1. Problem Definition
  2. PreProcessing
  3. Solving
  4. Visualizing Vector Plot on the Symmetry Plane

This second simulation is that of a turbulent, incompressible flow of water at 20C using the same rectangular duct as in the first part (Re ~ 100000).

To simplify the problem, the grid will not be generated since it already exists in CFX tutorials files. Therefore, the preprocessing file is loaded as previously by copying the directory and folder TASC_DIR/Examples/tutorials/bcc/rct.turb/*. The mesh is represented below :

Fig. 1 : Visualization of the grid

Click here to animate the grid (MPEG file - 8447 Ko).

Click here to see the grid and the problem parameters.

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As in the first example, liquid water ( Water@STP(SI) ) is selected in the selection list of the "Select a Material" sub-panel. Furthermore, a turbulence model has to be specified for the turbulent flow. If the "Turbulence" button in the "Models and Material Properties" is on, the following panel appears (fig. 2) :

Fig. 2 : Turbulence Model Setup Panel

 

In this problem, the "k-epsilon models" is used to simulate a turbulent flow in a rectangular duct, and the near wall model is set to "High-Re Models".

 

Creating Boundary Conditions

Boundary conditions for a turbulent flow are created in the same way as the first example.

 

Generating an Initial Guess

Pressure value is set to 0 and U value is set to 0.33.

Check PreProcessing and save information in a file as in the first example.

 

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Setting Solver Parameters

The fluid time step toggle is turned ON and the fluid time step value is set to 6.

The Solver Monitor is run like in the first party.

 

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Fig. 3 : Some results obtained with the visualizer

- Top left-hand corner : Contour map of pressure

- Top right-hand corner :Velocity vector plot on the symmetry plane

- Bottom : Velocity vector plot on cross-sections.

Since the flow is turbulent, the velocity gradient near the centre of the duct is rather small, whereas it increases near the walls. The pressure decreases along the duct like in the laminar flow case.

 

Creating a profile

In the "Visualization Object Manager" panel,an "XY Graph" object is created. The type of 1D Region is set to User Defined. The region must be selected : in this example, the coordinates are [15,1:10,8]. In other words,the x-coordinate is 15,the z-coordinate is 8 and the y-coordinate varies from 1 to 10.

 

The wall is located to Y=0,2 m and the symmetry boundary condition is set to Y=0m in the following pictures.

Fig. 4 : Velocity profile in a cross-section

The figure shows that the velocity is null in the region near the wall, while its gradient is very high. The velocity quickly reaches its maximum value in the middle of the duct. Hence, the profile seems to be approximatively correct.

 

Fig. 5 : Production of turbulence in a cross-section

The production gradient is rather big near the wall. The value of the turbulent production is very high near the wall, whereas it remains small in the middle of the duct : therefore, the profile seems to be in agreement with the theory.

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