CFX Tutorial Ductflow Laminar v4p00

7/28/2019 CFX Tutorial Ductflow Laminar v4p00

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ANSYS CFX Tutorial Laminar Flow in a Rectangular Duct 22 January 2013 V4.00

Department of Mechanical Engineering Page 1 of 25University of Manitoba

ANSYS CFX Tutorial

Laminar Flow in a Rectangular Duct

Scott J. Ormiston

Jeffrey R. Berg

Department of Mechanical Engineering

University of Manitoba

Winnipeg, Manitoba

Canada

V4.00

22 January 2013


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IntroductionThis tutorial has been adapted from a tutorial created by Jeff Berg (M.Sc. student) in 2004. That tutorial was bas

on running the CFX-TASCflow (V2.11) rct.lam tutorial in CFX-5 (v5.7).

Geometry Nomenclature

The duct has a length xL , a height yL , and a depth zL .The duct length is aligned with thex axis, the depth w

they axis, and the height with thezaxis. The flow is assumed to be symmetric about anx-zplane that bisects t

duct in they direction and therefore only half the duct is modelled. One corner of the duct is assumed to lie at torigin. Figure 1 below shows the duct geometry. When the geometry was defined in the creation of t

computational mesh, all faces of the domain were assigned names. The names of the inlet and outlet planes (

0x andxLx ) are RCT_W and RCT_E, respectively. The names of the planes at 0y and yLy are RCT_

and RCT_N, respectively. The names of the planes at 0z and zLz are RCT_B and RCT_T, respectively.

Figure 1: Rectangular Duct Geometry

Problem DefinitionThe problem is a laminar, incompressible, constant property flow of water in a rectangular duct. The code will run with the heat transfer model turned off (even though an alternative approach would be to run the code with t

heat transfer model as isothermal and specify the desired temperature for an isothermal flow). The flow

modelled with a rectilinear uniform grid for half the domain using symmetry in the y direction.

The problem parameters are:

Mass flow = 3.962 x 10-2

[kg / s] for the full duct. The mass flow rate at the inlet of the half duct is therefo1.981 x 10

-2[kg / s].

Density = 997.0 [kg / m3].

Viscosity = 8.899 x 10-4 [kg / m s].

Duct length = 2.00 m ( xL ).

Duct height = 0.40 m ( yL ). The actual grid height is 0.20 m due to symmetry.

Duct depth = 0.30 m ( zL ).

Hydraulic diameter of the duct, hD , is 0.34286 m.

Reynolds number based on the hydraulic diameter is 127.2.

FeaturesThis tutorial demonstrates how to:


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Import a grid (created using ICEM CFD)

Specify boundary conditions

Solve the flow problem

Do some post-processing of the results

SetupFirst, create a new directory called cfx-tutorial. Make sure that the path to this directory does not contain

any space characters. Spaces in a directory name or path will cause an error message in CFX (in addition, ahyphen cannot be used in the simulation name). Make this new directory your current directory (i.e., cd to thadirectory).

The grid for this tutorial has been pre-generated. It was created in software called ICEM CFD. For the purposes

this tutorial, the completed grid will be imported into CFX. The completed grid is in a file called duct.cfx5that can be copied to your current directory using:

cp -p ~engsjo/pub/mech-4822/cfx-tutorial/duct.cfx5 ./

or it can be downloaded (it is inside a zip file called cfxtutorial_duct_cfx5.zip) from a link in the

following web page:

http://home.cc.umanitoba.ca/~engsjo/teaching/Tutorials/index.htm#cfxtutorial

You can also use the grid that you created if you did the ICEM CFD tutorial: Simple Duct Grid.This grid has uniform mesh spacing and 41, 11, and 16 nodes in each of thex, y, andzdirections, respectively.

Assumptions about Running CFXThese instructions assume that:

1. The user has modified (customised) his/her Unix account as specified in the Linux/Unix Hands On tutorinotes used in MECH 4822.2. The user is connected to a Linux-based server or workstation usingVNCviewer. Examples of suitable

Linux machines (with suffix .cc.umanitoba.ca) are mars, venus, jupiter, cc01, cc02,cc03, cc04, and moon.

3. The version of the software is ANSYS CFX v14.0.

The CFX launcher can be started by typing:

cfx5 &

and then using the buttons for CFX-Pre, CFX-Solver, and CFD-Post.

In the past, two synonyms were used for running the pre-processor (cfx5pre) and the post-processor

(cfx5post) in aVNCviewer environment:

vnc-cfxpre (which is equivalent to cfx5pre -gr mesa&)

vnc-cfxpost (which is equivalent to cfx5post -gr mesa& )

to obtain correct graphical images when usingVNCviewer. These can still be used as an alternative to thelauncher.


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Defining the Simulation in CFX-PreTo begin using CFX-Pre, start the program by typing

vnc-cfxpre

1. Creating a New SimulationSelect File > NewSimulationSimulation Typedefault is General (click on General in the window and then click OK)

Also click on OK in the following window:

To name the simulation:Select File > SaveCase

In the window, set File name to rct_lam.cfx and clickSave.


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2. Importing the MeshSelect File > Import > MeshFiles of type: Select ICEM CFD

File name: Enter (or browse for) duct.cfx5ClickOpen

3. Domain SpecificationSelect Insert > DomainName: enterductClickOK


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Under the Domain: duct tab in the Basic Settings tab, click on and then in the Selection Dialog bothat appears, click on DUCT and then ClickOK

Still under the Basic Settings tab:Location: this should be DUCTDomain Type: this should be Fluid DomainCoordinate Frame: this should be Coord 0Fluid and Particle Definitions this should be Fluid 1Fluid 1: Option: this should be Material Library

Material: select WaterMorphology: Option: this should be Continuous Fluid

Do not click Minimum Volume Fraction.

Domain ModelsPressure: Reference Pressure: this should be 1 [atm]Buoyancy Model: Option: this should be Non BuoyantDomain Motion: Option: this should be StationaryMesh Deformation: Option: this should be None


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ClickApply

Under the Fluid Models tab:Heat Transfer: Option: select NoneTurbulence Model: Option: select None (Laminar)Combustion: Option: this should be NoneThermal Radiation: Option: this should be NoneDo not click Electromagnetic Model.

ClickApply

Under the Initialization tab:ClickDomain Initialization boxClickInitial Conditions box


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Leave all the values as the default values.

Now, ClickOK

4. Defining the Inlet Boundary ConditionSelect Insert > Boundary

Name: enterinlet

ClickOK

UnderBoundary: inlet tab:Basic Settings tab:Boundary Type: select InletLocation: select RCT_W


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UnderBoundary: outlet tab:Basic Settings tab:Boundary Type: select OutletLocation: select RCT_E

Boundary Details tab:Flow Regime: Option:SubsonicMass and Momentum: Option: Average Static PressureClick on space beside Relative Pressureand enter: 0.0

Leave Pres. Profile Blend at 0.05

Pressure Averaging: Option:Average Over Whole OutletClickOK


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6. Defining the Symmetry Plane Boundary ConditionSelect Insert > Boundary ConditionName: entersymmetry

ClickOK

UnderBoundary: symmetry tab:Basic Settings tab:Boundary Type: select SymmetryLocation: select RCT_SClickOK

7. Defining the Walls Boundary ConditionSelect Insert > Boundary ConditionName: enterwalls

ClickOK

UnderBoundary: walls tab:Basic Settings tab:Boundary Type: select Wall

Location: click on the icon. In the Selection Dialog window, click on RCT_B, then, while

holding down the Ctrl key, click on RCT_N and RCT_T. ClickOK.


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Boundary Details tab:Mass And Momentum: Option: select No Slip WallDo not check the box by Wall VelocityClickOK

The overall image of the domain should now appear as:


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Note that there is no duct domain default in the list. This means that all surfaces have been assigned

boundary condition.

8. Setting the Solver ControlsSelect Insert > Solver > Solver ControlUnderSolver Control tab:Details ofSolver Control in Flow Analysis 1 tab:Basic Settings tab:

Advection Scheme: Option: High ResolutionConvergence Control:

Min. Iterations: 1Max. Iterations: 100

Fluid Timescale Control:Timescale Control: select Physical TimescaleLength Scale Option: select Physical TimescalePhysical Timescale: click in the box and enter6000

Convergence Criteria:Residual Type: RMSResidual Target: 1.E-4

Leave the boxes unchecked for Conservation Target, Elapsed Wall Clock Time Control, and Interrupt

Control.

ClickOK


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9. Writing the Solver Definition FileSelect Tools > Solve > Write Solver Input FileAlternatively, you can click on the icon:

In the window that appears:

File name: rct_lam.defFiles of type: CFX-Solver Input Files (*.def)ClickSave


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10.Saving the SimulationSelect File > Save Case

11.Ending the CFX-Pre SessionSelect File > Quit

Obtaining a Solution Using the CFX-SolverTo start the solver, at the command line, type:

cfx5solve &

When the solver window comes up, if it is narrow, widen it by dragging the right edge of the window.

1. Defining the RunSelect File > Define RunIn the Define Run Window:

Solver Input File:browse for and select rct_lam.def

Run Definition tab:

Leave the box unchecked for Initial Values SpecificationType of Run:FullClick the box by Double PrecisionParallel Environment:Run Mode: select Platform MPI Local Parallel

You should see your host name appear in a table of Host Name and Partitions. Click the on theright to set the number of partitions to 4:


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ClickStart Run

The calculation should proceed with text information in one window and the residuals of the equationsa second window. In this case there should be a print-out of 12 outer loop iterations and then some

summary information, followed by a Solver Run Finished Normally window that pops up. In thiswindow there is some run information. ClickOK.

This solver run created the textual record of the run: rct_lam_001.outand the results file that can be post-processed: rct_lam_001.res.


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2. Ending the Solver SessionSelect File > Quit

Viewing the Results using CFD-Post

As simple examples of post-processing, this tutorial illustrates how to create a graph of a velocity profile at theduct exit and a velocity vector plot on the plane of symmetry. There are many other features available in CFD-

Post. For more details on these features, consult the course instructor and teaching assistant, as well as the on-liCFD-Post help.To begin using CFD-Post type:

vnc-cfxpost

1. Loading the Results FileSelect File > Load ResultsIn the file browser window, click on rct_lam_001.res and then clickOpen.

2. Creating a Line at the Exit PlaneSelect Insert > Location > LineName: enterExit Line

ClickOK


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A sidebar entitled Details ofExit Line should appear.

Geometry tab:Domains: All DomainsDefinition:Method: Two PointsPoint 1: enter2, 0, 0Point 2: enter2, 0, 0.3Line Type: click on circle forCutClick on Apply

(Aside: In the future, we will use Line Type Sample and specify a number of points to sample.)A yellow line will appear at the end of the duct image in the 3D viewer.

After zooming, it should appear like:


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In order to zoom in, you can use some of the icons at the top of the 3D viewer window:

To zoom click the (zoom) or (zoom box) icons. You can also use the pan icon: to mov

the image around and a scroll wheel on a mouse to zoom. You can also change the view by right

clicking on the 3D viewer window and choosing a Predefined Camera. If you want to see the entire duc

again, click on the fit view icon: .

3. Creating a Graph (Chart) of a Velocity Profile at the ExitSelect Insert > ChartName: U Velocity versus zClickOKUnderDetails of U Velocity versus z:

General tab:Type: XYTitle: U Velocity at the ExitCaption: Exit Velocity Graph

Data Series tab:ForSeries 1:Name: click in the box and enterExit Line ProfileLocation: select Exit Line

X Axis tab:


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Variable: select ZClick on the circle forHybridLeave the box checked for Determine ranges automatically

Y Axis tab:Variable: select Velocity uClick on the circle forHybridLeave the box checked for Determine ranges automaticallyClick on Apply

You should see the chart shown below in the right window (Chart Viewer).

The data used in this chart can also be exported to a spreadsheet program by using the export feature.To do this:

ClickExport

File name: enteru_exit_profile.csvFile Type: Comma Separated Values (*.csv)Click on Save


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The file created, when loaded into Excel (and formatted with more decimals for column A andscientific notation for column B), looks like:

These data can also be exported in a text file format for plotting with gnuplot or other plotting software

4. Creating a Velocity Vector PlotClick on the3D Viewertab.Select Insert > VectorName: enterSymm Plane VectorsClickOKA sidebar entitled Details ofSymm Plane Vectors should appear.


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Geometry tab:Domains: All DomainsDefinition:Locations: select symmetrySampling: VertexReduction: Reduction FactorFactor: select 1.0Variable: Velocity

Boundary Data: Click on the circle forHybridProjection: NoneClick on Apply

The vector plot below should appear in the 3D Viewer window. The domain was zoomed in for the

image.


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3. Ending the CFD-Post SessionSelect File > QuitClick on Save & Quit

File name: enterrct_lam.cst

Files of type:CFD-Post State (*.cst)Click on Save


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The state file that was saved (rct_lam.cst) has saved the new objects created in the previous CFD-Post

session. When examining the same results file another time in Post, those setting can be re-loaded using File >Load State. Another powerful feature is that the same state file can be loaded when viewing a different set ofresults on the same geometry and all plots (charts, vectors, etc.) are re-computed automatically for the new resul

Further Exploration

In order to get more experience using ANSYS CFX, you can try the following additional tasks.

1. Restart the flow calculation and converge to a tighter tolerance.a) Re-start CFX-Pre and re-load rct_lam.cfx.b) Go to the solution controls and change:

Maximum iterations to 500

Residual type to maxium

Residual target to 0.000001 (1.E-6)

c) Save the case filed) Write a new rct_lam.def file.

e) Start the Solver and define a new run Select the rct_lam.def file just created

Click on the box for Initial Values Specification

For Initial Values 1: for File Name, browse for rct_lam_001.res

Set up a Platform MPI Local Parallel run again with 4 partitions

Start the run and then close the solver after it is finished.f) Start CFD-Post and load the new results file.g) Load the rct_lam.cst file and examine the results.


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2. Add energy equation calculation and thermal boundary conditions.a) Re-start CFX-Pre and re-load rct_lam.cfx.b) In the Outline below Analysis Type, double click on duct. Under Domain: duct, click on the

Fluid Models tab. Change the Heat Transfer Option to Thermal Energy. Click OK.

You will see an error message appear that refers to boundary conditions. This means you need toadd thermal boundary conditions. You will add an inlet temperature and a wall temperature. The

symmetry and outlet conditions do not need to be changed.

c) Double click on inlet below duct under Analysis Type.

Click on the Boundary Details tab. Under Heat Transfer Option, select Static TemperatureThen, click in the box beside Static Temperature and enter 300 (the units should be K).

Click OK.d) Double click on walls below duct under Analysis Type.

Click on the Boundary Details tab. Under Heat Transfer Option, select Heat Flux. Then,click in the box beside Heat Flux in and enter 2000 (the units should be W/m

2). Click OK

e) Use File > Save Case As to save the current setup as rct_lam_thermal.cfx.f) Write a Solver Input File: rct_lam_thermal.def.g) Start the Solver and define a new run

Select the rct_lam_thermal.def file just created Do not use Initial Values Specification

Use double precision

Set up a Platform MPI Local Parallel run again with 4 partitions

Start the run and then close the solver after it is finished.h) Start CFD-Post and load the new results file.i) Load the rct_lam.cst file and examine the results.j) Try creating a contour plot of Temperature at the outlet face.k) Save the modified state as rct_lam_thermal.cst.l) Examine the temperature results. Create a new chart that is the temperature profile at the Exit Lin

created earlier.m)Create a new line that goes down the centre of the duct. Create a chart that plots the temperature

along this line.

Documents

CFX Tutorial Ductflow Laminar v4p00