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This 4 page Class Notes was uploaded by Dr. Kim Rosenbaum on Saturday September 19, 2015. The Class Notes belongs to CHEG341 at University of Delaware taught by Staff in Fall. Since its upload, it has received 44 views. For similar materials see /class/207186/cheg341-university-of-delaware in Chemical Engineering at University of Delaware.
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Date Created: 09/19/15
CHEG 341 CFD MODELING USING FLUENT Fluent consists of two separate programs 7 Gambit and Fluent Gambit is used to construct the flow geometry along with the mesh for solving the equations of motion and continuity at fixed points Fluent 6216 is the program which actually solves the equations for the geometries constructed using Gambit GAMBIT OPERATION 1 Before using either Fluent or Gambit in eCALC or the Dell Lab you must set the environment Using the Windows dropdown menus enter Start 9 Programs 9 Fluent Inc Products9 Fluent 6216 9 Set Environment Answer Yes and OK when prompted 2 Create a new geometry file using Gambit Go to Start 9 Run and type cmd At the Z prompt type C At the C prompt type gambit This will create a new gambit file After about 10 seconds a large black screen with a Cartesian coordinate system will appear MINIMIZE but DO NOT CLOSE the smaller black command win ow 3 In Gambint click on the grey cube the Geometry button Then click on the fourth button which looks like a white cube A new button menu called Volume should appear 4 RIGHT click on the icon that looks like a white shoebox the second button This will reveal a drop down menu of different 3D shapes Select Frustrum for this practice session Create a frustrum with a height of 27 with Radius 1 and Radius 2 both 155 and Radius 3 10 we will insert units later in Fluent rather than Gambit Make the Axis Location Centered X Label the tube LAD When you have entered all this info click the Apply button If you want your tube to fill the entire screen Click the button with the triangle near the bottom of the screen under the word Active 5 Right click on the frustrum symbol and select Torus as the next geometry to build Choose Radius 1 as 1275 and Radius 2 as 05 Label this Stenosis and center it on the X axis Click Apply 6 Right click on the icon of the two intersecting circles and select Subtract from the drop down menu Pick LAD as the Volume and Stenosis as the Subtract Volumes Click Apply Your tube will now have a toroidal chunk missing from its center If you mess up the Undo button is the curved blue arrow at the bottom right hand of the screen 7 Choose Solver 9 Fluent 56 DO NOT SKIP THIS STEP 8 From the Operation menu at the top select Zones third button from the left a bluegreen divided cube You will now label the boundaries of your LAD Click the left cube white one with blue side to specify boundary types Click the dark black arrow next to the yellow box to bring up the list of boundaries Select facel from the Available list and transfer it to the Picked list using the arrow Facel will then be highlighted in red You can also pick an entity by leftclicking on it while you hold down the Shift key If this is your exit face name it Outlet and select OutletiVent as the Type from the dropdown menu 9 Repeat step 6 for the inlet Type is MassiFlowiInlet The remaining faces are considered Walls as default 10 Next create the mesh Click on the Yellow cube in the Operation menu Click the White cube 43911 button under Mesh and then the first button Cube with Pencil under Volume Pick LAD as your Volumes and choose TetHybrid as your Elements Enter 04 as your Spacing then click the Apply button Note that 04 mm will be too coarse of a mesh for your actual study but we will use a coarse mesh here so that the calculations take less time ll Finally Export your mesh Choose File 9 Export 9 Mesh from the top menu bar Browse for the location on the scratch drive where you store your personal files eg your thumb drive and save the file with filenamemsh be sure to include the msh You may now exit Gambit FLUEN T OPERATION 1 Before using either Fluent or Gambit in eCALC or the Dell Lab you must set the environment Using the Windows dropdown menus enter Start 9 All Programs 9 Fluent Inc 9 Set Environment Answer Yes and OK when prompted You do not need to repeat this step if you already did it once since sitting down at the computer 2 Start up Fluent Start 9 Programs 9 Fluent Inc 9 Fluent 6216 Select 3d to RUN 3 Read in the desired geometry From the Fluent drop down menu choose File 9 Read 9 Case 9 Look in Location where you stored your mesh C is default Fluent will then open the mesh file which you created 4 Check the grid Grid 9 Check 5 Set the Scale of the grid to be mm This is important because the default is meters and larger diameters will result in larger Reynolds numbers Select Grid 9 Scale and enter mm as the unit the grid was created in then click SCALE and close the window 5 Display the grid Diplay 9 Grid Be sure the surfaces are highlighted and click Display The grid should come up in a black window on your screen 6 Tell Fluent that your fluid is blood Select Define9Materials Change the name from air to Blood and replace the density and viscosity of air with those of blood Click ChangeCreate and agree when Fluent asks if it s OK to overwrite air Close the dialog box 7 Tell Fluent this will be a laminar flow problem Choose Define 9 Models 9 Viscous 9 Laminar 9 8 Input the correct inlet flow rate by selecting Define 9 Boundary Conditions 9 Inlet 9 Mass Flow Inlet 9 Set Enter 000167 kgsec as the mass flow rate Click OK 8 Initialize the grid with numerical values You only have to do this the FIRST time you run a calculation with a new Fluent geometry For all subsequent times you can just start from the last calculated values which is usually faster Choose Solve 9 Initialize 9 Initialize 9 Init You can accept the default values Close the window when finished initializing 9 Solve the problem If you want to monitor the progress of the solver as it works first choose Solve 9 Monitors 9 Residual and check both the Print and Plot boxes and click OK This is useful in order to catch any mistakes you may have made in solving a problem Then choose Solve 9 Iterate and select the number of iterations to be 200 then click Iterate If it takes more than 200 iterations simply click the Iterate button again until you get a Solution is converged message on the screen 10 Display the pressure profiles Choose Display 9 Contours 9 Pressure Select defaultinterior from the Surfaces list Choose the options you prefer 7 Filled is quite impressive Click Display A color image of the pressure contours will be displayed You can rotate this image to see inside the pipe by drigging any point on the screen ll View the outlet velocity profile by rotating the pipe to look at it endon or by returning to the Contours window and selecting outlet as the surface remember to deselect defaultinterior 12 You may wish to save your case and data to your own folder by choosing File 9 Write 9 Case and 13 You can copy pictures for your report as a JPEG or other formats Choose File 9 Hard Copy 9 JPEG Choose the Color option and the Reverse ForegroundBackground option to avoid wasting the expensive color toner on the black background 14 You can save numerical data from a fixed surface by choosing File 9 Write 9 Profile and then selecting the surface and value you wish to record eg Outlet and Velocity Save the file as filenamecsv this format can be opened in Excel In your csv file you will get a single column listing first all the x coordinates for your chosen surface followed by the ycoordinates the z coordinates and the value eg velocity associated with the respective xyz coordinates You will probably want to convert this data into a 4column listing so that each row contains the x y and z coordinates followed by the velocity MODIFYING AN EXISTING GEOMETRY BEFORE MESHIN G You should use this procedure when modeling the bifurcation in the left main coronary artery Because geometry construction in Gambit is tedious we have built for you a model of the healthy no stenosis bifurcation For your project you will have to mesh this geometry as well as add stenoses and bypass grafts These instructions explain how to open the file in Gambit l The geometry file of the healthy bifurcation is located on the classfiles Y drive in eCALC and the Dell Lab as YCHEG34l2005Fluentartery7healthydbs However because students do not have write privileges to this drive you must first copy the file onto your own accessible media eg your C drive Otherwise you will NOT be able to open the file 2 Start up Gambit using the instructions 1 and 2 under Gambit Operation above Open the geometry file File 9 Open In the ID box type the address of your file eg Farteryhealthy DO NOT append the dbs to the arteryhealthy name Agree to save the current session 4 Continue with step 10 under Gambit Operation A L V SOME COMJVIENTS ON BOUNDARY CONDITIONS When you have a single inlet and a single outlet as we had in the example above you can either specify the inlet flow rate or velocity and Fluent will calculate the pressure at the inlet or else you can specify the inlet pressure and Fluent will calculate the velocity Note that specifying an Outlent Vent as an exit BC sets the pressure there to 0 gage When you model a geometry in Fluent which has more than one outlet you must provide boundary conditions for each inlet and outlet Unfortunately you can provide a mass flow BC m for inlet boundaries outlet BCs must be specified as pressure BCs Hence a trialanderror procedure is necessary even for a stenosisfree bifurcation Using the known pressure at the LM inlet you must guess the pressures at the LCX and the LAD exits these will likely be different until your Fluent simulation reports the correct flow rates for the healthy LCX and LAD A note about numerical simulations Fluent solves the flow equations to within a certain tolerance 7 the default value of the residual is 0001 So if you are solving for the pressure drop across a vessel where the entrance pressure is 10000 Pa Fluent will consider the problem solved when the exit pressure is calculated to within 10 Pa 10 Pa 10000 Pa 0001 However if the total pressure drop across your vessel is supposed to be 20 Pa note that Fluent will only be able to calculate the pressure drop to within 50 ie to 20 Pa 10 Pa Although you can fix this problem by lowering the tolerance say to 000001 this Will dramatically increase the computation time and may even run up against the limit of single precision computations So a trick for avoiding this problem is to set the initial pressure to a value that is lower than What you know the true value is say to 100 Pa instead of 10000 Pa Then Fluent Will calculate your exit pressure precise to 01 Pa at a tolerance of 0001 so your AP will be 20 01 Pa which is correct to Within 05 ONLIN E USER S MANUAL FOR FLUEN T If you Wish additional information about using Fluent you can access the User s Manual on the X Drive in eCALC or the Dell Lab In eCALC go to X applications on archim edes 9Fluentlnc9Fluent609help9index htm and click on the User s Guide In the Dell Lab go to ChE quot 39 Fluemluc 1fluent62D 39 and click User s Guide