Carnegie Mellon

Mechanical Engineering

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FEM/ANSYS

 

F3 - Heat Analogy - Seepage
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Fluid #2: Flow Around An Airfoil USING HEAT ANALOGY

 

Introduction: In this example you will model air flow over an airfoil.

Physical Problem: Compute and plot the velocity distribution over the airfoil shown below.

Problem Description:

         The chord of the airfoil has dimensions and orientation as shown in the figure.

         The flow velocity of air is 2m/s.

         Objective:

            To plot the velocity profile around the airfoil.

            To graph the velocity distribution above and below the airfoil.

         You are required to hand in print outs for the above.

         Figure:

 

 

IMPORTANT: Convert all dimensions and forces into SI units.

 

Starting ANSYS:

 

         Click on ANSYS 6.1 in the programs menu.

         Select Interactive.

         The following menu comes up. Enter the working directory. All your files will be stored in this directory. Also under Use Default Memory Model make sure the values 64 for Total Workspace, and 32 for Database are entered.  To change these values unclick Use Default Memory Model.

 

 

         Click RUN

 

MODELING THE STRUCTURE

 

         Go to the ANSYS Utility Menu

        Click Workplane>WP Settings

        The following window will appear:

 

 

         Check the Cartesian and Grid Only buttons

         Enter the values shown in the figure above.

         Go to the ANSYS Main Menu.

        Click Preprocessor>-Modeling-> and create a rectangle of dimensions 3mX2m. The created rectangle should look like the figure below:

 

 

        Creating the airfoil.

         Go to Main Menu>Preprocessor>-Modeling->Create>Keypoints.

         Create the keypoints as shown in the figure below:

 

 

         Note that I have only plotted the boundary lines of the rectangle created before to make it easier to see the keypoints.

        Go to Main Menu>Preprocessor>-Modeling->Create>-Lines->Splines thru Keypoints

         Create two splines through the top three and the bottom three splines. The figure should look like the one below:

 

 

          

         Now create an area enclosed by the two splines. Go to Modeling>Create>Areas>Arbitrary>By Lines.

         Pick the two splines and click OK. The model should look like the one below.

 

         Now subtract this airfoil area from the rectangle area.  Go to Postprocessor>Modeling>Operate>Booleans>Subtract>Areas and then select the larger area, then the airfoil.  The figure should look like the following:

 

         The modeling of the problem is done.

 

ELEMENT 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:

 

 

         Type 1 in the Element type reference number.

         Click on Thermal Solid and select 8node. Click OK. Close the 'Element types' window.

         So now we have selected Element type 1 to be a thermal solid 8node element. The component will now be modeled with thermal solid 8node elements. This finishes the selection of element type.

 

MATERIAL PROPERTIES

 

         We will model the fluid flow problem as a thermal conduction problem. The flow corresponds to heat flux, pressure corresponds to temperature difference and permeability corresponds to conductance.

         Go to the ANSYS Main Menu

         Click Preprocessor>Material Props>-Constant->Isotropic. The following window will appear

 

 

         Enter 1 for the Material Property Number and click OK. The following window will appear:

 

 

         Fill in 1 for Thermal conductivity. Click OK.

         Now Material 1 has the properties defined in the above table. This represents the material properties for the fluid in the channel.

 

MESHING:

 

         DIVIDING THE CHANNEL INTO ELEMENTS:

         Go to Preprocessor>Meshing>Size Cntrls>ManualSize>Global>Size. In the window that appears type 0.1 in the field for 'Element edge length'.

 

 

         Click on OK. Now when you mesh the figure ANSYS will automatically create a mesh, whose elements have a edge length of 0.1 m.

         Now go to Preprocessor>-Meshing->Mesh>Areas>Free. Click Pick All. The mesh should look like the following:

 

 

BOUNDARY CONDITIONS AND CONSTRAINTS

 

         Go to  Preprocessor>Loads>-Load->Apply>-Thermal->Heat Flux>On lines. Pick the left line of the rectangle. Click OK. The following window will appear:

 

 

         Enter 2 in the HFLUX value field and click OK. the 2 corresponds to the inlet velocity of 2 m/s.

         Repeat the above for the right line of the rectangle. Remember this is negative since it is leaving this side of the system, so include the negative sign.

         The figure should look like the one below:

 

 

         Now the Modeling of the problem is done.

         Go to Utility Menu>PlotCtrls>Symbols. The following window will appear:

 

 

         Fill in the values as shown and click OK. This sets up the arrow symbol to denote the heat fluxes, which in turn represent the fluid velocity.

         Now go to the Utility Menu>Plot>Lines.

 

SOLUTION

 

         Go to ANSYS Main Menu>Solution>-Analysis Type->New Analysis.

         Select "Steady State" and click on OK.

         Go to Solution>-Solve->Current LS.

         Wait for ANSYS to solve the problem.

         Click on OK and close the 'Information' window.

 

POST-PROCESSING

 

         Plotting the velocity vectors

         Now go to General Postproc>Plot Results>-Vector Plot->Predefined. The following window will appear:

 

 

         Enter the values as shown and click OK. The plot of velocities will look as follows.

 

 

         To plot the graph of variation of the velocity along the airfoil.

         Go to Utility Menu>Plot>Areas.

         Go to Main Menu>General Postproc>Path Operations>Define Path>On Working Plane

         Pick points at the ends of the elbow as shown. We will graph the velocity distribution along the line joining these two points.

 

         The following window will appear:

 

 

         Enter the values as shown.

         Now go to Main Menu>General Postproc>Path Operations>Map onto Path. The following window will appear:

 

 

         Now go to Main Menu>General Postproc>Path Operations>-Plot Path Items->On Graph. The following window will appear. Select VELOCITY and click OK.

 

 

         The graph should look as follows:

 

s>Define Path>On Working Plane+   

       

 

 

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