Journal of Marine Science and Technology最新文献

These new mechanisms for suppressing vibration in large diameter intake pipes and pumping systems has been developed for use in Floating Liquefied Natural Gas (FLNG) facilities. Like existing devices such as fairings and strakes, this mechanism is designed for practical use and has a flow path inside the pipe. The mechanism is designed to suppress vibration caused by both float motion and vortex-induced vibration (VIV) due to currents. Experiments were conducted using a scale model, and numerical calculations were used to evaluate the mechanism’s ability to reduce vibration. As a result, the natural frequencies of the pipes were analyzed, and it was found that the vibration damping mechanism, when installed at appropriate locations, can provide effective vibration suppression against the motion of the upper float and vibration caused by VIV due to currents over the entire length of the pipe, even at limited installation locations. On the other hand, it was found that the vibration suppression effect could not be achieved without appropriate positioning, and that the longer the pipe length, the more limited the vibration damping capability.

With the development of the large and high-speed ships, the cavitation and radiated noise of marine propellers have been more and more concerned. This paper presents a numerical simulation of marine propellers with the ridge structures, inspired by the airfoils with the bionic ridge surfaces. The bionic method is applied to redesign the blade sections of a marine propeller, and the ridge structures are arranged between the leading edge and the thickest point. Four bionic propellers are established by changing the style and distribution area of the ridge structures on the blade surface. The cavitation morphology, pressure distribution and open water characteristics are analyzed with the software STAR CCM + . The numerical model is validated with the test data and the results show that the ridged structures can make the low-pressure area on the blade surface more dispersed and suppress cavitation. Compared with the prototype propeller, the four bionic propellers with the ridge structures can reduce the cavitation area by 26% (with an advance speed coefficient of 0.5) to 30% (with an advance speed coefficient of 0.3) at medium and low advance speeds. Besides, one of the bionic propellers can improve the thrust and efficiency by 14.93% and 1.61% respectively at high advance speeds.

Although it is known from previous studies that porpoising in high-speed planing craft is a Hopf bifurcation, this study examined its occurrence and disappearance using the motion model identified from full-scale test data. Herein, we first analyzed the stability of the system linearized near the equilibrium point. From its results, we reconfirmed the knowledge that porpoising occurs when the system becomes unstable in the vicinity of the equilibrium point. We also found that the system became unstable as the thrust or trim angle of the outboard motor decreased. This finding was consistent with the results of a full-scale craft test performed in a previous study. Second, we confirmed that the limit cycle which is a result of the nonlinearity of the system was stable. The two analyses indicate that porpoising corresponds to the supercritical Hopf bifurcation. Furthermore, in the vicinity of the bifurcation point, it was found that stable equilibrium points and stable limit cycles can coexist. Finally, we confirmed this phenomenon in the full-scale test.