introduction to ansys fluenthome.agh.edu.pl/~jaszczur/doc/cfd/species.pdf• open workbench –...

© 2014 ANSYS, Inc. February 28, 2014 1 Release 15.0

15.0 Release

Introduction to ANSYS Fluent

Workshop 03 Multi-Species Flow and Post-Processing


• In this workshop you will analyze the release of heat and combustion gases from a single car with an engine fire in a ventilated parking garage

• The simulation will be run steady state assuming the fire has reached a stable developed stage

• Simulation Physics & Boundary Conditions – Mixture of N2, O2, CO2 and H20 – 6 m/s exhaust, 0.1 kg/s combustion gases (H2O and CO2) at 1200 K – 80 N/m3 momentum source in jets

Introduction

Fresh Air Inlet

‘velocity_inlet_fresh_air ‘

Jet Fan

‘fluid_jet_fan‘

Heat and gas release from fire

‘mass_flow_inlet_car_fire_source‘

Air Outlet

‘pressure_outlet_all_air ‘

Symmetry Plane

‘symmetry‘

Introduction Part 1: Long Version Setup Solving Proceed to Part 2.


Important Note

NOTE:

• This workshop has been designed to be completed in one of two ways

– Please check with your trainer on whether you are to take the short or long option

• [Short Option] This workshop can be used just to demonstrate post-processing in CFD-Post

– Pre-prepared results files are supplied, so please jump straight to page 35 for post-processing

• [Long Option] Follow all the instructions, which will demonstrate how to set up a multi-species simulation of a car fire

– Once the model is set up, you can choose to wait for it to converge, or then replace your results with the supplied pre-prepared set



If doing the short version (post-processing only) please jump to page 35 now



Learning Aims:

The first part of this workshop will show how to set up a multi-species problem. The domain will contain a blend of several different gases (nitrogen, oxygen, carbon dioxide, water vapor).

Other topics that will be introduced are:

• Including gravitational (buoyancy) effects

• Setting a momentum source term to account for a jet fan

Learning Objectives:

To understand how Fluent can be used to simulate mixtures of fluids, and account for buoyancy effects. Note that a multi-species problem like this assumes that the components are mixed at a molecular level (as normally happens with gases). The alternative is a multi-phase problem where there is an identifiable boundary between the components (either droplets / particles / bubbles, or a free-surface). Multi-phase workshops include Workshop 2 (DPM) and Workshop 8 (Tank Flush).

I Objectives (Flow Simulation Part)



Starting Workbench • Open Workbench

– Start > Programs > ANSYS 15.0 > Workbench 15.0

– Drag a Fluent Component System into the project schematic

– Rename the system to ‘Garage’ (RMB on cell A1 to rename the system)



Starting Fluent in Workbench • Start Fluent

– Double click on the Setup cell to open Fluent

– Choose 3D and Double Precision under Options and retain the other default settings (if your computer has 2 or more nodes and parallel licenses are available, you also could start Fluent parallel)



• Import the existing mesh file

– Under the Fluent File menu select Import > Mesh

– Select the file ‘car_and_garage.msh’ and click OK to import the mesh

– To check for any problems in the mesh click Check. There should be no problems reported in the TUI window

– Reorder the mesh using Mesh > Reorder > Domain (from the menu)

– Reordering the domain can improve the computational performance of the solver by rearranging the nodes, faces and cells in memory.

– Retain defaults for the solver

– Enable gravity and set z = -9.81 m/s2

Import Mesh and General Setup



Setting Physics (Turbulence) • Specify turbulence model

– Select Models in the navigation pane

– Double click Viscous in the model selection pane. The Viscous Model panel will open

– Select k-epsilon (2 eqn) under Model, Realizable under k-epsilon Model and Enhanced Wall Treatment under Near-Wall Treatment

Turbulence modeling, as with all physics modeling, is a complex area. There are many application specific options. The k-epsilon model is a simple but robust model. The documentation provides further guidance on which models to use for specific applications.

– Click OK



Setting Physics (Species / Mixture) • Specify species model

– Double click Species in the model selection pane to open the Species Model panel

In this workshop the products of combustion (heat & gases) will be modeled rather than the reaction itself. You can see in the panel that if it was desired to model fire by including the reactions, there are several combustion models available.

– Select Species Transport and click OK – Switching on the species model will introduce

new material properties

– An information box will appear. Click OK to accept this

– This setup will enable the tracking of non-reacting chemical species

The species model requires the definition of a ‘mixture’ representing the chemical species of interest. The default mixture contains nitrogen, oxygen and water vapour.



• Specify mixture

– Select Materials in the navigation pane.

– Note that under Mixture in the Materials pane the default mixture is listed as containing nitrogen, oxygen and water-vapor.=

– Double click mixture-template, this will open the Create/Edit Materials panel with the mixture preselected

Defining Materials (1)



Defining Materials (2) • Add a mixture species

– In the Create/Edit Materials panel click on Fluent Database

– The Fluent Database Materials panel will appear

– Select fluid as Material Type

– All predefined fluids materials will be listed under Fluent Fluid Materials

– Select carbon-dioxide (co2)

– Click Copy

– Close the Fluent Database Materials panel



Defining Materials (3) • Copying carbon-dioxide(co2) from the fluid database has made the species available to

the simulation, now add it to the mixture

– In the Materials panel ensure Material Type is set to Mixture

– Alongside ‘Mixture Species’, click Edit

– In the Species panel select co2 from the Available Materials list and select Add

– The Selected Species defines the component species of the mixture

– The order of the species listed under Selected Species is important. The most abundant species should be listed last

– Use the Remove button to Remove n2, followed by the Add button to replace n2 as the last species. Click OK (but don’t close the mixture panel yet)



Defining Materials (4) • Specify mixture

– Modify the existing settings for Thermal Conductivity and Viscosity to be ‘mass-weighted-mixing-law’

– Click Change/Create to apply the changes



Defining Materials (5) • Specify solid material

– Select Solid under Material Type. The default solid material is aluminum (al)

– Modify aluminum (Name, Chemical Formula & Properties) as shown below

– Click Change/Create and choose No for overwriting

Selecting No preserves the original material (aluminum) and adds the new material

– Close the Create/Edit Materials panel



Cell Zone Conditions • Set Cell Zone Conditions

– Select Cell Zone Conditions from the Navigation Pane

– Select fluid_jet_fan, then edit

– Activate Source Terms

– Select the Source Terms tab and click on Edit for Y Momentum

– Add 1 source, select Constant and enter a value of -80 (N/m3)

– Click OK in both panels


We need to account for the air movement produced by the ceiling jet fan. Here we have done this by adding momentum to the cell zone local to the jet. The advantage of this technique (over using a pair of velocity boundary conditions) is that we preserve the species (smoke) concentration through the fan. If we had used velocity boundary conditions, we would have needed a UDF to find the concentration at the intake to the jet fan and apply that to the jet fan discharge.


• Set Boundary Conditions

– Select Boundary Conditions from the Navigation Pane

– Open ‘velocity_inlet_fresh_air’ from the Zone list

– Apply Momentum and Thermal settings as shown

– Continued on next slide.....

Boundary Conditions (air inlet)



Boundary Conditions (air inlet) • Set Boundary Conditions.... cont

– Apply Species settings as shown

– Click OK


The mixture species contains 4 components (h2o, o2, co2, and n2). The most abundant species (n2) was entered last when the mixture was defined. You do not need to enter a mass fraction for n2. It will automatically account for the remaining fraction not used by the first three (in this case 0.77).


Boundary Conditions (air outlet) • Set Boundary Conditions

– Open ‘pressure_outlet_all_air” from the Zone list

– For Momentum, in Turbulence set intensity to 5% and viscosity ratio to 5

– For Thermal, set the temperature to 293.15 K (as for previous BC)

– For Species, set the o2 concentration to 0.23

– Click OK


So long as there is only flow out of the domain here, these values for turbulence, temperature and species will not be needed. However during the solution process there may be some inflow though this boundary, and therefore Fluent needs to know what values to apply.


• Set Boundary Conditions – Open “mass_flow_inlet_car_fire_source” from the Zone list.

– Momentum: Mass Flow Rate to 0.1 kg/s, Normal to Boundary

– Turbulence: 5% Turbulent Intensity, Turbulent Viscosity Ratio = 5

– Thermal: 1200K

– Species: set ‘specify in mole fractions’ with 0.65 h2o and 0.35 co2

Boundary Conditions (fire source)



Boundary Conditions (walls 1) • Set Boundary Conditions

– Open ‘walls_outer’ from the Zone list

– Set the temperature to 300 K and click Shell Conduction

– Enter the Shell Conduction Model Settings as shown

– Click OK in both panels

The Shell Conduction option

enables thin walls to solve for heat

transfer in both the normal and

planar directions without the need to

volume mesh them.


After Shell Conduction is selected, certain fields in the Wall

panel are greyed out and their entries are superseded by the

entries in the Shell Conduction Model Settings Panel.


Boundary Conditions (walls 2) • Set Boundary Conditions

– Select Copy from the Boundary Conditions Pane

– Select ‘walls_outer’ in the From Boundary Zone list and ‘wall_ceiling’ and ‘wall_floor’ in the To Boundary Zones list

– Click Copy, click OK in the question dialog box then Close

– This will copy all boundary settings from the boundary zone walls_outer to both wall_ceiling and wall_floor



Operating Conditions • Set operating conditions

– Select Operating Conditions

– Apply Specified Operating Density settings as shown

– Click OK



Operating Conditions • Notes

– ANSYS Fluent avoids the problem of round-off error by subtracting the operating pressure (generally a large pressure roughly equal to the average absolute pressure in the flow) from the absolute pressure, and using the result (termed the gauge pressure). The absolute pressure is simply the sum of the operating pressure and the gauge pressure.

– Operating temperature is only used when using the Boussinesq density model, so in this case, it has no meaning.

– Operating density is also a value for avoiding round-off errors. For simulations where pressure boundary conditions are present it is important to set the value correctly otherwise the pressure at the boundary will be incorrect and may lead to unphysical flow conditions. Here you have to set it to the density for the conditions at the pressure-inlet - a gas at 293.15 K with 23% O2 and 77% N2. You can initialize your flow field with these conditions to get the value for the operating density from the post-processor (e. g. Reports -> Volume Integral). See the Users Guide “Natural Convection and Buoyancy-Driven Flows” for more details.



Solution Methods • Set solution methods

– Select Solution Methods from the Navigation Pane

– For Pressure-Velocity Scheme, set to ‘Coupled’

– Under Spatial Discretization set Pressure to Body Force Weighted

– The Body Force Weighted scheme is recommended for problems involving large body forces



Solution Controls • Set solution controls

– Select Solution Controls from the Navigation Pane

– Set the values shown below – Flow Courant Number = 50 – Explicit Relaxation Factors 0.5 (Momentum & Pressure) – Density = 1 – Body Forces = 1 – Turbulent Kinetic Energy = 0.5 – Turbulent Dissipation Rate = 0.5 – Turbulent Viscosity = 0.7 – h2o = 1 – o2 = 1 – co2 = 1 – Energy = 1

Lower Under-Relaxation Factors will reduce the solution change between iterations, leading to more stable convergence though requiring more iterations to reach convergence.



Monitors (1) • Set solution monitors

– Select Monitors from the Navigation Pane

– Click Edit and set the Residual Monitors as shown below

By default ANSYS Fluent will plot residuals to the window and print to the console. The default setting for the convergence criterion is Absolute which means that the solver will continue until all residuals fall below the Absolute Criteria values specified in the Equations box. Switching the Convergence Criterion to none will cause the solver to continue until a maximum number of iterations is reached.

– Click OK



Monitors (2) • Set surface monitors

– Under Surface Monitors, click Create

It is important to ensure that solution variables have converged to sensible stable values. Creating Surface Monitors enables solution values of interest to be monitored on specific surfaces within the domain.

– Set the Surface Monitor as shown below and click OK



Monitors (3) • Set surface monitors

There are many different types of calculations that can be performed over surfaces listed under Report Type.

– Create a second monitor for the integral of the Total Surface Heat Flux on the surface “wall_ceiling” plotting to window 3 and printing to the console, as shown below



Monitors (3) • Notes on monitors

Buoyancy driven flows often show transient behavior. For this reason, the residuals will often oscillate. Because of this, convergence should always be judged by solution variable monitors and flux reports. The residuals will however give an indication of overall convergence behavior and stability.

In cases of an oscillating steady state solution, a common approach is to continue the simulation in transient mode. In many cases the oscillations will reduce significantly after a few time steps.

The use of surface/volume monitors combined with residuals will provide the best overall judge of solution convergence.



• Set initialization values – Select Solution Initialization and Hybrid Initialization

‘Hybrid Initialization’ performs a basic flow simulation (Laplace equation) to set up the initial flow field. Simplified momentum and pressure equations are solved, and so the general flow field can be quickly determined (unlike standard initialization which puts a constant value in each grid cell). By having a more realistic starting point, less work will be needed by the solver to converge the model.

– Select ‘More Settings’, and for species settings, define the initial o2 concentration to be 0.23, then ‘OK’

– Only flow and pressure equations are being solved with the ‘Hybrid’ method, so we need to set a realistic, although constant, value for species

– Select ‘Initialize’, and watch the TUI window for progress

Solution Initialization



Running the Simulation This simulation will take a long time to compute a converged solution – this is not unusual

for such ventilation / natural convection cases. There are natural unsteady features in the flow and the equation set is somewhat ‘stiff’ to converge.

• If you want to run the simulation yourself, set the case to run for 1000 iterations in Run Calculation, and keep an eye on the solution progress

• Alternatively, just load the results (data) file supplied with this workshop

File > Import > Data > car_and_garage_1000its.dat.gz

• You can reproduce the residual graph (but not the surface monitors) shown below by Monitors > Residuals > Select ‘Residuals’ then Edit, then press the ‘Plot’ button



Check Convergence • Select Reports from the Navigation Pane and select Fluxes under Reports • Click Set Up • In the Flux Reports panel select Mass Flow Rate and select the inlet/outlet boundaries (shown below) then click compute Note that the Net Results indicate a good mass balance, but the monitors show the solution is not completely converged. It is likely that there are some unsteady effects present that may necessitate going to a transient (time dependent) simulation. This will be discussed in a later lecture. Note that the energy balance can be checked in a similar way by selecting ‘Total Heat Transfer Rate’



Exit Close Fluent

Close ANSYS Fluent by selecting File>Close Fluent

ANSYS Fluent contains basic built in post-processing capabilities which can be used to quickly assess results graphically and numerically during and after the solution

The remainder of this workshop will introduce some features of CFD-Post, which is a powerful, separate post-processing application containing many more advanced features than Fluent



Post-Processing in CFD-Post

Part 2: Post-Processing Colour Images Line Graphs Expressions and Integrals Reports Summary


Learning Aims:

This workshop is designed to teach a range of skills in post-processing Fluent results files using CFD-Post. Topics to be covered include:

• Creating surface groups - Creating line graphs

• Creating isosurfaces - Creating expressions (CEL)

• Creating portable (.cvf) images - Performing integrals

• Creating automatic reports - Volume rendering

Learning Objectives:

To understand the ways in which CFD-Post can be used both for high quality images, as well as producing quantitative data from volume/surface integrals, and writing custom functions.

I Objectives (Post-Processing)



Starting CFD-Post If you are doing the long version of this workshop:

• Go to the ANSYS Workbench project page • Under ‘Component Systems’, pick ‘Results’ and drag onto the desktop • Left click in the Solution cell (A3) and without releasing the mouse button, drag the

pointer on to the Solution Cell of the Results system (B2) (see image)

• Start CFD-Post by clicking on the Results cell If you are doing the short (post-processing only) version of this workshop: • Start CFD-Post from the Start menu:

– Start Menu > ANSYS 15.0 > Fluid Dynamics > CFD-Post 15.0 • Within CFD-Post, select File > Load Results and pick the supplied file: car_and_garage_1000its.dat.gz

• OK to the pop-up window



Post-Processing – Wall Temperature (1) • Add a Location representing a group of surfaces

This lets you group a selection of entities (in this case walls) and apply the same post-processing treatment to all items in the group

Select Location > Surface Group, and enter the name ‘Walls’



Post-Processing – Wall Temperature (2) • Define Location details

The details of the new location will be displayed in the bottom left pane

Select Locations, click on “…” and select all walls EXCEPT wall_car (CTRL click to multiple select)

Click OK in the Location Selector



Apply contour plot to Location

Select the Color tab, click on Mode and select:

Variable: Temperature

Range: Local

Apply

Modify the Legend

Select ‘Default Legend’

Right click, Edit

Under ‘Appearance’:

Precision 1

Change ‘Scientific‘

to ‘Fixed’

Apply

Post-Processing – Wall Temperature (3) This option has allowed us to produce a temperature contour plot of identical colour range on a group of surfaces.



Generate a figure for use later in a report

- Click the Figure icon

- Enter name “Figure 1 Wall Temperature”

- OK to the Insert Figure panel

Post-Processing – Generating a Figure

Remember where this option is (you will be asked to use it several times on subsequent slides). We will be creating several figures as we progress through this workshop. Later on we will demonstrate how to use these figures to automate report-writing.



Post-Processing – Section Plane (1) • Add a Location representing a section plane

Select Location > Isosurface

In the pop-up window Enter name ‘xzplane’ and click OK.

Under ‘Geometry’ tab set Definition Variable to Y

Value to 9 [m]

Click ‘Apply’

In the model outline tree,

de-select the ‘Walls’ option

so only the new slice plane

is visible

Continued on next slide…..



Post-Processing – Section Plane (2) Under ‘Color’ set the Variable to ‘Temperature’

Under ‘Render’ disable Lighting, then ‘Apply’ to display

Generate a new Figure, and name it “Figure 2 Temperature Slice”



Post-Processing – Quick Animation The slice through the model gives us a useful indication of what is happening in the

middle of the domain. A ‘quick animation’ will transverse this slice through the model so we can see what is happening elsewhere.

Click the ‘Animation’ button in the top toolbar

Select ‘Quick Animation’

Highlight the ‘xzplane’ object.

Press the Play button , and watch the display

When finished, use the stop button then Close

If required, this animation could be saved to disk in MPEG / AVI formats. The alternative to ‘Quick Animation’ is ‘Keyframe Animation’. To use this you set a series of key animation frames. These might have different objects visible, different points in a transient simulation, or might have the model rotated at a different viewing angle. The animation will progress smoothly between these states.



Post-Processing – 3d Isosurface [1] First, change the look of the display:

Hide the plane ‘xzplane’ previously created by un-ticking in the model tree

Expand the top of the model tree, expanding ‘fluid_main_garage’

Double-click on wall_car

Make sure the details box shows (wall_car), if not you will be modifying the wrong object!

For ‘Color’ select constant, and pick yellow

Apply



Post-Processing – 3d Isosurface [2] Add a 3D Isosurface representing gases

Select Location > Isosurfaces

Keep the default name ‘Isosurface 1’

Set the Variable to ‘Co2.Mass Fraction’ and the value to 0.001

Set ‘Color’ to be constant (in the Color tab) (keep default grey colour), then ‘Apply’

Generate a new Figure: ‘Figure 3 CO2 Isosurface’



Add 3D streamlines to visualize flow

Hide the isosurface created in the previous step (un-tick in model tree)

Click the Streamline button and keep default name (“Streamline 1”)

Start From: “velocity_inlect_fresh_air”

# Points: 100

Under the ‘Color’ tab set

Mode: ‘Use Plot Variable’

Range: Local

Apply

Generate a new Figure of this image

“Figure 4 Streamlines”

Post-Processing – Streamlines



Post-Processing – Portable CVF Files [1]

Not only can CFD-Post export regular image formats (jpg /png), but in addition 3D images can be saved. These images have the file extension .cvf

They can be viewed using a free CFD viewer that can be downloaded from the ANSYS website. (Go to www.ansys.com, and search for ‘CFD Post Viewer’)

No license is required to use the viewer, so you can install this on any computer (e.g. laptop used for presentations, or ask your client/customer to also download and install a copy).

The 3D image can be viewed using rotate / pan / zoom functionality just as in CFD-Post, and can also be embedded in MS-Powerpoint. However the user cannot modify the image, they cannot add/remove objects from the image, or alter color ranges.

This is a really powerful tool for when you come to present your project work. In many cases a 2D jpeg image cannot explain 3D flow features. However rotating the model ‘live’ in front of your audience will help convey your findings.


http://www.ansys.com/



• In CFD-Post, click the camera icon:

• For Format, select ‘CFD-Viewer State(3D)

• Click the folder icon to the right of the filename

– Pick the directory you are working in (remember this!)

– Save as filename car-streamlines.cvf

– Click ‘Save’ in both windows




• Minimise CFD-Post, and use Windows Explorer to browse to the folder used on the last slide

• Note how this file (car-streamlines.cvf) is quite small (in this case about 170kB, and therefore easy to email to your client or manager)

• Double-click to open this file (it will take a few moments to launch the viewer application)

If you have ANSYS R15.0 installed on your machine, your computer will already have the viewer, and will recognise this file extension. You only need to do a separate installation of CFD-Post Viewer (from the ANSYS website) on machines that do not have Workbench installed.




• CFD-Post Viewer will look just like the graphical window of CFD-Post

• Use left mouse button to rotate

• Middle mouse button (or wheel) to zoom

• Right mouse button to translate

• Type question mark ‘?’ for a list of hotkeys



Post-Processing – Volume Rendering • Close the standalone viewer and return to CFD-Post

• Hide the streamlines plot by un-ticking in model view

• Select Volume Rendering, and use name ‘Gas Cloud’

Under Geometry, set ‘Co2.Mass Fraction’

Keep range as ‘Global’, then Apply

• To make it easier to see the image, change the screen background colour to white:

Edit > Options > CFD-Post > Viewer

Set ‘Color Type’ to ‘Solid’

Pick White from the color options

OK

This option applies a variable colour and transparency to each grid cell depending on the plot variable. For applications like this involving smoke movement it makes it easy for the eye to assess where the cloud is concentrated.



Post-Processing – Quantitative

Until now we have used CFD-Post to create colour images to help interpret the CFD results. Next we will look at some quantitative techniques for extracting numerical data (volume integrals), and producing line graphs. It is also possible to write your own arithmetic expressions for custom post-processing.

Create a line through the model: • Hide the ‘Gas Cloud’ volume rendering • Location > Line and keep default name ‘Line 1’ • Set Method Two Points

• From: X=18 Y=3 Z=2 • To: X=18 Y=18 Z=2

• Set 40 samples along this line • Apply

This has created a horizontal line through the model, passing above the front of the car.



Post-Processing – Line Graph [1]

• Select the Chart Icon from the top Toolbar • Keep the Default name ‘Chart 1’ then OK

• Under ‘General’, set the Title to “Temperature Profile” • Under ‘Data Series’ Set location to ‘Line 1’ • Under ‘X Axis’, set variable to Y • Under ‘Y Axis’ set variable to Temperature • ‘Apply’



Post-Processing – Line Graph [2]

The resulting graph looks like this:

The computation domain exists from 3m < y < 18m

The car (and fire source) is located at

8m < y < 10m

The jet fan is located at 13m < y < 15m

Notice that the peak temperature is located not above the middle of the car (y=9m) but moved some distance to the left (circa y= 6.5 - 8m). This is a direct effect of the air movement from the jet fan.



Post-Processing – Volume Integrals

The only source of carbon dioxide (co2) in the model is from the car fire source (the inlet just comprises oxygen and nitrogen). We will perform a volume integral to find out how much co2 is present in the model. • Select ‘Calculators’ tab • Select Function Calculator and double click • Function: volumeInt (for Volume Integral) • Location: fluid_main_garage • Variable: Co2.Mass.Fraction • Press ‘Calculate’

The result is about 0.57m3 of CO2.



Post-Processing – Expressions [1a]

It is possible to write your own arithmetic functions for post-processing, making use of the data exported by the solver. The resulting expression may either return a single value (first example, below), or produce a quantity that varies spatially for use in a contour plot / line graph (second example, to follow). • Select ‘Expressions’ tab • Right click in the window and select ‘New’ • Enter name ‘PressureDrop’ then OK • Enter the expression exactly as shown below, then Apply • The answer is approximately 35Pa If you get any errors look at the next slide now, which will help you understand why



Post-Processing – Expressions [1b]

• Note how ‘Pressure’ turns to italics as soon as you type it • It is important to make the first letter a capital ‘P’ • For a full list of available variables, right click in this window and select Variables

• This is the name of the boundary that you are performing the average function on • For a full list of locations, right-click and select ‘Locations’ • Note that to compute the pressure drop, we did not need to add the outlet boundary ...- ave(Pressure)@pressure_outlet_all_air The outlet boundary was set to be a pressure outlet in Fluent with a pressure of 0 Pa. This term would return a zero value – try it if you like!

• ave returns the average at the location specified by the ‘@’ • For a list of functions available, right-click in the window and select Functions > CFD-Post



Post-Processing – Expressions [2a]

The procedure to create an expression that can be plotted spatially is very similar, there is just one additional step to assign it to a variable As a simple example, let’s convert temperature from K to °C and plot this on the graph • Expressions > (Right click) New > name ‘TemperatureConversion’ • Enter the expression Temperature/1[K] – 273.15 then Apply

Note: • Initial capitals for Temperature

– It will turn to italics if correct • Expressions must balance dimensionally

– We cannot just enter ‘Temperature – 273.15’ since Temperature has a unit [K] – By dividing by 1 [K] we remove the temperature unit – We could instead enter ‘Temperature – 273.15[K]’

• This expression is valid, but would return a value with units [K] which would be misleading



Post-Processing – Expressions [2b]

Expressions cannot be plotted directly, they need to be assigned to a Variable

• Variables Tab > (Right click) New Enter name “TemperatureC”

• From the pull down list, select the expression “TemperatureConversion” created on the last slide

• Select Apply



Post-Processing – Expressions [2c]

Use this new temperature on the Chart created earlier

• (Top-Left) select ‘Outline’ then double-click on ‘Chart 1’

• Under ‘Y Axis’ change the variable by clicking on the ‘...’ icon

• Select this expression ‘TemperatureC’

• OK then Apply

• At the bottom of the screen change from ‘3D Viewer’ to ‘Chart Viewer’



Post-Processing – Expressions [2d]

The graph now shows the result of our expression:



Post-Processing – Reports

• In the options below the graphic window, select ‘Report Viewer’ • Select ‘Refresh’

• Review what is shown in the report

window. You can see: – Names of the results file – Mesh summary – List of boundary conditions – All the Figures and Charts produced

during this workshop

• If you select ‘Publish’ this will be written out in html format, along with copies of all the results images



State Files and Optional Extra Work

• Use File > Save state as... and save to your working directory The state file stores all the post-processing settings you have created. If CFD-Post is used from WB, this is done automatically when closing CFD-Post. If you have done the long version of this workshop, you will recall that we ran for a fixed number of iterations, and wanted to examine the results to help us determine if the model had converged or not. (The residuals were ‘stuck’ and further iterations would not lower the residuals). It might be be necessary to revisit the model setup, by moving to a transient scheme, or modifying the modeling settings. A useful assessment of convergence is to see if the results of interest remain unchanged as the solver settings are enhanced. The big advantage of having the state file is that if you choose to modify the solver settings and re-run the model, you can quickly reproduce the equivalent post-processing images. Simply load the new results file, then load the state file. Likewise, it is common in project work to have run a series of models to test different operating conditions. This technique will let you generate equivalent images so as to produce a good like-for-like comparison in your presentation / report.



State Files and Optional Extra Work

• The ‘Reports’ feature just demonstrated will let you customise the format of your report

• If you have finished this exercise ahead of the rest of the class, try experimenting with

the ‘Report’ options in the left-hand toolbar

• You can choose which objects are visible, add your own company logo, or add lines of text to explain the content of the report



Wrap Up

• CFD-Post is a very powerful post-processing tool, and capable of producing high quality images quickly and easily

• In this workshop we have shown how to produce contour plots, streamlines, and isosurfaces (as seen in some other workshops for this course)

• In addition you have used CFD-Post to perform volume integrals, create line graphs, and to create your own arithmetic expressions for post-processing

• 3D images can be saved to disk, and viewed in a freeware viewer. This adds much impact to presentations, and can be run on any computer (no license needed)

• CFD-Post can also automate the report generation process

• Post-processing is best learned by practice. If you have time now, try exploring the other buttons in the interface


introduction to ansys fluenthome.agh.edu.pl/~jaszczur/doc/cfd/species.pdf• open workbench –...

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