II. Example: Simulation of a flow out

from a water reservoir.

Figure 1. Reservoir with liquid

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

pressure is acting over the liquid. The

other geometrical sizes are shown in

figure 2.

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

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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

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Figure 6

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

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Figure 11

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)

Area

Relative pressure pr=0 Pa

B5 Air outlet (opening boundary)

Area

Relative pressure pr=0 Pa

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

3.4 Solution of the model

Figure 22

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 24

The status of the solution is shown on the

screen (figure 25)

Figure 23

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Figure 25

3.5 Post processing of the results

Figure 26

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

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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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