Vertical distribution of the Kuroshio velocity in the Pollution Nagasaki section and its formative mechanism

The seasonal and interannual variations of the vertical distribution of the Kuroshio velocity and its formative mechanism were studied by analyzing the Global Ocean Reanalysis Simulation 2 （GLORYS2） dataset in the Pollution Nagasaki （PN） section （126.0°E-128.2°, at depths less than 1000 m）. The results indicated that： 1） the maximum transport in the PN section occurs in summer, followed by spring, and the minimum transport occurs in fall and winter; the maximum velocities are located at the subsurface in both winter and summer and velocities are relatively larger and at a shallower depth in summer; and the velocity core is located at the surface in spring and fall. The isopycnic line has a clear depression around the Kuroshio axis in winter. The depth of maximum velocity and the zero horizontal density gradients both exhibit substantial seasonal and interannual variations, and the interannual variations are larger. 2） The distributions of velocity and density are in accordance with the therma~ wind relation. Although Kuroshio transport is determined by the large-scale wind field and mesoscale motion in the Pacific Ocean; local heat flux and thermohaline circulation influence the density field, modify the vertical structure of the Kuroshio velocity, and adjust the allocation of water fluxes and nutrients transport. 3） Shelf-water offshore transport into the Kuroshio upper layer induced by southwest monsoons might contribute to the maximum velocity up to the surface in summer. Nonlinear and nongeostrophic processes are not considered in the present study, and the thermal wind relation accounts for part of the vertical structure of the Kuroshio velocity.

An approach to determine coeffi cients of logarithmic velocity vertical profile in the bottom boundary layer

Velocity vertical profiles in the bottom boundary layer are important to understand the oceanic circulation.The logarithmic vertical profile,u=A ln z+B,is the universal profile for the horizontal velocity in the boundary layer,in which two coefficients(A and B)need to be determined.The two coefficients are the functions of the friction velocity(u_(*))and the roughness length(z_(0)),and they are calculated using u_(*)and z_(0).However,the measurement of u_(*)and z_(0) is a challenge.In the present study,an approach is developed to estimate the two coefficients(A and B)by using a series of fl ume laboratory experiments with fl at boundary and regularly distributed cylinders as the rough boundaries.An acoustic doppler velocimeter(ADV)is used to measure the velocity vertical profiles of the steady flow.Using the measured velocity data,the regressed logarithmic profiles are obtained.Based on the series of the A and B values,the mathematical formula for A and B are statistically established as the function of the cylinder height,inflow velocity,and the water depth,which avoids the measurement of the friction velocity and the roughness length.

Analysis of sand particles’ lift-off and incident velocities in wind-blown sand flux

In the research of windblown sand movement,the lift-off and incident velocities of saltating sand particles play a significant role in bridging the spatial and temporal scales from single sand particle＇s motion to windblown sand flux.In this paper,we achieved wind tunnel measurements of the movement of sand particles near sand bed through improving the wind tunnel experimental scheme of paticle image velocimetry（PIV） and data processing method.And then the influence of observation height on the probability distributions of lift-off and incident velocities of sand particles was analyzed.The results demonstrate that the observation height has no obvious influence on the distribution pattern of the lift-off and incident velocities of sand particles,i.e.,the probability distribution of horizontal and vertical velocities of lift-off and incident sand particles follow a Gaussian distribution and a negative exponential distribution,respectively.However,it influences the center of the Gaussian distribution,the decay constant and the amplitude of the negative exponential distribution.

Smoothed Particle Hydrodynamic Modelling of Hydraulic Jumps: Bulk Parameters and Free Surface Fluctuations

A hydraulic jump is a rapid transition from supercritical flow to subcritical flow characterized by the development of large scale turbulence, surface waves, spray, energy dissipation and considerable air entrainment. Hydraulic jumps can be found in waterways such as spillways connected to hydropower plants and are an effective way to eliminate problems caused by high velocity flow, e.g. erosion. Due to the importance of the hydropower sector as a major contributor to the Swedish electricity production, the present study focuses on Smoothed Particle Hydrodynamic (SPH) modelling of 2D hydraulic jumps in horizontal open channels. Four cases with different spatial resolution of the SPH particles were investigated by comparing the conjugate depth in the subcritical section with theoretical results. These showed generally good agreement with theory. The coarsest case was run for a longer time and a quasi-stationary state was achieved, which facilitated an extended study of additional variables. The mean vertical velocity distribution in the horizontal direction compared favorably with experiments and the maximum velocity for the SPH-simulations indicated a too rapid decrease in the horizontal direction and poor agreement to experiments was obtained. Furthermore, the mean and the standard deviation of the free surface fluctuation showed generally good agreement with experimental results even though some discrepancies were found regarding the peak in the maximum standard deviation. The free surface fluctuation frequencies were over predicted and the model could not capture the decay of the fluctuations in the horizontal direction.