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II. Example: Simulation of a flow out from a water reservoir. 1. Problem description A reservoir, full with liquid, is emptied through an outlet, as it is shown in figure 1. The reservoir is with cube form, with sizes 0,5 x 0,5 x 0,5 m. The outlet diameter is 0,05 m. The upper side of the reservoir is opened, so the atmospheric Figure 1. Reservoir with liquid pressure is acting over the liquid. The other geometrical sizes are shown in figure 2. 2. Problem aim: An investigation of the flow out of different liquids. 3. Problem solution with ANSYS/CFX 3.1 Creation of the model geometry The geometrical model is created in Ansys Design Modeler (figure 3). The geometrical model is consisted of two parts – a reservoir with cube form and an Figure 2 1 outlet with a cylindrical form. They share common face. The following steps lead to the creation of the geometrical model: Figure 3 Figure 4 1. Start Design Modeler Geometry and select desire length units (figure 4). 2. Create the cube (figure 5). 3. Create the cylinder (figure 6). 4. Save the project and exit. 2 Figure 5 Figure 6 3 3.2. Creation of the mesh of the model The discritization of the model is made in module CFX – Mesh. That module can be started for the existing geometry file by the steps in ANSYS Workbanch’s menu: Select “Return to the project” Mark the *.agdb file Start “Generate CFX – Mesh”. Diskritizate the outlet area with little elements: Mesh/Spacing/Insert face spacing 1 with right mouse button/ Apply the minimal and maximal sizes of the elements according figure 7/ Pick the surface outlet. Create surface mash (figure 8). Create volume mesh and save the file (figure 8). Figure 7 4 Figure 8 3.3 Creation of the model. The model, that must be solved to obtain the simulation of the flow out from the reservoir is made in ANSIS CFX interface. To start it: Close CFX Mesh Select “Return to the project” Mark the *.gtm file Start “Create CFD Simulation with Mesh 3.3.1 Selection of the simulation type Select the “Simulation type” from the Flow tree and select transient analysis, with time duration 15 s, time step 0.1s, and initial time 0 s , as it is shown in figure 9. 5 Figure 9 3.3.2 Selection of the fluid domain type, fluid models and initial conditions Main Menu/ Create/ Flow object/ Domain/ Domain 1 (figure 10) Edit fluid domain 1/ General options/ Fluid list/ Select water at 25°C and air at 25°С (figure 11) Select option Buoyant and gravity: gx=0; gy=-9,81m/s2; gz=0 (figure 12) OK Edit fluid domain 1/ Fluid models/ Homogenous model/ Free surface model – standard/ Isothermal homogenous model at 298K/ k-ε model of turbulence OK Edit fluid domain 1/ Initialization/ Initial velocity U=0; V=0; W=0/ Relative pressure =0/ Initial turbulent kinetic energy=0/ Initial energy dissipation=0/Initial water fraction=1/Initial air fraction=0 (figure 13, table 1) OK 6 A water flow out is demonstrated in that example. Other liquids can be specified by the material menu, or can be definite by the path: Create /Library object /Material /Liquid Figure 10 Figure 11 7 Figure 12 8 Figure 13 Figure 14 9 3.3.3 Boundary conditions The boundary conditions for the investigated process are given in table 4. The water is flow out from the reservoir because of the open inlet and the gravitation. The water velocity on the outlet will decrease with subsiding of the level in the reservoir. That velocity and the air velocity on the top of the reservoir are results. Only the pressure must be specified on those boundaries. Table 4 N Boundaries Boundary conditions B1 Walls (figure 12). Areas: Vx, Vy, Vz=0 B4 Water inlet (opening boundary) Relative pressure pr=0 Pa Area B5 Air outlet (opening boundary) Relative pressure pr=0 Pa Area The boundary conditions are specified at the following steps: A) Specifying the walls. Main Menu /Create/ Flow object/Boundary conditions – Boundary 1(figure 15) /Select walls/Pick F19, F20, F22, F23, F24, F25 (figure 16) B) Specifying the opening boundaries. Pick with right mouse button on the boundary “Domain 1 – Default 2D region” /Basic settings/ Opening/ Select F21, F26 (figure 17) Boundary details / Opening pressure =0 Pa (figure 18) Fluid values / Volume fraction of the water = 0 / Volume fraction of the air =1 10 Figure 15 Figure 16 11 Figure 17 Figure 18 12 Figure 19 3.3.4 Output options Pick on output options and create the file for the transient results. Select the time interval for the writing of the results (figure 20). 13 In “Solver control” select the number of iterations on time step =5 (figure 21) Figure 20 Figure 22 3.4 Solution of the model The “Solver file” must to be written and started for solution (figure 22 and 23). Start “Run” (figure 24) 14 Figure 22 Figure 23 The status of the solution is shown on the screen (figure 25) Figure 24 15 Figure 25 3.5 Post processing of the results After the final of the solution the results can be viewed with the postprocessor (figure 25). The results may be viewed on the boundaries with creation of the Figure 26 16 “contours” (figure 26 and 27), on the planes, as vectors or in another ways (figure 28 and 29) With “time step selector” the results can be viewed for the different time intervals (figure 28) Figure 27. Contour. 17 Figure 28. Plane. Figure 29. Contour and vectors. 18 19