Journal of Hydrodynamics, Ser. B

In order to study the effects of the confining pressure on the erosion characteristics of the self-resonating cavitating jet under wellbore and deep-water conditions, experiments are conducted on aluminum specimens impinged by the organ pipe cavitation nozzle and the conical nozzle with the confining pressure in the range 0 MPa–10.0 MPa. Meanwhile, through the numerical simulation of the collapsing process of the cavitation bubble and the noise test, the cavitation erosion mechanism is analyzed. The experimental results show that the optimal standoff distance and the confining pressure can be obtained for the maximum erosion quantities, and the optimal standoff distance is 5 to 7 times greater than the equivalent nozzle outlet diameter and the confining pressure is about 2.0 MPa. Under the same conditions, the erosion caused by the cavitation nozzle is up to 2 times larger than that caused by the conical nozzle. According to the numerical simulation and the noise test, the cavitation erosion on the aluminum specimens is mostly caused by mechanical forces due to the high-frequency pressure pulse generated during the collapse of cavitation bubbles, while just a small part is caused by micro-jets.

There are two types of floating bridge such as discrete-pontoon floating bridges and continuous-pontoon floating bridges. Analytical models of both floating bridges subjected by moving loads are presented to study the dynamic responses with hydrodynamic influence coefficients for different water depths. The beam theory and potential theory are introduced to produce the models. The hydrodynamic coefficients and dynamic responses of bridges are evaluated by the boundary element method and by the Galerkin method of weighted residuals, respectively. Considering causal relationship between the frequencies of the oscillation of floating bridges and the added mass coefficients, an iteration method is introduced to compute hydrodynamic frequencies. The results indicate that water depth has little influence upon the dynamic responses of both types of floating bridges, so that the effect of water depth can be neglected during the course of designing floating bridges.

For sudden expansion pipes, experiments were carried out to study the cavitation inception for various enlargement ratios in high speed flows. The flow velocity of the prototype reaches 50 m/s in laboratory. The relationship between the expansion ratio and the incipient cavitation number is obtained. The scale and velocity effects are revealed. It is shown that Keller’s revised formula should be modified to calculate the incipient cavitation number when the forecasted velocity of the flows in the prototype exceeds the experimental velocity.

The stability of the laminar flat plate boundary layer is investigated numerically by solving the linear Orr-Sommerfeld equations for the disturbulence amplitude function. These equations include the terms of viscosity, density stratification, and diffusion. Neutral stability curve and the critical Re numbers are computed for various Richardson (Ri) numbers and Schmidt (Sc) numbers. The results show that the larger the Ri, the larger the critical Re for Sc < 10. The flow is stable for Ri < 0, when Sc is very small or the mass diffusion coefficient is very large. But for Ri > 0, the effects of diffusion are reversed for Sc < 10. For Sc > 10, the critical Re rapidly decreases to zero as the Sc increases for a given Ri number. The critical Re rapidly decreases as the Ri increases.

The main focus of this study is on the development of an efficient and effective hull surface modification technique for the CFD-based hull form optimization. Two approaches are utilized. One is based on the radial basis function interpolation, and the other the sectional area curve of the hull. Both local and global modifications of hull forms can be achieved by combining these two approaches. The hull surface modification technique developed in this study is used to vary the hull forms during the optimization process, in which the objective functions associated with the resistance is evaluated by a practical design-oriented CFD tool (SSF), and a multi-objective genetic algorithm is adopted to allow for multi-design speeds. For the purpose of illustration, the KRISO container ship (KCS) is taken as an initial hull to be optimized for reduced drag at given design speeds. Numerical results obtained in this study have shown that the present hull surface modification technique can produce smooth hull forms with reduced drag effectively and efficiently in the CFD-based hull form optimization.

Cavitation bubbles behind a convex body were experimentally studied by a high speed camera and a hydrophone synchronously. The experiments were conducted in a circulating water tunnel with five various contraction ratios: β = 0.497, β = 0.6, β = 0.697, β = 0.751, and β = 0.799. The distributions of the cavitation bubble collapse positions behind the five different convex bodies were obtained by combining the images taken by the high speed camera. According to the collapse positions, it was found that no cavitation bubble was collapsed in the region near the wall until the ratio of the water head loss over the convex body height was larger than 20, which can be used to predict if the cavitation damage would occur in the tunnel with orifice energy dissipaters.