Search Results (6)

With the development of modern fluid machinery, the energy density of pumps is gradually being improved, and at the same time, higher demands are being placed on the cavitation performance, hence the introduction of the inducer and centrifugal impeller to form a dynamic–dynamic series structure. However, there are strict constraints on the axial size of pumps in fields such as firefighting and aerospace. The traditional empirical formula no longer satisfies the need to fit the axial dimensions between the induced wheel and the impeller at high velocities. Therefore, based on the wave-piercing theory, the drag reduction coefficient is introduced to explore the optimal axial fit size from the perspective of energy characteristics. This paper focuses on the influence of the inducer’s wake on the energy characteristics of downstream impellers, and conducts the following research: by adjusting the axial matching dimensions between the upstream inducer and the centrifugal impeller in the initial model, ten sets of axial distance models with matching dimensions of KD are designed, and the drag reduction coefficient is embedded to determine the optimal axial distance. The results show that the optimal axial distance is 0.2D, which is far lower than the axial distance value of 0.42D obtained from the traditional empirical formula for axial matching dimensions. Meanwhile, this paper uses tangential velocity, the inlet flow angle of the impeller, entropy production theory, and other indicators to analyze the internal energy loss of the high-speed vehicular fire pumps one by one. All of them confirm that the impeller in the high-speed vehicular fire pump has the lowest energy loss and optimal performance at an axial distance of 0.2D. Specifically, at this axial distance, the head can reach 259 m, and the hydraulic efficiency is as high as 83.62%. Thus, the feasibility of determining the axial placement of the impeller using the drag coefficient is validated. This research provides new insights into determining the axial coordination dimensions between the inducer and the impeller. Full article

In the present study, the diffraction and the radiation problems of water waves by a surface-piercing porous cylindrical body are considered. The idea conceived is based on the capability of porous structures to dissipate the wave energy and to minimize the environmental impact, developing wave attenuation and protection. In the context of linear wave theory, a three-dimensional solution based on the eigenfunction expansion method is developed for the determination of the velocity potential of the flow field around the cylindrical body. Numerical results are presented and discussed concerning the wave elevation and the hydrodynamic forces on the examined body for various values of porosity coefficients. The results revealed that porosity plays a key role in reducing/controlling the wave loads on the structure and the wave run-up, hence porous barriers can be set up to protect a marine structure against wave attack. Full article

The present study explores the performance of an array of cylindrical oscillating water column (OWC) devices, having a vertical symmetry axis, placed in front of a bottom seated, surface piercing, vertical breakwater. The main goal of this study is the investigation of a possible increase in the power efficiency of an OWC array by applying, in the vicinity of the array, a barrier to the wave propagation, aiming at amplifying the scattered and reflected waves originating from the presence of the devices and the wall. To cope with the set goal, a theoretical analysis is presented in the framework of linear potential theory, based on the solution of the proper diffraction, and pressure-radiation problems in the frequency domain, using the image theory, the matched axisymmetric eigenfunction expansion formulation, and the multiple scattering approach. Numerical results are presented and discussed in terms of the expected power absorption by the OWCs comparing different array’s characteristics i.e.,: (a) angle of incidence of the incoming wave train; (b) distances from the breakwater; and (c) geometric characteristics of the different arrangements. The results show that compared to the isolated OWC array (i.e., no presence of the wall), the power efficiency of the OWCs in front of a breakwater is amplified at specific frequency ranges. Full article

The Bragg reflections of oblique water waves by periodic surface-piercing structures over periodic bottoms are investigated using the eigenfunction matching method (EMM). Based on the assumption of small wave amplitude, the linear wave theory is employed in the solution procedure. In the step approximation, the surface-piercing structures and the bottom profiles are sliced into shelves separated by abrupt steps. For each shelf, the solution is composed of eigenfunctions with unknown coefficients representing the wave amplitudes. Upon applying the conservations of mass and momentum, a system of linear equations is obtained and is then solved by a sparse-matrix solver. The proposed EMM is validated by several examples in the literature. Then, the method is applied to solve Bragg reflections of oblique water waves by various surface-piercing structures over periodic bottoms. From the numerical experiments, Bragg’s law of oblique waves was used to predict the occurrences of Bragg resonance. Full article

The wave-structure interaction for surface-piercing bodies is a challenging problem in both coastal and ocean engineering. In the present study, a two-dimensional numerical wave flume that is based on a newly-developed meshless scheme with the generalized finite difference method (GFDM) is constructed in order to investigate the characteristics of the hydrodynamic loads acting on a surface-piercing body caused by the second-order Stokes waves. Within the framework of the potential flow theory, the second-order Runge-Kutta method (RKM2) in conjunction with the semi-Lagrangian approach is carried out to discretize the temporal variable of governing equations. At each time step, the GFDM is employed to solve the spatial variable of the Laplace’s equation for the deformable computational domain. The results show that the developed numerical method has good performance in the simulation of wave-structure interaction, which suggests that the proposed “RKM2-GFDM” meshless scheme can be a feasible tool for such and more complicated hydrodynamic problems in practical engineering. Full article

With the increase of battlefield target diversity and protection mobility, the disadvantages of traditional armor piercing warheads have gradually become prominent. The conception of the PELE (penetration with enhanced lateral efficiency) projectile was thus proposed. The axial residual velocity of the projectile is a very important indicator of a PELE projectile, which mainly reflects the penetration ability of the PELE projectile. The PELE projectile is a symmetrical structure, so the collision problem can be simplified to plane collision. Furthermore, the two-dimensional plane is axisymmetric, and so it can be further simplified to one-dimensional collision. Based on simplification and assumptions, the mechanism of a PELE projectile penetrating a thin metal target plate was studied using the shock wave theory, and a theoretical model of axial residual velocity has been established in this article. The energy loss during the penetration process was divided into the following parts: the kinetic energy increment of the target plug in the impact region, the internal energy increment of the outer casing and inner core, and the shear energy dissipation of the projectile against the target plate. In addition, the specific methods of determining the energy loss of each part are given in detail. According to the conservation of energy, the approximate calculation formulae of the axial residual velocity of a PELE projectile have been deduced. Finally, the theoretical results were compared with the experimental results under different working conditions, and the results were in good agreement. Therefore, the theoretical model has application value and guiding significance in the field of engineering. Full article