The flow characteristics in a meander channel are fully three-dimensional. With the primary flow in the streamwise direction, the secondary flow in the transverse and vertical directions induced by the channel bends are significant in the analyses of the turbulent structure. Some of the analyses in the straight channel, for instance, the quadrant analysis for the bursting phenomena, are inadequate in investigating the meander channel, since the flow in the transverse direction is not taken into account. In order to reveal the flow characteristics in a multi-bend meander channel, especially, the bursting process, experiments are conducted in the present study. With the three-dimensional quadrant analysis, the influence of the transverse flow velocity during the bursting process could be correctly addressed. The analyses and discussions are presented in this paper.
The meander channel is one of the most common channel patterns in nature.The characteristics of the flow and sediment in a meander channel which have significant effect on the development of watercourse are important subjects in river dynamics.The transition of the flow patterns in a meander channel concerns with the development mode of the channel pattern and the river regime including the generation conditions of the three-dimensional coherent vortex and secondary flow,the hierarchical scale of coherent vortex in different flow conditions,the large-scale turbulent eddy structure adapted to a meander,etc.In this paper we study the laminar flow instability of the two-dimensional channel in a meander channel.It is essentially different from that in a straight channel:The neutral curve will move forward and the critical Reynolds number will decrease.The flow is unstable in response to a wider range of the disturbance wave number,or the laminar flow instability can happen more easily.The above results could not be obtained in the traditional hydrodynamic stability theory so that our work in this paper would make up for the deficiency and blank in this aspect.
In a meandering river,a certain scale of turbulent vortex dominates the development of river morphology,making the river bend with s particular curvature.This kind of vortex is denoted as "bend-forming vortex".The coordinated relationship of bend-forming vortex and meandering river channel is then known as "self-adaption feature" of rivers.With these two concepts,this paper investigated the stability and self-adaption character of coherent vortex in the U-shape river bend with a constant curvature.On the basis of fluid mechanics theory and in consideration of turbulent coherent vortex as disturbance,the growth rate and the wave number response range of coherent vortex in meandering rivers with different curvatures were calculated in this paper.Moreover,the responses of different scales of coherent turbulence structure to river bend parameters were analyzed to explain the mechanism of river bend maintenance.These methods could provide a theoretical basis for further investigation on river meandering.
From the group movement of the bed load within the bottom layer,details of the nonlinear dynamic characteristics of bed load movement are discussed in this paper.Whether the sediment is initiated into motion corresponds to whether the constant term in the equation is equal to zero.If constant term is zero and no dispersive force is considered,the equation represents the traditional Shields initiation curve,and if constant term is zero without the dispersive force being considered,then a new Shields curve which is much lower than the traditional one is got.The fixed point of the equation corresponds to the equilibrium sediment transport of bed load.In the mutation analysis,we have found that the inflection point is the demarcation point of breaking.In theory,the breaking point corresponds to the dividing boundary line,across which the bed form changes from flat bed to sand ripple or sand dune.Compared with the experimental data of Chatou Hydraulic Lab in France,the conclusions are verified.
