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The statistical law that governs the drift velocity of tropical cyclones in the Northwest Pacific Ocean is investigated. The investigation is based on data published by China Meteorological Administration for historical tracks of 2146 cyclone events that occurred during 1949-2012. Empirical formulae are obtained to relate the magnitude, the direction, the meridional and zonal components of the averaged cyclone drift velocity with latitude. As the latitude effect is excluded, it is found that the cyclone drift velocity is governed by simple statistical laws, i.e. the magnitude and direction of the deviated drift velocity approximately satisfy a gamma distribution and a symmetric bimodal distribution, respectively, while the meridional and zonal components of the deviated drift velocity satisfy the same type of symmetric probability distribution represented by the hyperbolic secant function but with different deviations. The results obtained are potentially applicable to the enhancement of current tropical cyclone track forecasting techniques. They are also useful in risk management over the coastal areas where tropical cyclones may cause serious damages.

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a b s t r a c tA numerical model for the general description of the sediment-laden flow is developed based on an Euler-Euler approach of the two-phase turbulent flow theory. The basic equations of the model are the Reynolds averaged equations of motion for both the fluid and the sediment phase in addition to the Reynolds averaged continuity equations for the mixture and for the sediment phase. The fluid phase and the sediment phase are coupled through their interaction forces including resistance force, inertia force, and lift force. Turbulence closure of the fluid phase is based on the conventional k-e model while an algebraic particle turbulence model is applied to the sediment phase. The numerical method is based on the modified SIMPLE scheme. The model is applied to the computation of saturated sediment-laden flows and also the non-equilibrium transport of sediment by unidirectional flows under simple erosion and simple deposition conditions. The numerical results are well verified by the available experimental data. The vertical velocity of the sediment phase is also shown to be in very good agreement with the fall velocity of the sediment particles, which strongly support the assumption of Rouse's diffusion theory for suspended sediment under steady state.

A numerical model for the general description of the sediment transport under oscillatory sheet flow conditions is developed based on a two-fluid representation of the two-phase turbulent flows. The governing equations of the model are the Reynolds averaged continuity equations and equations of motion for both the fluid and the sediment phases. The two phases are coupled by the interphase forces including the resistance force, the inertia force, and the lift force. Turbulence closure of the fluid phase is based on a slightly modified k-ε model while an algebraic particle-turbulence model is applied to the sediment phase. The numerical method is based on the modified SIMPLE scheme and an improved time stepping technique. The model is validated by the published data for the symmetrical oscillatory sheet flows generated in an oscillatory flow tunnel at the University of Tokyo and for both the symmetrical and the asymmetrical oscillatory sheet flows generated in an oscillatory flow tunnel at University of Aberdeen. The numerical results on the temporal and spacial variation of the sediment concentration, the horizontal velocities of the two phases, the horizontal and vertical fluxes of the sediment phase, as well as the thickness of the sheet flow layer all show satisfactory agreement with the laboratory data. The model is also shown to predict the net sediment transport rate with a reasonably good accuracy.