2012 ANSYS, Inc. November 13, 2012 1 Release 14.5

14.5 Release

Introduction to ANSYS Fluent

Workshop 01 Fluid Flow and Heat Transfer in a Mixing Tee

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Workshop Description:

The flow simulated is in a T-section of a pipeline. Fluid enters at two different temperatures, and the computation simulates the mixing process.

Learning Aims:

This workshop aims to teach basic skills in the use of the Fluent interface. The entire simulation approach is covered, including:

reading a mesh, - setting up solution monitors,

selecting material properties, - running the simulation,

defining boundary conditions, - post-processing

Learning Objectives:

To show the basic techniques used in performing a Fluent simulation. You will need to use these steps in all your Fluent simulations.

I Introduction

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Simulation to be performed

The task is to simulate mixing of hot and cold water in a T-piece.

Note that if you have just attended our pre-processing course, this example will be making use of the same mesh that you generated in ANSYS Meshing / WS 1.

We have provided a fresh copy of this mesh here. You are welcome to use your existing mesh if you like.

The simulation is being performed to determine: How well do the fluids mix? What are the pressure drops? Its a good idea to identify the

key simulation outcomes from the start. You can use these to monitor solution progress.

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Jump to next slide if you would prefer to use the supplied mesh

If you want to use the mesh you created during the ANSYS Meshing course: Open up the previous project from the meshing course (Meshing Workshop 1). Drag a Fluent component system onto the Mesh cell B3.

Workbench will then generate a new Fluent system (column C), linked to the mesh component in B3.

- Right-click on the Update symbol [ ]in cell B3 and select Update. The icon will shortly change to a tick [ ]. - Double-click on Setup to launch Fluent then click OK on the Fluent Launcher screen. - Ignore the instructions on the next slide.

Loading a mesh and starting Fluent

Connecting Mesh to Fluent tells Workbench which mesh format is required. During the mesh update step, the mesh is then exported in a format suitable for Fluent . The mesh is not being re-generated, simply converted for the Fluent solver.

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These instructions are for users wanting to use the supplied mesh. Start a new workbench session. Drag a Fluent component system onto the project. Right-click on Setup, and select Import Fluent Case, and Browse.

In the pop-up window, change the filter (bottom-right) from case to Fluent Mesh File. Browse to and select the file mixing_tee.msh.

Click OK on the Fluent Launcher

screen

Loading a mesh and starting Fluent

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Fluent interface

The main commands are reached from the navigation pane

Each item in the navigation pane brings up a new task page. A typical workflow will tackle these in order

One or more graphics windows will be available (shown here with reduced size)

The console window displays text, and can accept TUI (Text User Interface) commands

Some useful commands have toolbar buttons

The Help button brings up context-sensitive help pages

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Meshing is available when Fluent (with Tgrid meshing) is used

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Mouse button functionality depends on the chosen solver (2D / 3D) and can be configured in the solver.

Default settings 2D Solver

Left button translates/pans (dolly) Middle button zooms Right button selects/probes

3D Solver Left button rotates about 2 axes Middle button zooms

Middle click on point in screen centers point in window

Right button selects/probes

Mouse functionality

Display Mouse Buttons

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The Workbench Defaults button will make the mouse behavior consistent with Workbench

When mouse-probe is enabled, clicking an object in the graphics window will retrieve detailed information about the flow field at the mouse position

The left mouse button functionality can also be controlled using the toolbar buttons

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Press Check

- review the text output

Press Report Quality

- review the text output

Mesh scale and check

Mesh quality is very important for getting a converged, accurate solution. The worst cells will have an orthogonal quality closer to 0, with the best cells closer to 1. The minimum orthogonal quality for all types of cells should be more than 0.01, with an average value that is significantly higher (0.2). The maximum aspect ratio is 18.0, which is high, but acceptable in inflation layers. If the mesh quality is unacceptable it is best to remesh before proceeding. There are other possible remedies in Fluent, such as conversion to polyhedral cells.

The mesh check ensures that each cell is in a correct format and connected to other cells as expected. It is recommended to check every mesh immediately after reading it. Failure of any check indicates a badly formed or corrupted mesh which will need repairs prior to simulation.

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Press Display

set Edge Type to Feature, press Display and then Close

Adjust the view if you like

in rotation mode:

drag left-mouse-button rotates

drag middle-mouse-button zooms (to zoom in, drag down and right) (to zoom out, drag up and left)

click middle-mouse-button centre origin on click

Display geometry

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Change units of temperature Click Units

select Temperature to be c (Celsius)

click Close

Fluent stores values in SI units. Most post-processing can be converted to other units.

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Double-click (or click and press Edit...) the following models:

Energy Equation: On

Viscous model: k-epsilon, Realizable

Activate models

Turbulence modelling is a complex area. The choice of model depends on the application. Here, the Realizable k-epsilon model is used. This is an improvement on the well-established Standard k-epsilon model. Accept the remaining default settings.

Activating the Energy equation allows temperature dependent problems to be solved.

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Define a new material In Materials, click Create/Edit...

click Fluent Database...

select water-liquid, press Copy, then close both windows

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The default available fluid is air. This step makes water available for the simulation. Note that it is the step on the next slide that makes water be actually used in the simulation.

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Cell Zone Conditions In Cell Zone Conditions, double-click the zone called fluid

Change Material Name to water-liquid

accept all other settings

Throughout the problem setup there are many options that are left to default settings as they are not relevant to this particular type of analysis.

Alternatively, click once on fluid to highlight it, and then click Edit....

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If you are using the mesh you created during the ANSYS Meshing course, the zone name may initially be solid. If so, in addition to changing the material as shown on this slide, also change the name of the cell zone to fluid before clicking OK.

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Boundary Conditions In Boundary Conditions, select the zone called inlet-y then Edit. Set the

following under the Momentum tab. Set Temperature under the Thermal tab.

Velocity Magnitude 3m/s Turbulent Intensity 5% Hydraulic Diameter 0.15m Temperature 10C

Inlet flows bring turbulence with them. The quantities depend on the upstream conditions so they are user inputs. For flow in pipes, turbulent intensity is typically 5 to 10%. The length-scale of the turbulence can be deduced from the pipe diameter.

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Boundary Conditions Select the zone called inlet-z, then Edit. Apply the following settings

Velocity Magnitude 1m/s Turbulent Intensity 5% Hydraulic Diameter 0.10m Temperature 90C

Select the zone called outlet, then Edit

for this problem the outlet gauge pressure is 0

turbulence must still be specified and is referred to as Backflow when specified on an outlet.

for backflow: Turbulent Intensity 5% Hydraulic Diameter 0.15m Temperature 30C

It is possible that during the solution process material may flow back into the domain though an outlet boundary. This could either be a genuine feature of the flow (and still present in the converged solution), or just a short-lived state reached during the convergence process. Either way, Fluent needs to know realistic conditions at this boundary to give to the incoming flow. If (as in this model) the converged model has only flow leaving at the outlet boundary then these values are not used and do not affect the final answer. Ideally, the geometry should be selected such that flow enters the model only at well-defined inlets, with no backflow occurring.

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Discretization schemes In Solution Methods

Change to the option Coupled for Pressure-Velocity Coupling and activate Pseudo Transient

Change Pressure, Turbulent Kinetic Energy and Turbulent Dissipation to Second Order

Tick the box to allow High Order Term Relaxation

Discretization schemes define how the solver calculates gradients and interpolates variables to non-stored locations. First-order schemes are more stable but less accurate than higher order schemes. This case is well defined and will be stable using second-order numerics from the start.

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Monitors In Monitors, press Create... for a Surface Monitor

Name p-inlet-y Plot in window 2 Area-Weighted Average Pressure inlet-y

By default, Fluent reports values of the residuals, which are indications of the errors in the current solution. These should decrease during the calculation. There are guidelines on the reductions that indicate a solution is converged. It is also recommended to observe other important solution quantities. In the current case, we will use pressure and temperature as monitors.

Accept Static pressure in the sub-category menu.

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Monitors In Monitors, press Create... for a Surface Monitor

Name p-inlet-z Plot in window 2 Area-Weighted Average Pressure inlet-z

In Monitors, press Create... for a Surface Monitor

Name tmax-outlet Plot in window 3 Vertex maximum Temperature outlet

Here is an instance where Fluent does not convert units. Click OK.

Not the default, 3 (which puts the new monitor in a new window).

Accept Static pressure and Static temperature in the sub-menu.

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Initialization In Solution Initialization

keep Hybrid Initialization under Initialization Methods

click Initialize

a warning message that convergence was not reached during hybrid initialization may appear, but it is only a warning, not an error, and it is safe to continue

Initialization creates the initial solution that the solver will iteratively improve. Generally, the same converged solution is reached whatever the initialization, but convergence will be faster if the starting point is more realistic. Basic initialization imposes the same values in all cells. You can improve on this in various ways for example, by patching different values into different zones. Several features, including patching and post-processing, are not available until after initialization. Hybrid Initialization Method is an efficient method of initializing the solution based purely on the setup of the simulation with no extra information required. This method produces a velocity field that conforms to complex domain geometries and a pressure field which smoothly connects high and low pressure values.

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Calculate In Run Calculation

click Check Case...

see No recommendations to make at this time.

set Number of Iterations to 150

click Calculate

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While calculating, review residuals and monitors

change graphics windows using the drop-down box

Calculating

An alternative way to stop calculation is to press CTRL-C.

In this simple case, approximately 100 iterations (or fewer) are enough to reach low residuals and stable values of monitors. Most cases require many more.

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This option will let you have several graphics windows visible simultaneously.

This option lets you choose what is in each window

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In Graphics and Animations, select Contours, press Set Up...

select Filled contours of Turbulence...Wall Yplus on wall-fluid

press Display

Set lights (see image)

Preliminary post-processing

Yplus is a measure of whether the mesh near the walls captures

turbulent effects. Standard wall functions work in the range 30

300. Smaller values require Enhanced Wall Treatment

(in the Models...Viscous panel).

The plot appears in the last active graphics window. You may want to revert to having one window, and

select fit to view

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3D images are best viewed with lighting enabled. Select Lights,

then scheme Gouraud with headlight on

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In Reports, select Fluxes and press Set Up...

compute Mass Flow Rate and Total Heat Transfer Rate for inlets and outlets check that Net Results are small

Check mass and heat balance

Checking that mass and energy are conserved (to acceptable accuracy) is simple and important.

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Exit Fluent Close Fluent simulation case and data files are written on exit

In Workbench: Save the Project (Save As....)

From Component Systems,

drag a Results object and drop

on the Fluent solution cell.

Double-click Results in Cell D2 in the Workbench Project Schematic window to launch

CFD-Post

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If you started with the supplied mesh file (rather than continuing the project started during the meshing course) then you will not have these blocks on your project page.

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CFD-Post The results are loaded

CFD-Post initially displays the outline (wireframe) of the model

viewer toolbar buttons allow you to manipulate the view

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Temperature contour plot Press the contour button

accept the default name Contour 1

set Locations to be wall fluid, and Variable to be Temperature

press Apply

Try changing the view by rotate, zoom and pan tools.

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Create a plane Hide the contour plot by unchecking it in the tree view

In the Location menu, select Plane

accept the default name Plane 1

set Method to be YZ Plane, accept X as 0.0 and press Apply

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Velocity vector plot Hide the plane by unchecking it in the tree view

Press the Vector button (accept default name)

set Locations to be Plane 1 and press Apply

The plane is used only as a location for the vector plot.

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Predefined Camera views and shortcuts Since the vector plot is on the YZ-plane, select a normal view

click with the right mouse-button in the view window

select Predefined Camera then View From +X

Alternatively, press x. Keyboard shortcuts are listed by pressing here.

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Streamline plot Hide the vector plot by unchecking it in the tree view

Press the Streamline button (accept default name)

set Start from to be inlet y and inlet z

in the Symbol tab, set Stream Type to be Ribbon

Press Apply

To select multiple locations, press the Location editor button, and press CTRL while clicking.

Ribbons give a 3-D representation of the

flow direction. In the current plot, the colour depends on the flow velocity.

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Velocity isosurface Hide the streamline plot by unchecking it in the tree view

In the Location menu, select Isosurface and accept the default name

in the Geometry tab, set Variable to Velocity and Value to 4 [m s^-1] Click Apply

The velocity magnitude is greater than 4 m/s inside the isosurface , and less than that outside it.

This is just one example you can try other values.

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Velocity isosurface By default, the isosurface is colored by velocity magnitude

In the Colour tab

select Mode to be Variable, Variable to be Temperature, Range to be Local, and press Apply

This is the end of the tutorial. To be able to revisit this problem, quit CFD-Post and save the project in Workbench.

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Further work There are many ways the simulation in this tutorial could be extended

Better inlet profiles

current boundary conditions (velocity inlets) assume uniform profiles

specify profiles (of velocity, turbulence, etc), or

extend the geometry so that inlets and outlets are further from junction

Mesh independence

check that results do not depend on mesh

re-run simulations with finer mesh(es)

generated in Meshing application, or

from adaptive meshing in Fluent

Temperature-dependent physical properties

density

differences could lead to buoyant forces (with gravity turned on)

quite small effects in this case

viscosity, etc

Actually, the current mesh is probably not fine enough one indication of this is that low-order discretization gives different answers.

Note that, by default, there is no gravity in the model this is a setting in the General task page.

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Wrap-up

This workshop has shown the basic steps that are applied in all CFD simulations: - Defining material properties. - Setting boundary conditions and solver settings. - Running a simulation whilst monitoring quantities of interest. - Post-processing the results, both in Fluent and CFD-Post. One of the important things to remember in your own work, before even starting the ANSYS software, is to think WHY you are performing the simulation: - What information are you looking for. - What do you know about the inlet conditions. In this case we were interested in checking the pressure drop, and assessing the amount of mixing present around this T-piece. Knowing your aims from the start will help you make sensible decisions of how much of the part to simulate, the level of mesh refinement needed, and which numerical schemes should be selected.

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