sub-layers, i.e., laminar, buffer, and turbulent, from the wall into the flow [5]. In the turbulent sub-layer, unsteady vortices appear on many scales and interact with each other, causing skin drag increase. Up to now, many theoretical and experimental investigations have been conducted to modify the turbulent structure for drag reduction [6, 7], such as microstructured surface [8−12], polymeric additives [13−16], and traveling wave [17]. Velocity gradient reduction is a reason for the decrease in skin friction drag [18−27]. Therefore, the turbulent sub-layer is normally selected to enlarge Friction 2(4): 295–309 (2014) 296 the thickness of boundary layer to decline the velocity gradient. Based on the investigation of shark scales, the longitudinal grooves (riblets) can be considered as a typical method to reduce the velocity gradient by affecting turbulent sub-layer structure. The height of riblets is several hundreds of micrometers, which is sufficient to affect the structure of turbulent layer [8, 18]. In addition, traveling waves can be used to modify the turbulent sub-layer structure for drag reduction, whose wave structures are vertical with flow direction and larger than riblets [8, 18]. However, when the structure in the turbulent sub-layer is modified, additional energy is essential to enhance the thickness of boundary layer due to energy dissipation in turbulent flow. Therefore, the drag reduction by modifying the structure of the turbulent sub-layer is limited. Various riblets and wave structures have been investigated, and a drag reduction rate of approximately 10% has been achieved [18−27]. 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]. When a liquid flows over the transverse microgrooved surface, vortexes can be formed in microgrooves, and on the upside of the vortex, the revolving direction is consistent with the main flow, which induces the flow shear rate reduction, as shown in Fig. 1. In this method, the scale of microgrooves is less than the thickness of laminar sub-layer. A drag reduction rate of 10% or more was achieved by a transverse microgrooved surface [5]. The above methods for drag reduction, by influencing boundary layer using riblets or other surface structures, can be applied in air or water [23, 28−31]. However, for an underwater vehicle, a gas lubricating Fig. 1 Velocity profile on transverse microgrooved surface in flowing water [5]. film on the solid surface can achieve much more effective drag reduction aided by significantly small viscosity of gas compared to water. Recently, several approaches, such as entrapped gas within superhydrophobic surfaces [32−34], gas injection [35], gas generation by electric field or heating [36], and gas generation induced by three-phase interface [37], have been suggested to achieve a gas layer on a surface. In this study, 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. 2 Entrapped gas underwater When free gas bubbles are formed in water, the pressure in bubbles is higher than that in liquid because of the Laplace pressure of the water–gas interface, resulting in the diffusion and disappearance of gas in the bubbles under the gas solubility limit [38]. Even when the gas solubility limit exceeds, since the Laplace pressure is higher around the smaller free bubbles, gas will diffuse from smaller to larger bubbles through Ostwald Ripening. Large free bubbles will eventually separate from water because (...truncated)