Download II. Example: Simulation of a flow out from a water reservoir. Figure 1

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
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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.
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Figure 5
Figure 6
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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
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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.
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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
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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
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Figure 12
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Figure 13
Figure 14
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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
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Figure 15
Figure 16
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Figure 17
Figure 18
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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).
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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)
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Figure 22
Figure 23
The status of the solution is shown on the
screen (figure 25)
Figure 24
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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
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“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.
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Figure 28. Plane.
Figure 29. Contour and vectors.
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