The exploration of superhydrophobic drag reduction has been and continues to be of significant interest to various industries. In the present work, direct numerical simulation (DNS) is utilized to investigate the effect of the parameters on the drag-reducing performance of superhydrophobic surfaces (SHS). Simulations with a friction Reynolds number of 180 were carried out at solid fraction values of ${\varphi }_s=\frac{1}{16},\frac{1}{11}$, and $\frac{1}{4}$, and three distribution shapes: aligned, staggered, and random. The top wall is the smooth one, and the bottom wall is a superhydrophobic surface (SHS). Drag reduction and Reynolds stress profiles are compared for all cases. The turbulent kinetic energy budget, including production, dissipation, and diffusion, is presented with respect to the solid fraction and type of distribution to investigate the drag reduction mechanism. The sizes of the longitudinal vortices and formation of hairpin vortices are investigated through the observation of coherent structures. The simulation of a post model is a useful method to study the drag reduction for different solid fraction values and distribution geometries. Our study demonstrates that the drag reduction could acquire 42% with the solid fraction value ${\varphi }_s=\frac{1}{16}$ and an aligned distribution shape for post superhydrophobic surface geometry. Our study also showed the relationship of the Reynolds stress component (R₁₁, R₂₂, and R₃₃) to the drag reduction with the differences in the solid fraction values and distribution geometry. In which, the R₁₁ component has the most change between an aligned distribution and a random one. The peak value of R₁₁ tends to shift away from the SHS wall. In addition, the analysis of the TKE budget over the superhydrophobic surface was performed, which can be adopted as a useful resource in turbulence modeling based on RANS methodology.

This paper presents the minimum submergence depth of an underwater vehicle that can remove the effect of free surface on the resistance of the underwater vehicle. The total resistance of the underwater vehicle in fully submerged modes comprises only viscous pressure and friction resistances, and no wave resistance should be present, based on the free surface effect. In a model test performed in this study, the resistance is measured in the range of 2 to 10 kn (1.03–5.14 m/s) under depth conditions of 850 mm (2.6D) and 1250 mm (3.8D), respectively, and the residual resistance coefficients are compared. Subsequently, resistance analysis is performed via computational fluid dynamics (CFD) simulation to investigate the free surface effect based on various submergence depths. First, the numerical analysis results in the absence of free surface conditions and the model test results are compared to show the tendency of the resistance coefficients and the reliability of the CFD simulation results. Subsequently, numerical analysis results of submergence depth presented in a reference paper are compared with the model test results. These two sets of results confirm that the resistance increased due to the free surface effect as the high speed and depth approach the free surface. Therefore, to identify a fully submerged depth that is not affected by the free surface effect, case studies for various depths are conducted via numerical analysis, and a correlation for the fully submerged depth based on the Froude number of an underwater vehicle is derived.

Submarines with pumpjet propulsors have recently been used in many countries to improve their propulsion and noise performance. The pumpjet has the advantage of improving noise and cavitation performance by increasing the pressure inside the duct, and it has good efficiency at a low advance ratio. This study analyzed the propulsive performance of a pumpjet propulsor affixed to a SUBOFF submarine as a function of variations in the design parameters using computational fluid dynamics analysis. The incidence angle and camber of the duct and the pitch angle of the stator were selected as the design parameters. To investigate the propulsion performance as a function of the design parameter variations, each case’s thrust, torque, and efficiency were compared, and the velocity and pressure fields of each case were analyzed. In addition, the efficiency was analyzed using the non-dimensional mass flow rate and area ratio difference for each case. The duct incidence angle contributed most dominantly to the dimensionless flow rate and the difference in area ratio, and these two factors resulted in high efficiency at certain values. It is expected that further research will be conducted in the near future that takes into account cavitation inception speed and cavitation.

The goal of this study involves developing an efficient and accurate parallel computation method for two-phase flow problems including complex moving foreign bodies. The proposed parallel computing techniques are based on the moving body-fitted grids’ overset on background multidomains with grid-overlapping at their interface. First, the cavitation flow over the hemispherical head form is investigated using the two-phase flow solver, which is validated by comparing the numerical and experimental results. Subsequently, the parallel computing technique based on the multidomain method that divides the computational domain into several smaller subdomains is proposed to facilitate more efficient numerical simulations. At the interface of the subdomains, the grid-overlapping method is proposed for more accurate simulations. The illustrative computations indicate that the accuracy of the parallel computation combined with the grid-overlapping method on multidomains is identical to that of the serial computation based on a single block, albeit with a significant reduction in the computation time. Finally, the moving overset grid technique is combined with the background multidomain method and applied to simulate the gust flow that is generated by the pitching motions of the twin hydrofoils. The overset grid technique includes the following three sequential steps: hole-cutting, finding donor cells, and bilinear interpolation. The prediction results for the inflow gust generated by oscillating hydrofoils closely follow the measured results.

A practical cavitator design method for straight-running-type supercavitating torpedoes was developed in this paper. Design requirements were first drawn in terms of torpedo performance characteristics, such as maximum range and motion stability. This method determines the optimum cavitator satisfying the design requirements that not only minimize the total drag of the torpedo, extending the maximum range, but also provide hydrodynamic forces required for straight level flight. The design procedure includes determining a design cavitation number and cavitator type (disk or cone) for obtaining the optimal cavitator that minimizes the total drag of a torpedo in straight level flight. To determine such an optimal cavitator, the equations of force and moment equilibrium for straight level flight were iteratively solved by the existing mathematical models that determine the cavity shapes generated by disk- and cone-shaped cavitators and hydrodynamic forces acting on the vehicle. For validation, model experiments on a small-scale supercavitating vehicle were conducted in a towing tank, and the results agree well with those of the mathematical models used in this study. A preliminary design based on the newly proposed method was also implemented for a realistic supercavitating vehicle. More precise computations using CFD should be conducted to investigate the physics in more detail in the near future.