Understanding the swimming characteristics of micro-organisms is significant for modelling the migration of motile cells and corresponding ecological risk assessments associated with harmful algae in oceans and estuaries. Presented in this paper is an experimental and numerical investigation of swimming characteristics of a typical gyrotactic micro-organism, Heterosigma akashiwo (H. akashiwo) in water, based on the technology of planar laser-induced fluorescence and the finite volume method. Two-dimensional swimming velocity of algal cells are obtained by analyzing cells’ trajectories in the vertical plane, and three-dimensional swimming velocity is reconstructed based on the assumption that cells’ swimming is isotropic in the horizontal plane. Four important parameters are given to reflect the swimming characteristics of gyrotactic cells in still water, including the mean swimming speed (V_s = 146 μm/s), the relative strength of reorientation by gravitational torque to rotational diffusion (λ = 1.96), the time scale of reorientation (B = 5.6 s), and rotational diffusivity (D_r = 0.046 rad²/s). A database of the ambient vorticity, mean swimming velocity and diffusivity tensor is established, by solving Fokker-Planck equation for the probability density function of cells’ swimming under the combined action of gravity, rotational diffusion, and the ambient vorticity. The mean swimming velocity and translational diffusion tensor of H. akashiwo are found to change with the horizontal and vertical vorticity. It is also shown that gyrotactic cells swim in a given direction for a weak horizontal vorticity, in contrast to cells’ tumbling and being trapped for a strong horizontal vorticity.

An experimental investigation of the cavitation characteristics of high-velocity flow over an offset-into-flow with various heights and chamfers both in the presence and absence of aeration was conducted in a cavitation tunnel of the Hydraulics Laboratory, Zhejiang University of Technology in China. The 2- and 5-mm abrupt and sloping offsets into flows were employed to be models of surface irregularities. Cavitation phenomena of high-velocity flow over the various offsets were carefully observed. A range of air concentrations was considered. Variations of wall pressures downstream and upstream of the offset were measured in terms of the air concentration. The effects of air concentration and chamfer on the cavitation characteristics of the offsets were analysed. In addition, the cavitation cloud over the offset-into-flow and the turbulent boundary layer over a flat plate are compared.

The discharge of natural rivers is one of the important hydrological factors that are considered when responding to ice-flood disasters during ice periods. Traditionally, holes need to be dug along the cross-section on the ice cover to gauge velocity distributions along the flow depth at each hole, and to calculate the cross-sectional flow discharge by integrating velocity profiles over the entire area. This method is time consuming, costly, and inefficient. The discharge measurement can be improved using the sectional flow-depth distribution and stream-tube methods. However, the selection of both the depth-averaged–velocity-estimation method and the typical survey-point position in the cross-section affects the estimation accuracy. This study first compared the estimation methods of the depth-averaged velocity, such as the one-, two-, three-, and six-point methods, and their estimation accuracy. Furthermore, the variations in relative-unit discharge distributions in common channels with cross-sectional topographies were analyzed, and the effects of the cross-section characteristic coefficient and typical survey-point position on the flow-discharge estimation accuracy were compared. The results show that the average errors of the depth-averaged velocity estimated by the one-point method at 0.5H, new three-point method, and six-point method were 1.96%, 1.22%, and 0.45%, respectively. The new three-point method is recommended if measurement workload and accuracy are key considerations. The cross-section characteristic coefficient is considered to be 0.5 and 0.25 for the natural river and artificial channel, respectively, and the maximum-flow-depth position in the mainstream area of the cross-section is selected as the typical survey-point position. Thus, the flow-discharge estimation accuracy can be improved. In conclusion, this study provides an improved stream-tube method for the measurement of flow discharge and velocity distribution in ice periods, which can be used as a reference during practical applications.