An Investigation On Numerical Simulation Of Free Water Surface And Entrainment Problems In Hydraulic Engineering

Many free fluid surface problems exist in engineering practice, so there is very importance engineering value and academic meaning to make scientific researches on free fluid surface. Numerical simulation is one of the primary method to study and solve engineering problems. Because of special quality of free surface, the disposal of free surface become more difficult and important in numerical simulation. Numerical simulation problems about free water surface and air entrained flows in hydraulic engineering are studied in this paper.First, the solving methods and exiting problems of free surface were reviewed and the numerical simulation methods were also brief summarized. Based on the shapes of free surface, the free surface is divided into a relative stable one and a rather fluctuant one. Water current problems including relative steady and rather fluctuant free surface were simulating numerically. Flow control equations were given, relative steady and rather fluctuant free surface were treated approximately with using rigidity cover assume, at the same time, control equations were closed and discretized and numerically solved. Flow field of upstream and downstream of Xiao gushanwater power station hinge discharge sluice were practical calculated examples. The results were analyzed and showed that rigidity cover assume was fitted to relative steady free surface flow upstream sluice. The velocity field and pressure field from the calculation accord with the fact. Based the results of numerical simulation, the bodily form of sluice parapet is optimized. But the elasticity assume existed dissatisfactory aspects to rather fluctuant free surface flow downstream sluice.To air-water two-phase flow including free surface, the control equations of two-fluid model were briefly listed, and the effect of the interfacial momentum transfer term was considered, then the equations were discretized on collocated grid. The open-channel aerated water flow was numerically simulated and the free surface was treated with improved rigidity cover assume. Based on simulation results which were satisfied, the effect of bubble diameter on entrainment concentration forcast result was analyzed. The bubble moving rule was found which includs the bubble is escaping upwards along the way, the diameter of the bubble is bigger, the velocity upwards is larger and the bladder remaining near channel bottom is smaller. To make best use of the function of the aerator, the demands of enough air entrainment quantity and suitable air bubble diameter are needed to be satisfied.The problems of free surface were dealed with by VOF which is more advanced method. The governing equations were listed and the open-channel flow and sluice upstream flow are simulated by using the method of VOF. From the simulation, the shape of air-water surface, the distributing of gas ratio, the velocity field, pressure field, turbulence momentum and turbulence moment dissipation ratio were gained. The results showed that the advantage was displayed by using method of VOF not only for relative steady water surface but for rather fluctuant one.The study on free surface showed the relative simple methods can be used for relative steady free water surface flows upstream the sluice, but more advanced methods must be adopted to rather fluctuant one downstream the sluice.