Tutorial 4: Flow over Infinite Cylinder 

ntroduction: In this example laminar fluid flow over an infinite cylinder.  Physical Problem: Compute and plot the velocity distribution of a flow of air over an infinite cylinder.  Problem Description:

Objective: o To plot the velocity profile

Dimensions: o The cylinder is .25 m in diametero Because of symmetry, the problem is simplified so that only half of the cylinder is computed. The results

are assumed to be the same on the other side of the x axis (axis of symmetry)o The flow area is  2 m by 0.5 m.  This arbitrary size serves to set up the boundary conditions of flow over

a cylinder.o The velocity of the air at infinite distance from the plate is 5 mm/s (.005m/s).  

Figure:

     

Basic Outline of the Problem: Preprocessing:1. Start ANSYS.

2. Create areas.3. Define the material properties.4. Define fluid element type. (2D Flotran 141 element, which is a 2-D element for fluid analysis.)5. Specify meshing controls / Mesh the areas to create nodes and elements.  Solution:6. Specify boundary conditions. 7. Specify number of solution iterations.8. Solve. Postprocessing:9. Plot the contour plot of the velocity distribution.10. Plot the velocity plot of the velocity distribution. Exit:11. Exit the ANSYS program, saving all data.  STARTING ANSYS 

         Click on ANSYS 5.6 in the programs menu.         Select Interactive.         The following menu that comes up. Enter the working directory. All your files will be stored in this directory.

Also enter 64 for Total Workspace and 32 for Database.         Click on Run.

 

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MODELING THE STRUCTURE: Go to the ANSYS Main Menu Click Preprocessor>Modeling>Create>Areas>Rectangle>By 2 Corners. The width is 2 and the

height is .5. Starting position is at (0,0).

 

To create the cylinder click Preprocessor>Modeling>Create>Areas>Circle>Solid Circle. The radius is .125 and the center is at (.5,0)

To subtract the circular area from the rectangle click Preprocessor>Modeling>Operate>Booleans>Subtract>Areas. Select the rectangle and click OK. Next select the circle and click Ok. You will be left with this area:

Modeling is now doneELEMENT PROPERTIES SELECTING ELEMENT TYPE:

         Click Preprocessor>Element Type>Add/Edit/Delete... In the 'Element Types' window that opens click on Add... The following window opens.

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          Type 1 in the Element type reference number.         Click on Flotran CFD and select 2D Flotran 141. Click OK. Close the Element types window.         So now we have selected Element type 1 to be solved using Flotran, the computational fluid dynamics portion

of ANSYS. This finishes the selection of element type. MESHING DIVIDING THE CHANNEL INTO ELEMENTS:

Go to Preprocessor>Meshing>Size Cntrls>ManualSize>Lines>Picked Lines. Click 'Pick all' or manually pick all the lines

In the window that comes up type .02 in the 'Element edge length' box:

Now Click OK.  Now go to Preprocessor>Meshing>Mesh>Areas>Free. Click the area and the OK.

Fluid Properties (air): 

Go to Preprocessor>Flotran Set Up>Fluid Properties. On the box, shown below, set the first two input fields to Air-SI. Then click on OK.

 

 BOUNDARY CONDITIONS AND CONSTRAINTS 

To make loading easier, go to Plot>Lines on the main file menu. Go to  Preprocessor>Loads>Define Loads>Apply>Fluid CFD>Velocity>On lines. Pick the left and top

edge of the rectangle and Click OK:

<!--[if !vml]--><!--[endif]--> Enter 0.005 in the VX field and 0 in the VY. Leave VZ blank. Then click OK. This number corresponds to the

velocity of 0.005 meter per second of air flowing from the left side:

To account for symmetry, the normal component to the axis of symmetry is set to zero. To set this go to Preprocessor>Loads>Define Loads>Apply>Fluid CFD>Velocity>On lines. and select the bottom line of

the rectangle. Enter 0 in the VY field. Leave VX and VZ blank. Repeat the above and set the Velocity to ZERO for the 2 lines that make up the half-circle.  (VX=VY=0, VZ

can be left blank). This is due to the no-slip condition along the cylinder. Your screen should look like this so far:

<!--[if !vml]--><!--[endif]--> The last step is to apply atmospheric pressure to the outlet of the fluid region. Go to

Preprocessor>Loads>Define Loads>Apply>Fluid CFD>Pressure DOF>On Lines and select the right most vertical line and click OK. In the window that appears, enter 0 for the constant pressure value and click OK:

Now the Modeling of the problem is done. The loads on the model will look like this:

SOLUTION 

Go to ANSYS Main Menu>Solution>Flotran Set Up>Execution Ctrl.          The following window appears.  Change the first input field value to 1000, as shown.  No other changes are

needed.  Click OK.

<!--[if !vml]--><!--[endif]--> Go to Solution>Run FLOTRAN. Wait for ANSYS to solve the problem.

 POST-PROCESSING 

Plotting the velocity distribution… Go to General Postproc>Read Results>Last Set. Then go to General Postproc>Plot Results>Contour Plot>Nodal Solution. The following window appears:

Select VSUM and click OK. The velocity distribution will look like this:

Now go to General Postproc>Plot Results>Vector Plot>Predefined and select Velocity. The vector plot looks like this:

To view the recirculation zone in more detail it helps to animate the results. To do this go to General

Postproc>Plot Results>Def Trace Pt. Select a few nodes in the entrance region (1) and select a few nodes in the recirculation region (2). Then click OK:

Now go to PlotCtrls>Animate>Particle Flow... Select VX. The other default options are fine. Click OK

ANSYS will then animate the flow. It will look similar to this (notice the recirculation region):