ANSYS AIM Tutorial Fluid Flow Through a Transition Duct

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1 ANSYS AIM Tutorial Fluid Flow Through a Transition Duct Author(s): Sebastian Vecchi, ANSYS Created using ANSYS AIM 18.1 Problem Specification Start Up Geometry Import Geometry Extracting Volume Suppress Mesh Set Mesh Size and Boundary Layer Generate Mesh Physics Set-Up Boundary Conditions / Forces Solution/Results Validation Reference

2 Problem Specification In this tutorial, the common transition duct pictured below will be the subject of our simulation. These are commonly found in office buildings and are sometimes used to vent air into the atmosphere. For this example, the temperature at the inlet of the transition duct is 73 degrees Fahrenheit. If air travels at 33 feet/sec at the inlet, find the velocity vectors inside the transition duct and find the pressure exerted on the transition duct from the air within.

3 Start Up A few words on the formatting on the following instructions: 1) Notes that require you to perform an action are colored in blue 2) General information is colored in black, but does not require any action 3) Words that are bolded are labels for items found in ANSYS AIM 4) Most important notes are colored in red We are ready begin simulating in ANSYS AIM. Open ANSYS AIM by going to Start > All Apps > ANSYS 18.1 > ANSYS AIM Once you are at the starting page of AIM, select the Fluid Flow template as shown below. In order to use the units given to us in the problem, press the Home menu button in the top left corner and select Units > U.S. Customary. You will be prompted by the Fluid Flow Template to either Define new geometry, Import geometry file, or Connect to active CAD session. Select Import geometry file and press Next.

4 Geometry Import Geometry Download the geometry from here and open it. Once successfully imported, enable the Compressible flow (ideal gas) option and press Finish. In most situations, air behaves closely enough to an ideal gas that it can be modeled as one. Extracting Volume Press Geometry in the Workflow panel and select Edit Geometry in the Geometry template. In the Prepare tab of the Model Editor, select the Volume Extract tool. Use the Select Faces tool to select the faces at the opposite ends of the duct. Lastly, use Select Seed Face to select a face inside the duct. This can be any face on the inside of the duct. Press the green checkmark and your interior flow volume will be extracted.

5 Below is what the flow volume will look like when the imported solid is hidden.

6 Suppress Now that the geometry of the flow volume has been created, we can suppress the imported solid geometry from the physics calculation. Right click on the Solid in the geometry Structure tree and select Suppress for Physics. Exit the modeler to continue with the simulation.

7 Mesh Initiate the meshing process by clicking on Mesh in the Workflow. Set Mesh Size and Boundary Layer Retain the default mesh settings. AIM will prompt you to fix the boundary layer before generating the mesh. Click on Boundary Layer under Mesh Controls. Select every face except for the inlet and outlet faces. Generate Mesh Return to the Mesh panel, then click Generate Mesh under Output or at the top of the screen by the status window for Mesh. AIM will detect that you are ready to generate the mesh and highlight the buttons in blue. Below is an image of the mesh.

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9 Physics Set-Up Select the Physics task in the Workflow. Boundary Conditions / Forces First, the inlet must be defined using the Fluid Flow Conditions. In the Add drop down menu by Fluid Flow Conditions, select Inlet. Then, using the face selection tool, define an inlet at the wide end of the duct. Input the Velocity magnitude as 33 [ft s^-1] and a Temperature of 73 degrees [F]. When entering the Velocity magnitude, be sure to enter the units correctly. After entering the value in ft s^-1, it will be automatically converted to the default units of in s^-1 and displayed accordingly. Once the inlet is defined, the outlet is next. In the same Add menu, define an Outlet at the small end of the duct. Assign a Gauge static pressure of 0 psi.

10 Next, a Wall condition must be added to all surfaces that are not already defined. Wall can be found in the same Add menu as the previous conditions. By default, the wall condition is thermally insulated. AIM will automatically create the Wall once the option is selected, automatically selecting every face that doesn't already have a constraint on it.

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12 Solution/Results Press the Results button in the Workflow to extract information from the simulation. In order to calculate the solution and find information that can be readily used, first press Evaluate Results. Once the evaluation is complete, AIM will automatically output a Vector in the Results section under Objects. The vectors show air velocity. Select the Vector to edit the settings with which it is defined. Change the Approximate number of points to 1000 to increase the density of vectors in the display, then change the Symbol length to Constant to make the smaller vectors easier to see. Press the Play button in the model window to see how these velocity vectors develop over time. To find the total pressure on the walls of the duct, r eturn to the Results task and select Contour from the Add dropdown menu. Select all of the outside faces of the flow volume (except the inlet and outlet) and change the Variable to Total Pressure. This will plot the pressure on the duct walls.

13 A temperature contour can be added by using the same process, but instead selecting Temperature as the Variable.

14 Streamlines are also an excellent way of visualizing the flow and are easily created in the Results section of the Workflow. Right click in an empty space of the model window and select Add > Results > Streamline. Change the Seed location to Inlet 1 and change the Approximate number of points to 100. Finally, press Evaluate to calculate and display the streamlines. A creative way of seeing how the streamline behaves over time is to change the appearance of it to show different things. For example, the line type can be changed in order to show how the streamlines rotate in response to the flow. Change the Line type to Ribbon and increase the Ribbon width to 2 or 3. Press the Play button in the model window to see how these streamlines develop over time.

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