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Step 4: Setup (Physics)

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Physics Setup

Launch FLUENT

We will be working within the ANSYS 12.1 workbenchWorkbench.   To launch FLUENT, double click on setup from the workbench. the Setup cell from the Project view. Make sure the Double Precision option is selected. This will use 64 bits (rather than 32) per floating point number, decreasing round-off errors.

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Once Fluent has opened, select Problem Setup > General > Display...

Make sure all 5 items under Surfaces are selected. Then click Display. Remember that we can zoom in using the middle mouse button. Zoom in and admire the mesh. How many divisions are there in the radial direction?

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Recall that you can look at specific components of the mesh by choosing the entities you wish to view under Surfaces (click to select and click again to deselect a specific boundary). Click Display again when you have selected your boundaries. Use this feature and make sure that the boundary labels correspond to the correct geometric entities.

Define Governing Equations

Problem Setup > General > Solver

Choose Axisymmetric under 2D Space. As in the laminar pipe flow tutorial, we'll use the defaults of Pressure-Based Type, Steady flow and Absolute Velocity Formulation.

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The energy equation can be turned off since this is an incompressible flow and we are not interested in the temperature. Make sure no tick mark appears next to Energy Equation Energy - Off appears.

Problem Setup > Models > Viscous - Laminar

Click Edit...Choose and choose k-epsilon (2eqn). Notice that the window expands and additional options are displayed on choosing the k-epsilon turbulence model. Under Near-Wall Treatment, pick Enhanced Wall Treatment so that we may get a more accurate result.
. This option uses a blended function to go between a two-layer model and standard wall functions. If the mesh near the wall is fine enough, the two-layer model is used. Otherwise, standard wall functions are used. You could alternately use Standard Wall Functions; this will work well when 30 < y+ < 100. Refer to the turbulence chapter in the FLUENT user manual.

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Click OK.

Problem Setup > Materials

Double click on air and change Density to 1.0 kg/m^3 and Viscosity to 2e-5 kg/(m*s). These are the values in the Problem Specification and are picked to give us a Reynolds number of 10,000. We'll take both as constant.

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Click Change/Create and close the window.

Define Boundary Conditions

Problem Setup > Boundary conditions > Operating Conditions...

Recall that for all flows, FLUENT uses the gauge pressure internally. Any time an absolute pressure is needed, it is generated by adding the operating pressure to the gauge pressure. We'll use the default value of 1 atm (101,325 Pa) as the Operating Pressure.

Click Cancel to leave the default in place.

We'll now set the value of the velocity setup the boundary conditions at the wall, centerline, inlet and pressure at the outlet. Now under

Problem Setup > Boundary conditions

The four types of boundaries we defined are specified in a window as zones. Recall that we We don't need to set any parameters for the centerline and wall zones pipewall zone. FLUENT will automatically detect that this location should be set as a wall based on its name. Verify this by selecting each of these two zones that zone and looking at their types its type in the drop down menu below the Zones..

Next, let's look at the centerline. Since we are solving an axisymmetric problem, we will set the centerline as the axis; this will impose symmetry at this boundary. Set centerline to axis boundary type, using the drop down menu. Click Yes and OK to confirm.

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Choose inlet and click on Edit..... This boundary is set to velocity-inlet type by default which is right in our case. Change the Velocity Specification Method to Magnitude, Normal to Boundary. Enter 1 m/sfor Velocity Magnitude. This indicates that the fluid is coming in normal to the inlet at the rate of 1 meter per second. Select Intensity and Hydraulic Diameter next to the Turbulence Specification Method. Then enter 1 1% for Turbulence Intensity and 0.22m for Hydraulic Diameter. Click OK to set the velocityboundary conditions for the inlet.

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The (absolute) pressure at the outlet is 1 atm. Since the operating pressure is set to 1 atm, the outlet gauge pressure = outlet absolute pressure - operating pressure = 0. Choose outlet under Zone. The Type of this boundary is pressure-outlet. Click on Edit. The default value of the Gauge Pressure is 0. Click Cancel to leave the defaults in place.

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This completes the boundary condition specification.

Reference Values

Let's setup the reference values, which will be used later on while viewing non-dimensional results (this setting doesn't affect the numerical solution).

Problem Setup > Reference Values
Select Compute from > inlet.

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