Tut 05 Blunt Body

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CFX-5 Tutorials Page 1

Master Contents Master Index Help On Help

CFX-5 Tutorials

Tutorial 5

Flow Around a Blunt Body

Sample files used in this tutorial can be copied to your working

directory from /examples. SeeWorking Directory (p.

and Sample Files (p. 3)for more information.

Sample files referenced by this tutorial include:

BluntBody.pre

BluntBodyDist.cse

BluntBodyMesh.gtm
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Flow Around a Blunt BodyIntroduction

Page 124 CFX-5 Tutorials


5.A: Introduction

5.A.1: Features explored in this tutorial

Introduction:This tutorial addresses the following features of CFX-5.

You learn about:

solving and post-processing a case where the geometry has been

omitted on one side of a symmetry plane

using free slip wall boundaries on the sides of and above the domain as

a compromise between accurate flow modelling and computational

grid size

Component Feature DetailsCFX-Pre User Mode General Mode

Simulation Type Steady State

Fluid Type Ideal Gas

Domain Type Single Domain

Turbulence Model Shear Stress Transport

Heat Transfer Isothermal

Boundary Conditions Inlet (Subsonic)

Outlet (Subsonic)

Symmetry Plane

Wall: No-Slip

Wall: Free-Slip

Timestep Physical Timescale

CFX-Solver Manager Restart

Parallel processing

CFX-Post Plots Default Locators

Outline Plot (Wireframe)

Sampling PlaneStreamline

Vector

Volume

Other Changing the colour range

Instancing Transformation

Lighting Adjustment

Symmetry

Viewing the Mesh
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Flow Around a Blunt BodyIntroducti



accurately modelling the near-wall flow using Shear Stress Transport

(SST) turbulence model

running the CFX-Solver in parallel (optional)

creating vector plots in CFX-Post with uniform spacing between the

vectors

creating a macro using power syntax in CFX-Post

5.A.2: Before beginning this tutorial

Introduction: It is necessary that you have a working directory and that

sample files have been copied to that directory. This procedure is detaile

in "Introduction to the CFX-5 Tutorials" on page 1.

Unless you review the introductory materials and perform required steps

including setting up a working directory and copying related sample files

the rest of this tutorial may not work correctly. It is recommended that yo

perform the tasks inTutorial 1,Tutorial 2 andTutorial 3 before working wi

other tutorials as these three tutorials detail specific procedures that are

simplified in subsequent tutorials.

5.A.3: Overview of the problem to solve

This example demonstrates external air flow over a generic vehicle body

Since both the geometry and the flow are symmetric about a vertical plan

only half of the geometry will be used to find the CFD solution.

Figure 1: External Air Flow Over a Generic Vehicle Body

1.44 m

5.2 m

air speed

15.0 m/s
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Flow Around a Blunt BodyDefining the Simulation in CFX-Pre



5.B: Defining the Simulation in CFX-Pre

This section describes the step-by-step definition of the flow physics in

CFX-Pre. If you wish, you can use the session file BluntBody.preto

complete this section for you and continue fromObtaining a Solution Using

the CFX-5 Solver (p. 133). See any of the earlier tutorials for instructions on

how to do this.

5.B.1: Creating a New Simulation

1. Start CFX-Pre.

2. Select File > New Simulation.

3. Select Generalmode.

4. Set File nameto BluntBodyand then clickSave.

5.B.2: Importing the Mesh

Tip:While we provide a mesh to use with this tutorial, you may want todevelop your own in the future. Instructions on how to create this meshin CFX-Mesh are available from the CFX Community Site. Please see"Mesh Generation" on page 3for details.

1. Copy the mesh file BluntBodyMesh.gtm, located in the examples

directory (/examples), to your working directory.2. Click the Meshtab to access the Mesh workspace.

3. Right-click in the Mesh Selector, then select Import.

4. Leave Mesh Formatset to CFX-5 GTM file.

5. Set Fileto BluntBodyMesh.gtm.

6. Click OKto import the mesh.

5.B.3: Creating the DomainThe flow in the domain is expected to be turbulent and approximately

isothermal. The Shear Stress Transport (SST) turbulence model with

automatic wall function treatment will be used because of its highly

accurate predictions of flow separation. To take advantage of the SST

model, the boundary layer should be resolved with at least 10 mesh nodes.

In order to reduce computational time, the mesh in this tutorial is much

coarser than that.
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Flow Around a Blunt BodyDefining the Simulation in CFX-P



See "The Shear Stress Transport (SST)k-w Based Model" on page 71 in the

document "CFX-5 Solver Theory"and "Automatic Near-Wall Treatment fo

k-w Based Models" on page 90 in the document "CFX-5 Solver Theory"fo

more details.

To create anew domain

1. Click Domain .

2. Set Nameto BluntBody.

3. Click OK.

Edit Domain: BluntBodyis displayed with General Optionsselecte

4. Set Locationto Assembly.

5. Leave Domain Typeset to Fluid Domain.

6. Retain Fluids Listas Air Ideal Gas.

7. Leave Coord Frameset to Coord 0.

8. Leave Reference Pressureset to 1 [atm].

Note:This tutorial uses an ideal gas as the fluid whereas previous tutoriahave used a General Fluid. When modelling a compressible flow using thideal gas approximation to calculate density variations, it is important to sa realistic Reference Pressure. This is because some fluid properties depen

on the absolute fluid pressure (calculated as the static pressure plus thereference pressure).

One atmosphere is equal to 1.0E+5 Pa. You can enter pressure values in avariety of units in CFX-Pre. Valid formats for scientific notation numbersinclude 1e5, 1E5, 1e+5, 1E+05and 1.0e+05. You should not use spaces

between any of the characters.

9. Under Buoyancy,leaveOptionset to Non Buoyant.

10. Under Domain Motionleave Optionset to Stationary.

11. Click the Fluid Modelstab, then:

a. Under Heat Transfer Model, leave Optionset to Isothermaland

set Fluid Temperatureto 288 [K].b. Under Turbulence Model, set Optionto Shear Stress Transpor

c. Under Turbulent Wall Functions, set Optionto Automatic.

The Initialisationpanel sets domain specific initial conditions, which are

not used in this tutorial. Global initialisation will be set later in the tutoria

12. ClickOKto create the domain.
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5.B.4: Creating Composite Regions

An imported mesh may contain many 2D regions. For the purpose of

creating boundary conditions, it can sometimes be useful to group several

2D regions together and apply a single boundary condition to the

composite 2D region. In this case, you are going to create a Union between

two regions that both require a free slip wall boundary condition.1. Click the Regions tab.

2. Right-click in the Region Selector, then select New.

3. In the Create Regionbox, enter FreeWalls and clickOK.

4. In the Region Editor, set Combinationto Union.

Note:The Aliasoption can be used to re-name a region. This can be veryuseful if you wish to apply the physics from one simulation to another withdifferent region names.

5. Set Dimensionto 2D.

6. In the Region List, hold down the key and select Free1and

Free2.

7. Click OKto create the new region.

The Region Editor closes. The Region Selector is updated to show that

the new region has been added.

5.B.5: Creating the Boundary Conditions

The simulation requires Inlet, Outlet, Wall (No Slip and Free Slip) and

symmetry plane boundary conditions. The regions for these boundary

conditions were defined when the mesh was created.

To Create theInlet BoundaryCondition

1. Click Boundary Condition .

2. In Create Boundary, set Nameto Inletand leave Domainset to

BluntBody.

3. Click OK.

Edit Boundary is displayed.

4. On the Basic Settingspanel, set:

a. Boundary Typeto Inlet

b. Locationto Inlet


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5. Click the Boundary Detailstab, then:

a. Under Flow Regime, confirm that Optionis set to Subsonic.

b. Under Mass and Momentum, confirm that Option issetto Norm

Speedand set Normal Speedto 15 [m s^-1].

c. Under Turbulenceset Optionto Intensity and Length Scale,

FractionalIntensity to0.05,and Eddy Length Scale to0.1 [m6. Click OKto create the boundary condition.

To Create theOutletBoundaryCondition


2. Set Name to Outlet, leave Domain set to BluntBody, and then clic

OK.


a. Boundary Typeto Outlet

b. Locationto Outlet

4. Click the Boundary Detailstab, then:

a. Under Flow Regime, confirm that Optionis set to Subsonic.

b. Under Mass and Momentumset Optionto Static Pressureand

Relative Pressureto 0 [Pa].

5. Click OKto create the boundary condition.

To Create Free

Slip WallBoundaryCondition

The top and side surfaces of the rectangular region will use free slip wall

boundary conditions.

On Free Slip Walls the shear stress is set to zero so that the fluid is

not retarded.

The velocity normal to the Wall is also set to zero.

The velocity parallel to the Wall is calculated during the solution.

This is not an ideal boundary condition for this situation since the flow

around the body will be affected by the close proximity to the walls. If th

case was modelling a wind tunnel experiment, the domain should mode

the size and shape of the wind tunnel and use no-slip walls. If this case wmodelling a blunt body open to the atmosphere, a much larger domain

should be used to minimise the effect of the walls.

There are other possible ways of dealing with, or eliminating, wall effects

but these are beyond the scope of this tutorial.


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You will apply a single boundary condition to both walls by using the

composite region defined earlier.


2. Set Nameto FreeWalls, then clickOK.


a. Boundary Typeto Wall

b. Locationto FreeWalls

4. Click the Boundary Detailstab, under Wall Influence on Flow, set

Optionto Free Slip.


To Create theSymmetryPlane

BoundaryCondition


2. Set Nameto SymP, then clickOK.


a. Boundary Typeto Symmetry

b. Locationto SymP


To Create aWall BoundaryCondition onthe Blunt BodySurface


2. Set Nameto Body, then clickOK.


a. Boundary Typeto Wall

b. Locationto Body

4. Click the Boundary Detailstab, then, under Wall Influence on Flow,

set Optionto No Slip.


The remaining 2D regions (in this case, just the low Z face) will be assigned

the default boundary conditionwhich is an adiabatic, no-slip wall condition.In this case, the name of the default boundary condition is BluntBody

Default. Although the boundary conditions Body and BluntBody Default

are identical (except for their locations), the Body boundary condition was

created so that, during post-processing, its location can by conveniently

distinguished from the other adiabatic, no-slip Wall surfaces.


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5.B.6: Setting Initial Values

1. Click Global Initialisation to display Global Initialisation.

2. Under Cartesian Velocity Components, set:

a. Optionto Automatic with Value

b. Uto15 [m s^-1]

c. Vto 0 [m s^-1]

d. Wto 0 [m s^-1]

3. Under Static Pressureand Turbulence Kinetic Energy leave Option

set to Automatic.

4. Turn on Turbulence Eddy Dissipationand leave Optionset to

Automatic.

5. Click OKto set the initialisation details.

5.B.7: Setting Solver Control

1. Click Solver Control .

Solver Controlis displayed.

2. Under Advection Scheme, set Optionto High Resolution.

3. Under Convergence Control, set:

a. Timescale Controlto Physical Timescale

b. Physical Timescaleto 2 [s]

This is the approximate dynamic time for the flow and is an

aggressive timestep for use with a turbulence model.

c. Max. No. Iterations to 60

4. Under Convergence Criteria, leave Residual Typeset to RMSand se

Residual Targetto 1e-05.

5. Click OKto set the solver control parameters.

5.B.8: Writing the Solver (.def) File

1. Click Write Solver (.def) File .

Write Solver Fileis displayed.

2. Leave Operationset to Start Solver Manager.

3. Leave Report Summary of Interface Connections turned off and Qu

CFX-Pre turned on.


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4. Click OK.

5. Click Yeswhen asked if you want to save the CFX file.


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Flow Around a Blunt BodyObtaining a Solution Using the CFX-5 Solv



5.C: Obtaining a Solution Using the CFX-5

Solver

This tutorial introduces the parallel solver capabilities of CFX-5. If you do n

want to solve this tutorial in parallel (on more than one processor) or, do n

have a licence to run the CFX-Solver in parallel, you can continue with thtutorial from this point to solve it in serial (as you have for previous tutoria

If you do not know if you have a license to run the CFX-Solver in parallel, yo

should either ask your system administrator, or query the license server a

described below.

The results produced will be identical, whether produced by a parallel or

serial run.

If you would like to solve this tutorial in parallel, continue with the tutoria

from Obtaining a Solution in Parallel (p. 134). To solve it in serial, continuefrom Obtaining a Solution in Serial (p. 134).

Querying theLicense Serverfor CFX-5Parallel

This section explains how to find out whether you have any licenses to ru

CFX-5 in parallel, by using the CFX License Manager.

1. Fromthe CFX-5 Launcher, selectTools > CFXLicense Manager to ope

the CFX License Manager.

This is a tool for managing the licensing of CFX-5. The top of the window

displays information about the system and license setup. The bottom half

a message window.

2. If running using a Windows platform select

Query > Licenses Available.

3. If running using a Unix platform select

Licenses > Features > Available.

The message window will display information about available licenses on

your system, in the following form:

CFX-5-SOLVER 5.200 100 30-Sep-2003 CFDSCFX-5-NOLIMIT 5.200 100 30-Sep-2003 CFDS

CFX-5-PAR-PROC 5.200 200 30-Sep-2003 CFDS

CFX-5-PARALLEL 5.200 100 30-Sep-2003 CFDS

CFX-5-COMBUSTION 5.200 100 30-Sep-2003 CFDS

CFX-5-MULTIFLUID 5.200 100 30-Sep-2003 CFDS

CFX-5-MFR 5.200 100 30-Sep-2003 CFDS


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Flow Around a Blunt BodyObtaining a Solution Using the CFX-5 Solver



Look for the presence ofCFX-5-PARALLEL type licenses. If you have this

type of license, you can run the CFX-5 Solver in parallel.

5.C.1: Obtaining a Solution in Serial

When the CFX-Solver Manager starts:

1. Click Start Run.The CFX-Solver calculates the solution to your CFD problem.

2. When the CFX-Solver has finished, clickOKin the message box.

To View theResults

3. Click Post-Process Results .

4. When Start CFX-Post appears, turn on Shut down Solver Manager

then clickOK.

Continue with this tutorial from Viewing the Results (p. 141).

5.C.2: Obtaining a Solution in Parallel

Using the parallel capability of the CFX-Solver allows you to divide a large

CFD problem so that it can run on more than one machine at once, saving

time and avoiding problems which arise when a CFD calculation requires

more memory than a single machine has available. The partition (division)

of the CFD problem is automatic but you need to set up the machines you

want to run on.


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Background toParallelRunning inCFX-5

A number of events occur when you set up a parallel run and then ask th

CFX-Solver to calculate the solution:

Your mesh will be divided into the number of partitions that you hav

chosen.

The CFX-Solver runs separatelyon eachof the partitions on the selecte

machine(s).

The results that one CFX-Solver process calculates affects the other

CFX-Solver processes at the interface between the different sections

the mesh.

All of the CFX-Solver processes are required to communicate with eac

other and this is handled by themasterprocess.

The master process always runs on the machine that you are logged

into when the parallel run starts. The other CFX-Solver processes are

slaveprocesses.

After the problem has been resolved, a single Results File is written. Iwill be identical to a Results File from the same problem run as a seria

process, with one exception: an extra variable Real partition numbe

will be available for the parallel run. This will be described later in the

tutorial.

Setting Up toRun in Parallel

Windows

If you are working on Windows, no set up is required if your system

administrator has followed the instructions in "Windows Parallel Setup" o

page 60 in the document "CFX-5 Installation"as part of the installationprocedure. You can move straight to"To Define a Parallel Run" on page 13

in the document "CFX-5 Tutorials".

UNIX

Follow the procedure below prior to running in parallel for the first time o

UNIX systems. Additional information can be found in "UNIX Parallel Setu

on page 47 in the document "CFX-5 Installation".

Note:The following procedure may not be needed for all systems. Yoursystem administrator will inform you if you need to create a.rhosts fito run in parallel.
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You need to know the hostname of the machine that you are currently

logged into. It will be used for the master process.

1. Type cfx5info -hostin a UNIX terminal window.

The output should be used as the hostname.

2. Create a file named.rhostsin your home directory.

Do this using any text editor.3. Put the following line into the.rhostsfile:

Where is the hostname found in step 1 above and

is your current user name.

For example, your user name issmith, and you are logged onto a

machine with the hostname ofmachine1. Then your .rhosts

file would contain the following line of text:

machine1 smith4. You now need to make this file readable by you only. This can be done

by typing the following in a UNIX terminal window:

chmod 600 ~/.rhosts

If you have a different home directory on a machine you want to use for a

slave process, you will have to create a.rhosts file on it in the same way,

using exactly the same text (for example, use the same hostname, not the

machines own hostname, for ).

Note: If you do not have the same user name, you will be unable to run in

parallel on this machine.

To Define aParallel Run

In CFX-Solver Manager, Define Runshould already be open.

1. Set Definition Fileto BluntBody.def.

If you had wanted to run the problem as a serial run (as you have done in

previous tutorials) you could have clicked Start Run. However, you are

going to run the problem in parallel instead.

2. Leave Type of Runset to Full.IfTypeofRun was instead set to Partitioner Only,yourmeshwouldbe

split into a number of partitions but would not be run in the CFX-Solver

afterwards.

3. Set Run Modeto PVM Distributed Parallel.

Run Mode can be set toone of several options. See"Run Mode" on page

12 in the document "CFX- Solver Manager"for details.
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The name of the machine that you are currently logged into should be in th

Host Namelist. You are going to run with two partitions on two different

machines, so another machine must be added.


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4. Click Insert Host to specify a new host machine.

The Select Parallel Hostspanel is displayed. This is where you

choose additional machines to run your processes.

Your system administrator should have set up a hosts file

containing a list of the machines that are available to run the parallel

CFX-5 Solver.

The Host Namecolumn displays names of available hosts.

The second column shows the number of processors on that

machine.

The third shows the relative processor speed: a processor on a

machine with a relative speed of 1 would typically be twice as fast

as a machine with a relative speed of 0.5.

The last column displays operating system information.

This information is read from the hosts file; if any information ismissing or incorrect your system administrator should correct the

hosts file.

Note:The # Processors, Rel. Speed and System information does not haveto be specified to be able to run on a host.

5. Select the name of another machine in the Host Namelist

Select a machine that you can log into.

6. Click Add.

The name of the machine is added to the Host Namecolumn.

Note: Ensure that the machine which you are currently logged into is in theHosts Namelist in the Define Runwindow.

7. Closethe Select Parallel Hostswindow.

8. Enable Show Advanced Controls.

9. Click the Partitionertab at the top of the panel.

10. Use the default MeTiSpartitioner.

Your model will be divided into two sections, with each section running

in its own CFX-Solver process. The default is the MeTiSpartitioner

because it produces more efficient partitions than either Recursive

Coordinate Bisectionor User Specified Direction. See "Setting Up

and Running a Parallel Run" on page 45 in the document "CFX- Solver

Manager"for details of the other settings on this panel.
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11. ClickStart Runto begin the parallel run.

The Text Output Areawill display information about the partitioning

job as below:

+--------------------------------------------------------------------+| Job Information | +--------------------------------------------------------------------+

Run mode: partitioning run

Host computer: fastmachine1Job started: Wed Nov 28 15:18:40 2000

This tells you that the information following is concerned with the

partitioning. After the partitioning job has finished, you will find:CPU-Time requirements:- Preparations 1.460E+00 seconds- Low-level mesh partitioning 1.000E-01 seconds

- Global partitioning information 3.100E-01 seconds- Vertex, element and face partitioning information 1.600E-01 seconds- Element and face set partitioning information 5.000E-02 seconds- Summed CPU-time for mesh partitioning 2.080E+00 seconds

+--------------------------------------------------------------------+| Job Information | +--------------------------------------------------------------------+

Host computer: fastmachine1Job finished: Wed Nov 28 15:19:16 1998Total CPU time: 1.143E+01 seconds

or: ( 0: 0: 0: 11.428 ) ( Days: Hours: Minutes: Seconds )

This marks the end of the partitioning job. The CFX-5 Solver now begi

to solve your parallel run:+--------------------------------------------------------------------+| Job Information | +--------------------------------------------------------------------+

Run mode: parallel run (PVM)

Host computer: fastmachine1Par. Process: Master running on mesh partition: 1Job started: Thu Nov 28 15:19:20 2000

Host computer: slowermachinePar. Process: Slave running on mesh partition: 2Job started: Thu Nov 28 15:24:55 2000

The machine which you are logged into runs the master process, and


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controls the overall simulation. The second machine selected will run

the slave process. If you had more than two processes, each additional

process is run as a slave process.

The master process in this example is running on the mesh partition

number 1 and the slave is running on partition number 2. You can find

out which nodes and elements are in each partition by using CFX-Post

later on in the tutorial.When the CFX-Solver finishes, the Output Filedisplays the following:

+--------------------------------------------------------------------+| Job Information | +--------------------------------------------------------------------+

Host computer: fastmachine1Par. Process: Master running on mesh partition: 1Job finished: Thu Nov 28 16:44:01 2000Total CPU time: 9.025E+02 seconds or: ( 0: 0: 15: 2.517 )

( Days: Hours: Minutes: Seconds )

Host computer: slowermachinePar. Process: Slave running on mesh partition: 2Job finished: Wed Nov 28 16:54:30 2000Total CPU time: 1.291E+03 seconds or: ( 0: 0: 21: 31.034 ) ( Days: Hours: Minutes: Seconds )--> Master-Partition Nr. 1 reaches final synchronization point!--> Slave-Partition Nr. 2 reaches final synchronization point!This run of the CFX-5 Solver has finished.

You may need to scroll up through the output file to view the region

shown above. More details on the contents of the Output Filecan be

found in "The CFX-5 Output File" on page 87 in the document

"CFX-Solver Manager".

The CFX-Solver displays the following at the end of the run:BluntBody_001 has completed normally. All RMS residuals and global

imbalance are below their target criteria.

12. ClickOK.

13. ClickPost-Process Results .14. When Start CFX-Post appears, turn on Shut down Solver Manager

then clickOK.
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Flow Around a Blunt BodyViewing the Resu



5.D: Viewing the Results

In this tutorial, a vector plot is created in CFX-Post. This will let you see ho

the flow behaves around the body. You will also use Symmetry Planes an

learn more about manipulating the geometry view in the 3D Viewer.

5.D.1: Using Symmetry Planes

Earlier in this tutorial you used a symmetry plane boundary condition

because the original blunt body had a symmetry plane. Due to this

symmetry, it was necessary to use only half of the full geometry to calcula

the CFD results. However, for visualisation purposes, it is helpful to use th

full blunt body. CFX-Post is able to recreate the full data set from the half

that was originally calculated. This is done by creating an Instance

Transformobject.

Manipulatingthe Geometry

You need to manipulate the geometry so that you will be able to see wha

happens when you use the symmetry plane. The CFX-Post features that yo

have used in earlier tutorials will not be described in detail. New features w

be described in detail.

1. Click View Toward +X by using the viewer icon drop-down men

Creating anInstanceTransform

Instance Transforms are used to visualise a full geometry representation

cases where the simulation took advantage of symmetry to solve for only

part of the geometry. There are three types of transforms that you can usRotation, Translation, Reflection. In this tutorial you will create a Reflectio

transform located on a plane.

1. Click Create plane .

2. Set Nameto Reflection Plane, then clickOK.

3. Under Geometry, set Methodto ZX Planeand Yto 0.

4. Disable Visibility.

5. Click Apply.This creates a Plane in the same location as the symmetry Plane defined

CFX-Pre. Now the instance transform can be created using this Plane:

6. Click Create instancing transformation and accept the default

name.


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Flow Around a Blunt BodyViewing the Results



7. On the Definitionpanel, fill in the settings as shown below.

8. Click Applyto create the Instance Transform.

Using theReflectionTransform

You can use the transform when creating or editing graphics objects. For

example, you can modify the Wireframe view to use it as follows:

1. Edit the Wireframeobject.

2. Set Transformto Instance Transform 1, retain the other settings as

their defaults and clickApply.Zoom in so that the geometry fills the Viewer.


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You will see the full blunt body. It is possible to select more than one

symmetry plane. More details can be found in"Instance Transform" on pag

120 in the document "CFX-Post".

5.D.2: Creating Vectors

You are now going to create a vector plot to show velocity vectors behinthe blunt body. You need to first create an object to act as a locator, whic

in this case, will be a Sampling Plane. Then create the vector plot itself.

Creating theSamplingPlane

A Sampling Plane is a plane with evenly spaced sampling points on it.

1. Click Create Plane .

2. In New Plane, set Nameto Sample, then clickOK.

3. Under Geometry:

a. Set Methodto Point and Normal.b. The following (x, y, z) values must be entered into the Pointboxe

to define the centre of the Sampling Plane:6, -0.001, 1.

c. For the Normalvector enter the values:0, 1, 0.

d. Expand the Plane Boundssection of the form.

e. Set Typeto Rectangular, X Sizeto 2.5 [m], and Y Sizeto

2.5 [m].

f. Expand the Plane Typesection of the form.

g. Turn on Sampleand set X Samplesand Y Samplesto 20.4. Click Applyto create the Sampling Plane.

5. Click View Toward +Y .

You will not see the location of the sampling points because Draw

Linesis turned off by default on the Rendertab.

6. Click the Render tab:

a. Turn on Draw Lines.

b. Turn off Draw Faces.7. Click Apply.

You can zoom in on the Sampling Plane to see the location of the samplin

points (where lines intersect). One way to do this is to hold while

middle clicking the area of interest. There are a total of 400 (20 * 20)

sampling points on the plane. A vector can be created at each sampling

point.
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8. Make the Sampling Plane invisible by turning offVisibilityand then

clicking Apply.

Creating aVector PlotUsing theSampling

Plane

1. Click Create Vector Plot .

2. In New Vector, clickOK to accept the default name.

3. Under Geometry, set:a. Domainsto All Domains

b. Locationsto Sample

c. Variableto Velocity

4. Click the Colourtab, then set Modeto Use Plot Variable.

This colours the Vectors by the variable shown on the Geometry panel

(Velocity).

5. Click the Symboltab, then set Symbol Sizeto 0.15.

This scales the length of the vectors.

6. Click Applyto create the Vector plot.

7. Zoom until the Vector plot is roughly the same size as the 3D Viewer.

The plot should look similar to the one below. (If you get lost, clickView

Toward +Y and zoom in again.).

You should be able to see a region of recirculation behind the blunt body.

Note: In this example, you may want to increase the density of the vectors

(by increasing the number of sampling points) to resolve the flow in thisregion. However, you could end up with poor results. This is because themesh has only 2-3 elements across the feature. This is insufficient to resolve


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the vortex properly. You would need to decrease the size of the meshelements (decrease the mesh length scale in this region) in order to improthe solution accuracy.

You can try the following before continuing to the next section.

1. Change the location of the Vector plot to SymPand clickApply.

2. Double-click the Body object. Set Mode to Variableand colour theBodyboundary object by Pressure and change the Transform(on th

Renderpanel) to the transformation previously created (Instance

Transform 1). Turn on Visibility

3. Try using different mouse methods of manipulating the geometry.

Examine the effect of Move Light (plusand use the

Arrow keys to move) and Zoom box .

4. Make the Bodyboundary object invisible and create a view of the

surface mesh on the SymP object by making it visible, turning on line

and turning off faces. You will be able to see the mesh around the blu

body, with the mesh length scale decreasing near the body, but still

coarse in the region of recirculation. By zooming in, you will be able t

see the layers of inflated elements near the body and ground.

If you want to know more about any feature, select Help >

Master Contentsfrom the main menu.

5.D.3: Creating Surface Streamlines

In order to show the path of air along the surface of the blunt body, surfa

streamlines can be made as follows:

1. Click Create plane .

2. In New Plane, set Nameto Starter, then clickOK.

3. Under Geometry:

a. Set Methodto YZ Plane.

b. Set Xto -0.1 [m].

4. Click Apply.

The plane appears just upstream of the blunt body.

5. Turn off Visibility, then clickApply.

This hides the plane from view, although the plane still exists.

6. Click Create streamline .


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7. In New Streamline, accept the default Nameby clicking OK.

8. On the Geometry panel:

a. Set Typeto Surface Streamline.

b. Under Definition, set Surfacesto Body, Start Fromto Locations,

Locationsto Starter.

9. Click Apply.

The surface streamlines appear on the surface of the blunt body. They start

near the upstream end because the starting points were formed by

projecting nodes from the plane to the blunt body.

5.D.4: Creating a Surface Plot of y+

The velocity next to a no-slip Wall boundary changes rapidly from a value of

zero at the wall to the free stream value a short distance away from the wall.

This layer of high velocity gradient is known as the boundary layer. Manymeshes are not fine enough near a wall to accurately resolve the velocity

profile in the boundary layer. Wall functions can be used in these cases to

apply an assumed functional shape of the velocity profile. Other grids are

fine enough that they do not require wall functions, and application of the

latter has little effect. The majority of casesfall somewhere in betweenthese

two extremes, where the boundary layer is partially resolved by nodes near

the wall and wall functions are used to supplement accuracy where the

nodes are not sufficiently clustered near the wall.

One indicator of the closeness of the first node to the wall is the

dimensionless wall distance y+. It is good practice to examine the values of

y+ at the end of your simulation. At the lower limit, a value of y+ less than or

equal to 11 indicates that the first node is within the laminar sublayer of the

boundary flow. Values larger than this indicate that an assumedlogarithmic

shape of the velocity profile is being used to model the boundary layer

portion between the wall and the first node. Ideally you should confirm that

there are several nodes (3 or more) resolving the boundary layer profile. If

this is not observed, it is highly recommended that more nodes be added

near the wall surfaces in order to improve simulation accuracy. In this

tutorial, a coarse mesh is used to speed the run time. Thus the grid is far too

coarse to resolve any of the boundary layer profile, and the solution is not

highly accurate.

More related information can be found in"Modelling Flow Near the Wall" on

page 88 in the document "CFX-5 Solver Theory", including the difference

between the solution Yplus and Solver Yplus.
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Creating aSurface Plot ofy+

A surface plot is one which colours a surface according to the values of a

variable: in this case, y+. A surface plot of y+ can be obtained as follows:

1. Turn off Visibilityfor all previous plots.

2. Click Perspective to enable a perspective view.

3. In the Objects Workspace, double-click the BluntBody Defaultobjec

then:

a. On the Colourpanel, set Modeto Variable.

b. Set Variableto Yplus.

Click to the right of the Variable dropdown menu to view a fu

list of variables, including Yplus.

c. Set Rangeto Local.

d. Click the Render tab, then use the reflection transform you create

earlier (Instance Transform 1).

e. Turn on Visibility.

4. Click Apply.

5. Produce a plot ofYpluson the Bodyobject in the same way.

5.D.5: Demonstrating Power Syntax

This section demonstrates a Power Syntax Macroused to evaluate the

variation of any variable in the direction of the X axis. This is an example o

Power Syntax programming in CFX-Post. For more information on PowerSyntax, please see "Power Syntax Overview" on page 238 in the documen

"CFX-Post". For information on CCL please see "Overview of the CFX

Command Language (CCL)" on page 200 in the document "CFX-Post".

1. Play the session file namedBluntBodyDist.cse.

A macro containing CCL and power syntax is loaded. The macro tells

CFX-Post to create slice planes, normal to the X axis, at 20 evenly-spaced

locations from the beginning to the end of the BluntBody domain. On eac

plane, it measures and prints the minimum, maximum, and average valu

for a specified variable (using Conservative values). The planes are coloure

using the specified variable.

This macro will be executed by entering a line of power syntax in the

Command Editor.
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Note:The CFX-Post engine can respond to CCL commands issued directly,or to commands issued using the graphical user interface. The CommandEditorcan be used to enter any valid CCL command directly. Please see"Command Editor" on page 169 in the document "CFX-Post"for moreinformation.

2. Position the geometry to look toward the -X axis.

3. From the Main menu clickTools > Command Editor.

4. Type the following line into the Command Editor (the quotation marks

and the semi-colon are required):

!BluntBodyDist("Velocity u");

5. Click Process.

The Minimum, Maximum and Average values of the variable at each X

location are written to the fileBluntBody.txt. The results can be

viewed by opening the file in a text editor.

You can also run the macro with a different variable.

To view the content of the session file (which contains explanatory

comments), open the session file in a text editor. It contains all of the CCL

and power syntax commands and will provide a better understanding of

how the macro works.

5.D.6: Viewing the Mesh Partitions (Parallel Only)

If you solved this tutorial in parallel then an additional variable named Real

partition numberwill be available in CFX-Post

1. Create an Isosurface ofReal partition numberequal to 1.

2. Create a second Isosurface ofReal partition numberequal to 1.999.

The two Isosurfaces show the edges of the two partitions. The gap between

the two plots shows the overlap nodes. These were contained in both

partitions 1 and 2.

When you have finished looking at the results, shut down CFX-Post.

Next TutorialPrevious Tutorial
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