CFD Modeling Using ANSYS FLUENT

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Familiarity with the UI of ANSYS FLUENT. – CFD numerics and ... What can ANSYS FLUENT be employed for? Across the ..... Tutorial HELP in ANSYS FLUENT.
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CFD Modeling Using ANSYS FLUENT

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Course Notes Not for sale. Strictly for private circulation. Published 2015

Before First Simulation Lesson 02

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Across the Table!!



The following background knowledge is expected from the candidates: – Familiarity with the UI of ANSYS FLUENT – CFD numerics and methodology



By the end of the lecture, you should be able to answer: – What and how to prepare before doing a simulation? – What can ANSYS FLUENT be employed for?

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Overview

• • • • • • •

Precision and dimension File handling Units and geometry scale Mesh modifications Mesh nomenclature Available features Support options

ANSYS FLUENT Launcher

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Single and double precision solver



For most cases single precision solvers are sufficiently accurate



Certain kind of problems need double precision – If your geometry has features of very disparate length scales – Mean pressure level is high but pressure driving the flow is small – For conjugate heat transfer problems involving high thermalconductivity ratios and/or high-aspect-ratio grids  2d (2D single precision )  2ddp ( 2D double precision)  3d (3D single precision)  3ddp (3D double precision)

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File I/O in ANSYS FLUENT

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Data handling in ANSYS FLUENT

U

MESH file

• Uns. mesh info • Coordinates • Names of nodes/cells

CASE file • Uns. mesh info • Problem set-up • Numerical set-up • Display settings

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DATA file • Dependent variables’ values at locations specified in the CASE file • u, v, w, p •T • Yi, k, e, …

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Compressed Files





Reading – – – –

Fluent can read files compressed using compress or gzip If you select a file with .gz, it invokes gunzip If you select file with .Z extension, it invokes zcat On Windows, files with only .gz extension can be read

Writing – To compress the file, append .Z or .gz extension to the file name – Fluent invokes the appropriate compression tool and gives following message in case you have added .gz • Writing "| gzip -cfv > flow.cas.gz"...

– On Windows, compression can be performed only with gzip. • .gz extension can compress the file • .Z does not compress the file

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Automatic Numbering

• •

You can include special characters in file name Provides shortcut for numbering the files based on various parameters (iteration no., time step etc.) – E.g. : For unsteady calculations, saving file that reflect the time step at which file is saved • contours-%t.ps : contours-0001.ps

– E.g. : File name which reflect the number of iteration • airfoil-%i : airfoil-005.cas

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Automatic Saving of Case & Data

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Automatic Saving of Case & Data



Case & Data file can be saved automatically at specified interval during calculation – – – –

Useful for time-dependent solution Useful to see how solution has progressed for steady solution Case file may or may not be written every time If Case file undergoes a change, it will always be saved along with the updated Data file



To Autosave



Frequency for saving case and data



In case of limited disc space

– File → Write → Autosave – For steady flow number of iterations – For unsteady flow number of time steps – Use Overwrite Existing File option – Specifies how many maximum files can be written – After saving specified number of files, earliest existing file will be overwritten

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Automatic Saving of Case & Data



Specify the root name of file – .cas or .dat will be added automatically – If .gz or .Z is given, files will be written in compressed format – Iteration number/time-step/flow-time automatically appended to the name, dependent on the choice

Frequency for file saving

In case of limited disk space

Path for saving

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Unit Systems



• • •

Fluent allows to work in any unit system, including inconsistent unit systems – You may work in British units with heat input in Watts (SI) OR you may work in SI units with length defined in inches This is accomplished by using appropriate conversion factors Fluent uses these conversion factors for input and output, internally storing all parameters and performing all calculations in SI units Units can be changed during problem setup or after calculation – If you have completed simulation in SI units but would like to see reports in any other units – Fluent converts all parameters in specified units and displays results Fluent always works in SI units internally and stores all results in SI units To be considered while generating Profile files, Journal files, UDFs, etc.

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Built-In Unit Systems in FLUENT



Four built in unit systems – British, SI, CGS, and "default'' – You can convert all units from one system to another – Define → Units

Default is same as SI, but uses degrees instead of radians for angles Clicking on any system will immediately change all units in selected system

All future inputs will be in selected system 54

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Customizing Units





Used in case of mixed unit systems – Select quantity, select unit from available list or specify your own conversion factor

Before changing any unit, you can list the units of all quantities that are currently used – Press List button in Set Unit form

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Geometry/Mesh scaling • •



Some grid generators allow to define set of units for the mesh dimensions But when mesh is read into Fluent, it always assumes unit of length as meters You can change the unit of grid using – Mesh → Scale

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Defining a New Unit •

To create new unit for selected quantity

– Select the quantity in list – Click New to open Define Unit panel – Set the name of new unit in Unit field – Conversion factor in Factor field – Offset in Offset field – Click OK

E.g. To use hour as unit of Time : Select Time, Select New, Enter Name as hour, Enter 3600 for factor

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Mesh modifications in ANSYS FLUENT

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Operations on Grid

• •

Fluent can import different types of gird Operations on grid – – – – – –

Modify the grid by translating or scaling node coordinates Partitioning the cells for parallel processing Reordering the cells in the domain to decrease bandwidth Merging or separating zones Adapt grid as per solution Diagnostic information on the grid • • • • •

Memory usage Simplex, topological, and domain information Number of nodes, faces, and cells in the grid Determine the minimum and maximum cell volumes in the domain Check for the proper number of nodes and faces per cell

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Grid Topology • Fluent is an unstructured solver – It can accept • • • • •

2D tri and quad 3D hex, tet, pyramid, and wedge Single and multi-block structured grid Hybrid grid Grid with hanging nodes

Hanging Node 60

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Choosing the Appropriate Grid Type



When choosing the mesh type, consider the following issues



Setup Time

– Setup time – Computational expense

– Creation of structured grid for complex geometries is time consuming if not impossible • Gives advantage to unstructured mesh

– If geometry is simple, setup time is almost same



Computational Expense



Mesh Quality

– Mesh generated using unstructured algorithm usually has a very high cell count

– Plays an important role in accuracy and stability of the numerical computation – Attributes associated with mesh quality are • Node point distribution • Smoothness • Skewness 61

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Choosing the Appropriate Grid Type



Why go the hard way: structured mesh? – Grid aligned along the flow direction • Least numerical diffusion – accurate flow prediction

– Great control over the mesh quality and cell count



• Boundary layer capturing

Why go the easy way: unstructured mesh? – Can capture complex geometry with minimum user intervention – If flow is rapidly changing direction, grid alignment along the flow not possible to maintain – Fast computational power available

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Choosing the Appropriate Grid Type L-grid type Multi-block Structured Mesh for flow through a Sector Grid aligned along the flow direction

Unstructured Mesh for flow through a Sector (Octree algorithm) Grid not aligned along the flow direction

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Non-Conformal Grids





In FLUENT it is possible to use a grid composed of cell zones with non-conformal boundaries What is non-conformality ? – Grid node locations are not identical at the boundaries where two subdomains meet

Completely Overlapping Grid Interface Intersection

Partially Overlapping Grid Interface Intersection 64

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Checking the Grid



The grid checking capability in FLUENT provides Domain extents Volume statistics Grid topology Periodic boundary information Verification of simplex counters, and (for axisymmetric cases) node position verification with respect to the axis – Accessible through – – – – –

• Grid → Check

– It is generally a good idea to check your grid right after reading it into the solver, in order to detect any grid trouble before you get started with the problem setup

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Mesh terminology in ANSYS FLUENT

• • • • •

Cell center and Node Edge Face Cell Zone or Thread – Collection of faces – Face zone – Collection of volumetric cells – Cell zone

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Basic Modeling capabilities in ANSYS FLUENT •

2D planar, 2D axi-symmetric, 2D axi-symmetric with swirl (rotationally symmetric), and 3D flows



Quadrilateral, triangular, hexahedral (brick), tetrahedral, prism (wedge), pyramid, and mixed element meshes with non-conformal interface



Steady-state or transient flows



Incompressible or compressible flows, including all speed regimes (low subsonic, transonic, supersonic, and hypersonic flows)



Inviscid, laminar, and turbulent flows



Heat transfer, including forced, natural, and mixed convection, conjugate

(solid/fluid) heat transfer



Multi-species and User-defined scalar/Additional variable simulation

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Advanced Modeling capabilities in ANSYS FLUENT •

Radiation heat transfer modeling, Solar Load Model



Multiple reference frame (MRF) and sliding mesh options for modeling multiple moving frames



Mixing-plane model for modeling rotor-stator interactions, torque converters, and similar turbo-machinery applications with options for mass conservation and swirl conservation



Dynamic mesh model for modeling domains with moving and deforming mesh (IC Engine simulations, reciprocating pumps, flow induced rotations, etc.)



Volumetric sources of mass, momentum, heat



Multiphase flow modeling



Reacting flow modeling – Kinetics and PDF based combustion models



Material property database 68

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Embedded HELP in ANSYS FLUENT

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Content HELP in ANSYS FLUENT

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Control Panel HELP in ANSYS FLUENT

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Model User Guide HELP in ANSYS FLUENT

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Theory HELP in ANSYS FLUENT

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Tutorial HELP in ANSYS FLUENT •

Required Mesh and related files are available from the ANSYS Customer Portal

www1.ansys.com/customer

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Summary



• • • •

Familiarity with file formats necessary to start with simulations FLUENT processes all data internally in SI unit system Appropriate mesh choice necessary for efficient and accurate simulations Mesh modifications possible in ANSYS FLUENT to some extent only ANSYS FLUENT offers on the fly HELP on theory and software specific topics

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