A Preliminary Study On Movement Characteristics Of Adverse-Slope Density Currents

When density currents forming in the main stream reach the junction zone of the main stream and the tributary,density currents may intrude into the tributary forming adverse-slope density currents.When the salt water at estuary intrude into the tributary,it may affect water security of both sides of the tributary.When the main stream of the reservoir intruded into the tributary,it may cause sedimentation at the entrance of the tributary and cause loss of reservoir capacity.At present,many studies have been carried out on studying the movement characteristics of flat slope and downslope density currents,but few researches have been implemented on investigating the movement characteristics of adverse-slope density currents.From the perspective of practical engineering applications,it is of great important to study the adverse-slope density current motions.This helps us better predict the movement process of stream in the junction zone of various types of density currents such as salt water density currents,temperature density currents,and sediment density currents,and clarify the formation mechanism of the sand bar at the entrance of tributaries.In this thesis,flume experiments and numerical simulations on the movement characteristics of adverse-slope density currents are carried out.The main research contents are listed as follows.(1)A 3D mathematical model of density current is established.In the model,the Godunov-type finite volume method based on the collocated grid is used to solve the three-dimensional Reynolds time-averaged Navier–Stokes equations.The Harten-Lax-van Leer-contact(HLLC)approximate Riemann solver is employed to solve the interface numerical flux in the horizontal direction to resolve the discontinuity in the interface of density currents,and the σ-coordinate transformation is employed to adapt to the irregular bed topography and the dynamically changing water surface.Besides,a buoyancy-corrected nonlinear K-(?) model with two equations is employed for turbulence closure.Three classic lock-exchange experiments of density currents propagating on flat and sloped beds are employed to verify the performance of the model.Results show that the developed model simulates the movement of density currents well on flat and uneven bed with high accuracy.(2)The movement characteristics of adverse-slope density currents in a single flume are studied through experiments and numerical simulation methods.Result shows that the propagation velocity of density current decreases along the sloped bed,and the thickness of the density currents gradually decreases.When density currents move for a certain distance,density currents may move back,and a separation inside the density currents occurs.there are similar phases in the movement process of adverse-slope density currents comparing with density currents moving on a flat bed.Three phases exist,i.e.,collapse phase,self-similar phase and viscous phase.Calculation formulas are established for the change of front position and propagation velocity of density currents with time under the influence of various factors.(3)The whole movement process of density currents in the main stream intruding into the tributary is simulated numerically.It is found that the movement process of density currents in the main stream intruding into the tributary can be roughly divided into four stages.In the first stage,density currents in the main stream has passed one side of the entrance of tributary but not reached the farther side,or the front of density currents in the main stream has reached the farther side of the entrance.In this stage,the peak surface of density currents in the tributary have not yet been influenced by currents accumulated on the far side of the entrance door.In the second stage,density currents in main stream accumulates on the farther side of the entrance,the front of density current changes from the proximal side of the tributary to the farther side of the tributary,and the velocity of the tributary density current increases rapidly.In the third stage,the main body of density current passes the father side of the entrance,and the propagation speed of the front of tributary density current decreases.In the fourth stage,after the main body of density currents passed the farther side of the entrance,the front velocity of density current decreases further.The greater the initial density difference between the denser water and surrounding water,the greater the acceleration of propagation speed of density current in the second stage,and the larger the tributary bottom slope,the faster deceleration process of propagation speed of density current occurs in the third and fourth stages.