|Yanuar||Department of Mechanical Engineering, Faculty of Engineering, Universitas Indonesia, Kampus UI Depok, Depok 16424, Indonesia|
|Made S.G. Putra||Department of Mechanical Engineering, Faculty of Engineering, Universitas Indonesia, Kampus UI Depok, Depok 16424, Indonesia|
|M. Akbar||Department of Mechanical Engineering, Faculty of Engineering, Universitas Indonesia, Kampus UI Depok, Depok 16424, Indonesia|
A new device for air lubrication called the Winged Air Induction Pipe (WAIP) is studied in the present work. The device, which consists of an angled hydrofoil, uses the low-pressure region produced above the hydrofoil as a ship moves forward. The low pressure drives the atmospheric air into the water at certain velocities when the pressure is negative compared to atmospheric pressure. A computational fluid dynamics approach is presented in the study of the effect of the hydrofoil clearance of the Winged Air Induction Pipe on the drag reduction experienced by the plate to which the WAIP is attached. The well-known 'volume of fluid' model and k-w SST (shear stress transport) turbulence closure model were used in the 2D numerical simulation in ANSYS Fluent. The numerical simulation was conducted with different hydrofoil clearance and angle of attack configurations, and the effects of these parameters on total drag force and drag reduction are reported. The reduction of drag force is found to increase by about 10% compared to the bare plate configuration.
Air lubrication; Computational fluid dynamics; Drag reduction; Hydrofoil; Multiphase flow
Methods of drag reduction using air lubrication are becoming a promising study area due to the increase in fuel efficiency produced as the result of reduced drag. The principle of the air lubrication method is to reduce the Reynolds shear stress in a turbulent boundary layer of the flow (Yanuar et al., 2012). The magnitude of such stress can be moderately changed by the dispersed phase for the dilute two-phase flow, but the distribution pattern remains unchanged (Muste et al., 2009). Kodama et al. (2000) obtained promising results using air lubrication in the form of microbubbles for drag reduction. It is well known that the presence of air in the turbulent boundary layer of the flow leads to drag reduction for two reasons: first, by lowering the average viscosity and density of the mixture flow, with the mixture of gas and liquid having a lower density and viscosity compared to the liquid itself; and second, by decreasing the magnitude of the Reynolds shear stress through the interaction of the air and liquid.
A numerical study has been made as an alternative to the experimental approach, as it requires less time, while still producing accurate results by first conducting validation of similar experimental results. Various numerical studies have been performed to calculate drag reduction using various air lubrication methods. Mohanarangam et al. (2009) studied the phenomenon of drag reduction by the injection of microbubbles into the turbulent boundary layer using an Eulerian–Eulerian two-fluid model. Pang et al. (2014) investigated microbubble drag reduction using the Euler–Lagrange two-way coupling method in order to understand the drag reduction mechanism from such bubbles. Shereena et al. (2014) conducted a numerical simulation using k-w SST to calculate the drag reduction produced by an air jets on axisymmetric-underwater vehicles. Air lubrication requires an injection to disperse air into the water; this requires energy due to the relatively high pressure in the water, particularly at certain depths in a ship’s bottom hull. Pressure from an air compressor is required in order to inject air into the water. However, the amount of energy required is large enough to cancel out the energy saved by the air lubrication. The injection of air into water at certain depths requires various sources of energy: first the adiabatic compression, the air generation in the water and mechanical losses at the air compressor (Kumagai et al., 2015). As a result, net power savings fall to as little as 5%.
Kumagai et al. (2015) developed a new device called the Winged Air Induction Pipe (WAIP). This consists of an air pipe and angled hydrofoil with a lower pressure at the upper surface due to the higher magnitudes of velocity. Previously, numerous studies on the effect of the hydrofoil on the air-water interface have been made. Duncan (1981) conducted an experiment on the breaking waves produced by a towed hydrofoil at constant depth and velocity, finding that the drag associated with the breaking was proportional to the downslope component of the weight of the breaking region. The wake survey measurement also showed that the drag associated with breaking reached more than three times the maximum drag that could theoretically be obtained with non-breaking waves. Kumagai et al. (2011) found that the hydrofoil also produced negative pressure that pulled air into the water as the hydrofoil was positioned near the water surface.
In this work, the WAIP from the previous work of Kumagai et al. (2015) is studied. The device produces natural air injection without using an air compressor at critical velocity Uc, which is defined as:
where g is gravity acceleration, H is the depth of the injection, a is the mean void fraction, CP is the pressure coefficient, and L, hb, CD and q are the hydrofoil chord length, the air-water mixture layer thickness, coefficient drag and the hydrofoil angle of attack, respectively. However, Shereena et al. (2014) found that in some cases hydrofoils develop problems regarding the clearance to the bottom plate where the is WAIP located.
Following the previous research of Kumagai et al. (2015), optimalization of the device is yet to be made. Therefore, it should be noted that the numerical simulation conducted in this work aims to analyze the influence of the hydrofoil clearance in the WAIP on the amount of drag reduction produced and the relationship between the angle of attack and clearance of the hydrofoil in the device.
The paper has numerically investigated and optimized a WAIP device for possible use on the hull of the ship, with the clearance or distance from the hydrofoil’s upper surface to the ship’s hull the main study parameter. Clearance plays an important role in producing the appropriate amount of drag reduction. However, in some configurations the flow characteristic of the device produced more drag due to the depth variation of the hydrofoil.
The Computational Fluid Dynamics approach to estimate the drag reduction by air lubrication using the Winged Air Induction Pipe (WAIP) was taken and reasonably validated by the experimental work. By using nine configurations to achieve the effect of hydrofoil clearance on drag reduction, it is concluded that the desired magnitude of reduction can be achieved when the contributing parameters, namely the angle of attack and hydrofoil clearance, are optimally chosen. The numerical results were validated with published experimental results. Good agreement between these proves the accuracy of the numerical method employed in the calculation of the air-water interface and the results obtained.
The numerical results reveal that the optimum range is achieved by modification of the parameters using trial and error. The unique flow characteristic produced by the hydrofoil interacts with the Part C in different ways, depending on the clearance between the hydrofoil and the bottom plate of the model. The application of WAIP gives a level of net drag reduction of up to 10%. In future work, 3D simulation is recommended to further study the effect of the size of the induction pipe at different positions and to explore the air-water interface phenomenon in its correlation to the drag reduction produced by the device.
The authors would like to thank Kementerian Riset, Teknologi dan Pendidikan Tinggi (KEMENRISTEKDIKTI). This work was supported by Indexed International Publication for Student Final Project NKB-1791/UN2.R3.1/HKP.05.00/2019.
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