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Training Manual
Chapter 2
The Example Problem
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Training Manual
• Flow of Air in a 2D duct….
• Objective: Peform laminar analysis of a relatively slow moving flow and then increase the flow rate dramatically.
Streamlines
An Example Problem !!
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Training ManualThe Geometry
• This is Duct which has a smooth transition to a larger area.
• Units of Length - Inches
– Inlet length 3.0
– Inlet height 0.5
– Transition length 1.0
– Outlet height 1.0
– Outlet length 4.0
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Training ManualProperties - Conditions
• Use PSI system of units
– Property type is AIR-IN
– Density will be 1.1214E-7 (lbf-s2/in4)
– Viscosity will be 2.6240E-9 (lbf-s/in2)
• Conditions
– Reference Pressure 14.7 psi
– Outlet Pressure 0 psi (relative pressure)
– Default Temperature used : 293K
• Flow
– Velocity of 10 inch/sec -> RE ~ 424 (laminar)
– Note in 2D the hydraulic diameter (used in the Reynolds Number) is twice the inlet height
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Training ManualSet Preferences
• Preferences provides a filter to prevent irrelevant information from being presented….
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Training Manual
1- Add
2 - Choose
3 - OK4 - Close
Establish Element Type
• Main Menu: Preprocessor-> Element Type->Add/Edit/Delete
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Training ManualGeometry - Create Inlet/Outlet Regions
• Preprocessor>Modeling>Create Areas> Rectangle (By Dimensions)
• First
– X1=0,X2=3
– Y1=0,Y2=0.5
– Click Apply
• Second
– X1=4,X2=8
– Y1=0,Y2=1
– Click OK
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Training ManualThe Two Rectangles
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Training ManualTransition Region Between Them
• Create a smooth transition line between the two
• Preprocessor>Modeling>Create Lines (Tangent to 2 Lines)
• Follow the Instructions carefully in the resulting PICKERS
– There will be four successive choices
• Check your result with the following page….
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Training ManualThe Smooth Transition Line
• Tangency to two lines requires choosing the proper endpoints…..
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Training ManualThe Transition Area
• Preprocessor>Modeling>Create> Area >Arbitrary
• Choose 4 keypoints in response to the PICKER, then OK
1 2
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Training ManualGeometry is Finished!!
Area Plot
Line Plot
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Training ManualBoundary Conditions
• Use Solid Model Boundary Conditions
– Do not require require re-application upon re-meshing
• Preprocessor>Loads>Apply>Velocity> Lines
• We will apply Velocities and Pressures
– Inlets are Velocity or Pressure
– Outlets are Pressure
– Walls: Velocities are zero
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Training Manual
Walls
Walls
Inlet:VX=10,VY=0
OutletPRES = 0
The Boundary Conditions
• These boundary conditions are typical
• Proper condition at boundary intersections is determined by FLOTRAN
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Training Manual
1-Lines
2-Pick These 6 Lines3-OK
4 - Input Values, Do Endpoints of lines…OK
Solid Model Boundary Conditions
• Example for Walls
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Training ManualRemaining Boundaries.
• Note that leaving a blank DOES NOT result in a zero condition being applied..
Inlet
Outlet
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Training ManualControl of the Display
This is a line plot after application of the Boundary Conditions
To prevent display of these symbols:Utility Menu: PlotCtrls>Symbols…
Choose NONE and OK
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Training ManualPreparation for Meshing
• Use the Mesh Tool
– Size Controls, Lines, Set
• PICKER shows up and you choose the lines,OK
– Set the number of divisions and the ratio, OK
• Use these settings for line divisions Line Divisions• Lines NDIV Ratio
• Transverse direction 12 -3
• Inlet Region - flow direction 16 -2
• Transition - flow direction 10 1
• Outlet - flow direction 18 2
• See next page for Mesh tool!
SAVE Database Before Meshing….
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Training ManualMesh Tool
1
2 - Choose Lines
3 - OK
Use FLIPif Line Biasis reversed
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Training ManualElement Size Box
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Training ManualProper Line Divisions
• Four Different Groups of Lines must be done for this problem…
• Remember to flip one of the outlet lines
• Generally, Avoid large adjacent element size changes
• The Four lines in the Y direction are the transverse lines
• Inlet Lines Transition Outlet Lines (Flipped!)
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Training Manual
1
2
3
4
5
Meshing Step
• Use the Mesh Tool
– 1: Choose Areas
– 2: Mapped
– 3: Quad
– 4: Mesh
• PICKER comes up
– Pick All
• (Meshing Occurs)
• 5: Close Meshtool
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Training ManualNow You Have a Mesh!
Picture Made with Reverse Video(PlotCtrls>Style>Color>ReverseVideo
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Training ManualNow for the FLOTRAN Input
• Enter FLOTRAN Setup through PREP7 or Solution
• (Depending on Program Setup, you may need to access “Unabridged Menu”)
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Training ManualFLOTRAN Setup
• We will be making changes to these portions of the Menu.
NOW - Our Initial Analysis
LATER - Follow On Work
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Training Manual
OK!
Execution Control
• Choose 50 Global Iterations to Start with
– We are not relying on the automatic termination criterion based on problem convergence
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Training ManualFluid Properties
• Choose AIR-IN for the property type for Density and Viscosity using scroll down menu….
• OK
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Training ManualResulting Screen (click OK)
(Thermal conductivity and Specific Heat not needed)
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Training ManualFlow Environment
• Reference Conditions are found as a subset:
• Pressure: 14.7Psi
• Nominal Temperature: 70F
• Offset Temperature: 460R
• OK!
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Training ManualFLOTRAN Execution
• Done in SOLUTION:
• Run FLOTRAN
• Execute 50 iterations, look at the results and then run 50 more…
• Convergence monitors indicate the normalized rate of change of the solution
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Training ManualConvergence Monitors
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Training Manual
More Convergence Monitors
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Training ManualPost-Processing
• FLOTRAN Post-Processing is fairly typical of ANSYS
– Explicity read in a set of results (not automatically loaded)
• Velocity Vectors
• Nodal Solution Plots
– Solid Color
– Line Contours
• Path Plots
• Particle Traces
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Training ManualVelocity Vectors
• Plot Results>Predefined Vector Plot..OK
(Use this for Nodal Solution Plots….)
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Training ManualVectors - Typical
50 Global Iterations
after 100 Global Iterations
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Training ManualNodal Results Plotting
• Show up as solid color plots or lines depending on the device chosen
– Utility Menu>Plot Ctrls>Device Options
• Shading or Contours
• Choose DOF
• OK
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Training ManualPressures (50, 100 Global Iterations)
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Training ManualResults
• We can’t tell the difference between the velocity vector plots, but it looks like the pressures have changed slightly.
• We also notice that the Convergence Monitors (Normalized rate of change of each DOF) have leveled off….
– This implies solution is slightly oscillatory
• We will modify the input slightly, choosing the SUPG (Streamline Upwind Petrov-Galerkin) formulation for the momentum equations…
– The SUPG algorithm is less diffusive and more accurate (but sometimes less robust) than the default algorithm (MSU - Monotone Streamline Upwind Method)
• Also, set the number of Global Iterations to 100
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Training ManualChanging Advection
• FLOTRAN Setup > Advection
OK!
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Training ManualConvergence with SUPG
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Training ManualResults
• We could continue, but you get the idea...
• Use of SUPG has given enhanced convergence.
• We should expect a less diffusive solution, and so the re-circulation region may be better defined.
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Training ManualComparison of Vectors at 100, 200 GI
Vectors at 200 GI show more extensive recirculation
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Training ManualPath Plots
• Look at the profile of VX along the outlet
• Procedure -Path Operations
– Define Path by Nodes
• Choose Nodes on either corner of the outlet
– Map Onto Path
• Choose VX and label it
– Plot
• On graph
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Training ManualSet up the Path Plot
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Training ManualPath plot
• Pick the corner nodes
• OK
• Name the Path
• OK
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Training Manual
• Read and dismiss the PDEF (path definition) box
• Map Onto Path, Choose VX, Give it a Name, OK
Path plot - Still More
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Training ManualPath Plot - Almost Done
• -Plot Path Item on Graph
• Choose DOF, OK
• And Then…...
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Training ManualPath Plot of the Outlet Velocity!
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Training ManualSome Discussion
• Modify the line colors as needed with the Utility Menu
– PlotCtrls>Style>Colors>Graphs
• Modify the plot controls as needed with Utility Menu
– PlotCtrls>Style>Graphs
• Result
– Fully developed flow would show the outlet velocity profile as a perfect parabola
– Therefore, the problem domain could be lengthened to provide room for more flow development
– Check the Mass Balance
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Training ManualThe Print File
• Use the Utility Menu to Look at the bottom of the jobname.pfl file
• List > Files> Other> (choose jobname.pfl file)
• Scan to the bottom
• Mass balance looks good!!!
• (If you had forgotten to put a No-Slip Boundary condition somewhere, there would be another outlet listed….)
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Training ManualMassless Particle traces
• Particle traces are based on the velocity field, not the stream function.
• For a steady state, perfectly converged problem on a perfect mesh, the streamlines and particle trace plots would be identical.
• Procedure: Plot Results>Flow Traces
– Define trace points with PICKER
– Plot Flow Traces
• Optionally color code trace with
• the value of a DOF
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Training ManualTrace Points
• The points defined on the Working Plane
– Ensure, for 3D models, that the WP is correctly located!
• The resolution of the trace point location is controlled by the currently set Snap Increment (Working plane controls)
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Training ManualParticle Trace
• Color Code According to PRES (or something else!)
• Note Maximum number of loops allowd
• OK
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Training ManualParticle Trace
• The maximum number of loops is exceeded in the recirculation region. Close the box
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Training ManualNew Analysis
• Increase the velocity from 10 to 200 (no other changes)
– This makes the Reynolds Number ~8500
• Solve the problem
– Very Shortly you will get a message that either the solution has diverged or that a negative value has been encountered in the coefficient matrix main diagonal.
• This is because the flow is now in the turbulence regime and a laminar solution will be unstable.
• So activate the turbulence model and again solve.
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Training ManualFLOTRAN Solution Options
• Activate Turbulence with Scroll Down Menu Option,OK
• SOLVE
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Training ManualConvergence Monitors
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Training ManualTurbulent Flow Results
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Training ManualDiscussion
• Note that the maximum value of pressure is no longer at the inlet. It has moved to the outlet !!!
• Consider Bernoulli’s equation and note that in our new, higher velocity problem the relative importance of the viscosity has decreased.
– The recoverable pressure due to the velocity change now dramatically outweighs the viscous losses.
unrec2c
22
21c
21
1 Pghρg2
VPghρ
g2
VP
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Training ManualNew Outlet Velocity Profile
• A Fully developed flow would have the maximum value in the center. This implies we should make the problem domain longer….
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Training Manual
Another New Analysis - Extend the Problem
• Outline of the steps required
– Add another rectangle to the outlet
• Try 15 additional inches
• Remember to Merge Keypoints
– Revise boundary conditions
• Delete old pressure boundary
• Pressure on new boundary
• Walls
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Training ManualNew Geometry
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Training Manual
As expected, the meshed line is kept
Completing the New Geometry
• Preprocessor>Numbering Ctrls>Merge Items> Keypoints
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Training ManualNew Boundary Conditions
• Preprocessor>Loads>Delete>
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Training ManualNew Boundary Conditions
• Add the walls as done previously
• Assign Zero pressure to the outlet line as done previously
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Training ManualMesh the New Outlet Area
• Use the Mesh tool to copy the transverse direction assignment to the new outlet boundary.
• PICKER asks for the line to be copied from (pick andOK) and then the line to be copied to (pick and OK).
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Training ManualSet Divisions Along Outlet
• Flip the line after setting (when necessary)
• Then Mesh the New Area as in previous fashion
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Training ManualThe Mesh
• Note that the symbols shown are the previously transferred Nodal Boundary Conditions. The solid model boundary conditions don’t show up on an element plot.
• The complete mesh with the symbols turned off..
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Training ManualExecute
• Note that if you have not changed the jobname, FLOTRAN will provide a notice to the effect that it has renamed the old results file to jobname.rfo
– This occurs because the number of nodes and elements in the case has changed.
– Other files such as the jobname.pfl are appended to, even though this is a new analysis
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Training ManualConvergence Monitors
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Training Manual
Convergence Monitors - more
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Training ManualResults
• Two representations of pressure, the second in the vector mode with 128 contours.
• Note that the pressure drop, once the flow has recovered, is very small.
• We expect a good outlet velocity profile.
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Training ManualRecirculation Region
• The recirculation region is captured despite the relatively coarse mesh.
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Training ManualBenefits of Extension
• The lines are superimposed onto the new velocity vector plot. It is clear that the flow is continuing to develop past the original boundary of the problem.
Old Outlet
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Training ManualOutlet Velocity Profile
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Training ManualFinal Check of Transverse Velocity
• The flow is very close to fully developed. The following plot of the transverse velocities at the outlet provides a measure of how close it is. Note the scale. The maximum transverse velocity is 0.11.
End ofProblem!
