An elliptic jet and a square jet flowing into a counterflow with different jet-to-current velocity ratios are investigated by using realizable k–ε model.Some computed mean velocity and scalar features agree reasonably well with experimental measurements,and more features are obtained by analyzing the computed results.After fluid issues from a nozzle,it entrains ambient fluid,and its velocity and concentration on the centerline decay with the distance downstream from the potential core(l0).The decay ratio increases with the decreasing jet-to-current velocity ratio α.For an elliptic jet,the evolution of the excess velocity half-width b and the concentration half-width bc merely remains constant near the jet exit on major-axis plane while they increase linearly on the minor-axis plane.However,the half-widths on the major-axis and minor-axis plane become proportional to the axial distance downstream after equaling each other.For a square jet,b and bc increase linearly with the distance downstream from the jet exit,but the spread ratio is larger on the middle plane than that on the diagonal plane before they equal each other.The radial extent of the dividing streamline rs or the mixing boundary rsc increases linearly downstream,and decreases exponentially after reaching a peak at xb.The ratio on the minor-axis plane is larger than that on the major-axis plane for an elliptic jet.The characteristics are the same for the square jet.b,bc,rs,and rsc on two corresponding planes become equal to each other more rapidly for the square jet than for the elliptic jet,because the sharp corner of the square nozzle induces secondary structures that are more intense.The distributions of the excess axial velocity and scalar concentration exhibit self-similarity for either the elliptic jet or square jet in the region of l0 < x < xb.On the cross section,four counter-rotating pairs of vortices,which enhance the entrainment between the jet and counterflow,form at the four corners of the square jet or at the two ends of the major-axis
The hydrodynamics of a single jet and four tandem jets in a cross flow are simulated by using the Computational Fluid Dynamics (CFD) software Fluent. The realizable model is used to close the Reynolds-Averaged equations. The flow characteristics of the jets, including the jet trajectory, the velocity field and the turbulent kinetic energy are obtained with various jet-to-cross flow velocity ratios in the range of 2.38-17.88. It is shown that a single jet penetrates slightly deeper than the first jet in a jet group at the same , although the difference decreases with the decrease of . It is also found that the way in which the velo-city decays along the centerline of the jet is similar for both a single jet and the first jet in a group, and the speed of the decay increases with the decrease of . The downstream jets in a group are found to behave differently due to the sheltering effect of the first jet in the group. Compared with the first jet, the downstream jets penetrate deeper into the cross flow, and the velocity decays more slowly. The circulation zone between the two upstream jets in the front is stronger than those formed between the downstream jets. The Turbulent Kinetic Energy (TKE) sees a distinct double-peak across the cross-sections close to each nozzle, with low values in the jet core and high values in the shear layers. The double-peak gradually vanishes, as the shear layers of the jet merge further away from the nozzle, where the TKE assumes peaks at the jet centerline.
The particle size of sediment is one of the main factors that influence the phosphorus physical adsorption on sediment. In order to eliminate the effect of other components of sediment on the phosphorus physical adsorption, the sediment mineral matrices were obtained by removing inorganic matter, metal oxides, and organic matter from natural sediments, which were collected from the Nantong reach of the Yangtze River. The results show that an exponential relationship exists between the median particle size (D50) and specific surface area (Sg) of the sediment mineral matrices, and the fine sediment mineral matrix sample has a larger specific surface area and pore volume than the coarse sediment particles. The kinetic equations were used to describe the phosphorus adsorption process of the sediment mineral matrices, including the Elovich equation, quasi-first-order adsorption kinetic equation, and quasi-second-order adsorption kinetic equation. The results show that the quasi-second-order adsorption kinetic equation has the best fitting effect. Using the mass conservation and Langmuir adsorption kinetic equations, a formula was deduced to calculate the equilibrium adsorption capacity of the sediment mineral matrices. The results of this study show that the phosphorus adsorption capacity decreases with the increase of D50 , indicating that the specific surface area and pore volume are the main factors in determining the phosphorus adsorption capacity of the sediment mineral matrices. This study will help understand the important role of sediment in the transformation of phosphorus in aquatic environments.
Yang XIAOXiao-lian ZHUHao-ke CHENGKai-jie LIQi LUDong-fang LIANG
