• Document: Setting up Oscillatory Moving Boundaries in Fluent using UDFs. Laboratory for Product and Process Design. By Madhawa Hettiarachchi
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Setting up Oscillatory Moving Boundaries in Fluent using UDFs Laboratory for Product and Process Design By Madhawa Hettiarachchi 10/31/2011 1 Moving Boundary: The moving boundary problems encountered in biological systems are periodic in nature. Therefore, when a motion of a periodically moving wall is simulated, the motion of the each node of the wall needs to be controlled independently. Thus, we want to update the position of each node based on the deflection due to fluid-structure interaction. In Fluent, we can define UDFs using DEFINE_GRID_MOTION as shown below to specify the motion of each node on a boundary or interface. DEFINE_GRID_MOTION( name, d, dt, time, dtime) Argument Type Description symbol name UDF name. Domain *d Pointer to domain. Dynamic_Thread *dt Pointer to structure that stores the dynamic mesh attributes that you have specified (or that are calculated by ANSYS FLUENT). real time Current time. real dtime Time step. Function returns void There are five arguments to DEFINE_GRID_MOTION: name, d, dt, time, and dtime. You supply name, the name of the UDF. d, dt, time, and dtime are variables that are passed by the ANSYS FLUENT solver to your UDF. NOTE: The UDFs that are defined using DEFINE_GRID_MOTION can be executed only as compiled UDFs. Please refer to fluent UDF manual for more information. 2 Method: Since the pulsatile nature of the wall motion, it is important that the node positions of the moving wall are returned to their original positions at the end of the cycle. It is very important to establish the original positions of the mesh at the end of each cycle, otherwise the mesh can be gradually deformed with time and lead into negative mesh volumes which could abruptly stop the simulations. Step1: Original node locations (coordinates) of the moving wall/interface need to be written to a file, which will be used to update the original positions of the nodes. This can be done in fluent after the mesh file is loaded. We will use the mesh file of the 3D brain CSF space as an example in this report. (Please see the mesh, fluent case and UDFs provided with this report. S:\For Madhawa\Reports). Forebrain Moving boundary Ventricle Moving boundary Choroid plexus CSF generation Lower brain Wall Sagittal plane Symmetry Fig.1 shows the 3D geometry of the Brain CSF space. The parenchyma wall shown in green color and ventricle surfaces shown blue color are moving boundaries. 1. Start Fluent (Win 32 system –start Fluent from visual studio command prompt , Win 64 system- start fluent from SDK command prompt. Go to command prompt and type fluent. Refer appendix 1 for more information) 2. Read the mesh file in to the fluent (3DBrainMWChroidArchnoidVilli2.msh). 3. Check the mesh for completeness. 3 4. The current dimension of the mesh is too large. Scale the mesh by a factor of 0.00094. It will bring the size of the mesh into more realistic values. Save the case file. 5. Go to File ExportSolution Data 6. Write the coordinates to a file. MovingParenchymacordinates.out. The format of the file is shown below. There are total of 35573 moving nodes. 7. Repeat the same steps for other moving walls. Create MovingVentriclecordinates.out file. There are total of 4669 moving nodes. 8. Setting the problem parameters. DefineGeneraltransient Model keep the default setting (viscous-laminar flow)  Materialselect Fluid Create/EditFluent Databaseselect Water- 4 liquid from the Fluent Fluid Materials and click Copy. Now you will see Fluid- water in the material panel under the Fluid. DefineCell Zone Conditionsselect Zone in panelEditMaterial Name— select Water-liquid from the drop down menu. Click OK. 9. Setting up boundary conditions Set the zone ‘symmetry’ to type symmetry. Set Choroid mass flow inlet to 3.33e-6

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