Computational Fluid Dynamics
Lecture 6
by
Dr. A. Nurye
Faculty of Mechanical Engineering
nurye@ump.edu.my
Stages of CFD Analysis
• Aims
– The aim of this chapter is to introduce students the main stages of
CFD analysis
• Expected Outcomes: At the end of this lecture, students
should be able to understand
– elements of CFD analysis
– how to generate computational grid
• References
Stages of CFD Analysis
Contents
Stages of CFD analysis
– Pre-processing
– Solving
– Post-processing
Stages of CFD Analysis
Introduction
There are many free and commercial CFD packages
available.
Popular non-commercial CFD solvers include
OpenFOAM (http://www.openfoam.com/)
CodeSaturne (http://code-saturne.org/cms/)
An excellent web portal for all things CFD is
http://www.cfd-online.com/.
For this course, the commercial software called
Ansys Fluent will be used for the lab exercise and
projects.
Stages of CFD Analysis
Some of the commercial CFD codes include :
Ansys CFD,
Comsol Multiphysics
Star-CD,
CFDRC, CFX/AEA, etc.
Stages of CFD Analysis 5
CFD solution includes three major processes
• Geometry creation
• Material properties selection
Pre-
• Model selection processing
• Setting of initial and boundary conditions
• Mesh generation
• Specification of numerical parameters
Solving
• Flow solution
• Results visualization and analysis
Post-
• Uncertainty assessment processing
Stages of CFD Analysis 6
Pre-processing
This stage is the first step in CFD analysis. It involves
I. definition and creation of the geometry of the flow
region;
II. mesh generation;
III. specifying the physics (equations to be solved) and
fluid properties;
IV. specifying the boundary conditions
Stages of CFD Analysis
I. Creating the computational domain (geometry)
It is representation of the real problem that is to be
solve.
Before creating/importing the computational domain,
necessary assumptions and simplifications need to be
done in relation to the intended analysis.
Simple geometries can be easily created by few geometric
parameters (e.g. circular pipe) using Ansys Workbench.
Complex geometries can be created CAD/CAE or
Solidworks or other similar software and importing the
geometry (e.g. shell and tube heat exchanger) into Ansys
Fluent.
Stages of CFD Analysis
A typical Ansys Fluent Geometry: DesignModeler
Stages of CFD Analysis
Material properties and Model selection
For a given problem, you will need to:
– Select appropriate physical models.
• Turbulence, combustion, multiphase, etc.
– Define material properties.
• Fluid
• Solid
• Mixture
– Prescribe operating conditions.
– Prescribe boundary conditions at all boundary zones.
– Provide an initial solution.
– Set up solver controls.
– Set up convergence monitors.
Stages of CFD Analysis
Initial and boundary conditions
Initial condition should not affect the final solution, only
convergence path, i.e. iteration numbers needed to get the
converged solution.
But more reasonable guess can speed up the
convergence.
Boundary conditions
– No-slip or slip-free on the wall, periodic, inlet (velocity
inlet, mass flow rate, constant pressure, etc.), outlet
(constant pressure, velocity convective, buffer zone,
zero-gradient), and non-reflecting (compressible flows,
such as acoustics), etc.
Stages of CFD Analysis 11
Shell surfaces
Cold gas outlet
Cold liquid inlet
Hot gas inlet
Hot Liquid outlet
Heat exchanger model using Ansys Fluent
Stages of CFD Analysis
II. mesh generation
Meshing is splitting of flow domains in to many
smaller sub-domains
The type of grid (mesh) used is different for 2D and
3D cases
Triangular or quadrilateral grids can be used in 2D meshing
Tetrahedral or hexahedral grids can be used in 3D meshing
Depending on the discretization scheme and
application of the simulation, either structured or
unstructured mesh type can be used.
Stages of CFD Analysis
In complex geometries unstructured grids can be
generated faster than structured grids. However, the
opposite is true for simple geometries.
In terms of accuracy, structured meshes are more
accurate for simpler problems. However, for more complex
flows, the adaptivity facilitated by an unstructured grid may
allow more accurate solutions.
Structured meshes are advantageous in terms of
calculation time as they take less time to calculate the
problem compared to unstructured meshes.
Example of
unstructured Mesh
Example of
structured Mesh
Stages of CFD Analysis
In some case mixture of structured and unstructured girds
called hybrid mesh can be used
https://commons.wikimedia.org/
wiki/File:Unstructured_Grid.jpg
Stages of CFD Analysis
Types of grid elements
2D
quadrilateral
Triangle
tetrahedron hexahedron
3D
pyramid prism/wedge
Stages of CFD Analysis
If you are not using commercial softwares, note that the
following mesh properties are desired for accurate flow
prediction:
i. Two adjacent grids should have the same node points
Wrong mesh correct mesh
ii. Finer meshes must be applied near a fluid boundary
layer and other similar boundaries.
A typical example of fine
mesh near an airfoil walls
generated by Ansys
Fluent
Stages of CFD Analysis
iii. Select meshes than can be easily generated.
iv. Pay attention to the orthogonality of the mesh edges.
Meshes with very small or very large angles may lead to
inaccurate results, and affect the convergence.
v. Try to minimize the number of meshes as much as
possible.
Stages of CFD Analysis
Exercise
Following the step-by-step guidelines provided in Kalam,
create geometry and mesh for the air flow through a circular
tube.
After finishing this exercise, you should be able
to understand the difference between
•sketching and modelling modes
•DesignModeller and Fluent-Mesh
•surface and volume Meshes
Stages of CFD Analysis
Dr. A. Nurye
Research interest:
- Computational Fluid Dynamics,
- Thermo-fluids,
- Multidisciplinary Numerical Modelling and Simulation
Contact:
Tel: +094246259
email: nurye@ump.edu.my
