水动力学研究与进展:英文版最新文献

This paper describes a novel sharp interface approach for modeling the cavitation phenomena in incompressible viscous flows. A one-field formulation is adopted for the vapor-liquid two-phase flow and the interface is tracked using a volume of fluid (VOF) method. Phase change at the interface is modeled using a simplification of the Rayleigh-Plesset equation. Interface jump conditions in velocity and pressure field are treated using a level set based ghost fluid method. The level set function is constructed from the volume fraction function. A marching cubes method is used to compute the interface area at the interface grid cells. A parallel fast marching method is employed to propagate interface information into the field. A description of the equations and numerical methods is presented. Results for a cavitating hydrofoil are compared with experimental data.

A simulation is carried out for the pressure fluctuation driven by the water hammer, based on a joint use of the one-dimensional method of characteristics (MOC) and the three-dimensional finite volume method (FVM). The three-dimensional visualization of the cavitation induced by the water hammer is implemented, and the temporal and spatial analyses of extreme regions are made. A practical case of the water hammer, with the minimum boundary pressure higher than the saturated vapor pressure condition, is simulated. The simulation prediction that the cavitation would occur in the front of the gasket could serve some guideline for the optimization of industrial designs.

The information of the wave loads on a wave energy device in operational waves is required for designing an efficient wave energy system with high survivability. It is also required as a reference for numerical modeling. In this paper, a novel system, which integrates an oscillating wave energy converter with a pile-restrained floating breakwater, is experimentally investigated in a 2-D wave flume. The measurements of the wave pressure on the wet-surface of the device are made as the function of the power take-off (PTO) damping force. It is shown that the wave pressure is significantly affected by the PTO system, in particular, at the edges, and the wave pressure varies under different wave conditions. From the results, conclusions can be drawn on how the PTO damping force and wave conditions affect the loads on the device, which is of engineering concern for constructing safe and reliable devices.

The influence of minute amounts of additives on pressure drop is an interesting fundamental phenomenon, potentially with important practical applications. Change of the pressure drop in a quasi-two-dimensional channel flow using various additives is experimentally investigated. Tests were conducted for a wide range of concentrations (100 ppm–500 ppm) and Reynolds numbers (16 000– 36 000) with two polymers and four rigid fibers used as additive. Maximum drag reduction of 22% was observed for xanthan gum. However, xanthan gum loses its drag-reducing property rapidly. It was also seen that drag reduction percentage of xanthan gum remains almost constant for different Reynolds numbers. Guar flour demonstrated good drag reduction property at high Reynolds numbers. Drag reduction of 17.5% at Re =33 200 using 300 ppm solution was observed. However, at low Reynolds numbers guar flour will cause an increase in pressure drop. Fiber fillers (aspect ratio=21) have been tested as well. In contrast to polymers, they increased the drag for the range of examined concentrations and Reynolds numbers. Polyacrylonitrile fiber with three different aspect ratios (106, 200, 400) was also used, which showed an increase in pressure drop at low aspect ratios. Polyacrylonitrile fibers of larger lengths (6 mm) demonstrated minor drag-reducing effects (up to 3%).

Ship wave pattern is a fascinating research topic in the fields of marine hydrodynamics and water waves. Within the pure-gravity wave theory, the ship wave pattern composed of transverse waves and divergent waves appearing on the downstream is confined within a sector symmetrical about the ship track with a half-angle . However, when the surface tension is accounted for, the wave pattern is greatly modified especially at a low translating speed. Besides the minimum speed of capillary waves c_min = 0.23 m/s below which waves cannot be generated, there is another critical speed c_div = 0.45 m/s associated with the disappearance of divergent waves. In the present paper, the wave patterns created by a steadily translating source are studied, and they are examined with the crestlines obtained from the asymptotic analysis.

A simple and highly-efficient method for numerically evaluating the waves created by a ship that travels at a constant speed in calm water, of large depth or of uniform depth, is given. The method, inspired by Kelvin's classical stationary-phase analysis, is suited for evaluating far-field as well as near-field waves. More generally, the method can be applied to a broad class of integrals with integrands that contain a rapidly oscillatory trigonometric function with a phase function whose first derivative (and possibly also higher derivatives) vanishes at one or several points, commonly called points of stationary phase, with the range of integration.

A vehicle-mounted three-dimensional underwater particle image velocimetry (PIV) device is used in a towing tank to measure the velocity distribution of the inlet duct of a waterjet ship model in a self-propulsion test. The following points are shown through a comparison of the influences of the stationary and free states of the ship model on the measured results: (1) during the test, the ship attitude will change, specifically, the ship model will heave and trim, (2) the degree of freedom disturbs the processing of the pixel images enough to distort the subsequent image processing, (3) the stationary state of the ship model is the optimal mode for measuring the velocity distribution using the PIV device, and (4) if the changes must be considered, the man-made heaving and trimming may be pre-applied, and be made a corrected stationary mode. In addition, the momentum effect coefficient and the energy effect coefficient are calculated in a non-uniform inflowing state, and the related factors affecting the two coefficients are analyzed. The test results show that the pumping action of the waterjet creates a transverse vector in the cross-sectional speed, which increases the non-uniformity of the inflow. These results could help to establish the design requirements for a waterjet-propelled ship type.

This paper aims to provide a better understanding of the interaction between solitary waves and vertical circular cylinders. This is achieved via process based numerical modelling using the parallel particle-in-cell based incompressible flow solver PICIN. The numerical model solves the Navier-Stokes equations for free-surface flows and incorporates a Cartesian cut cell method for fluid-structure interaction. Solitary waves are generated using a piston-type wave paddle. The PICIN model is first validated using a test case that involves solitary wave scattering by a single vertical cylinder. Comparisons between the present results and experimental data show good agreement for the free surface elevations around the cylinder and the horizontal wave force on the cylinder. The model is then employed to investigate solitary wave interaction with a group of eleven vertical cylinders. The wave run-up and wave forces on the cylinders are discussed.

The centrifugal pumps usually work at various rotational speeds. The variation in the rotational speeds will affect the internal flow, the external performance, and the anti-cavitation performance of the pump. In order to improve the anti-cavitation performance of the centrifugal pumps, variable-pitch inducers are placed upstream of the impeller. Because the rotational speeds directly affect the flow and the performance of the pump, it is essential to characterize the performance of the pump with a variable-pitch inducer at various rotational speeds. In this paper, the simulations and the experimental tests of a centrifugal pump with a variable-pitch inducer are designed and carried out under various rotational speed conditions. Navier-Stokes equations, coupled with a Reynolds average simulation approach, are used in the simulations. In the experimental tests, the external and anti-cavitation performances of the pump are investigated in a closed system. The following results are obtained from the simulations. Firstly, the velocity in the passage of the inducer rises with the increase of the rotational speed. Secondly, the static pressure escalates on the inducer and the impeller with the increase of the rotational speed. Thirdly, the static pressure distribution on the inducer and the impeller is asymmetric. Fourthly, the anti-cavitation performance of the pump deteriorates with the increase of the rotational speed. Additional results are gathered from an analysis of the experiments. H − Q curves are similar parabolas at various rotational speeds, while η − Q curves are similar parabolas only when n ≤ 6 000 r/min. The anti-cavitation performance of the pump deteriorates with the increase of the rotational speed. Finally, the simulation results are found to be consistent with the experimental results.