Computational fluid dynamics

1. An Introduction to
Computational Fluid Dynamics
CFD

2. 2
Outline
 What is CFD?
 Why use CFD?
 What are the advantages and limitations of CFD?
 How does CFD Work?
 Where is CFD used?
 Physics
 Modeling
 Numeric
 CFD process
 Resources

3. 3
What is CFD?
 What is CFD and its objective?
– Computational Fluid Dynamics
– Historically Analytical Fluid Dynamics (AFD) and EFD
(Experimental Fluid Dynamics) was used. CFD has become
feasible due to the advent of high speed digital computers.
– Analysis of such a system based on computer simulation for
prediction fluid-flow, heat transfer or chemical reactions
phenomena.
– The objective of CFD is to model the continuous fluids with
Partial Differential Equations (PDEs) and discretize PDEs into an
algebra problem (Taylor series), solve it, validate it and achieve
simulation based design.

4. 4
Why use CFD?
 Why use CFD?
– Analysis and Design
 Simulation-based design instead of “build & test”
– More cost effectively and more rapidly than with experiments
– CFD solution provides high-fidelity database for interrogation of
flow field
 Simulation of physical fluid phenomena that are difficult to be
measured by experiments
– Scale simulations (e.g., full-scale ships, airplanes)
– Hazards (e.g., explosions, radiation, pollution)
– Physics (e.g., weather prediction, planetary boundary layer,
stellar evolution)
– Knowledge and exploration of flow physics

5. 5
Advantages of CFD
 Relatively low cost.
– Using physical experiments and tests to get essential
engineering data for design can be expensive.
– CFD simulations are relatively inexpensive, and costs are
likely to decrease as computers become more powerful.
 Speed.
– CFD simulations can be executed in a short period of time.
– Quick turnaround means engineering data can be introduced
early in the design process.
 Ability to simulate real conditions.
– Many flow and heat transfer processes can not be (easily)
tested, e.g. hypersonic flow.
– CFD provides the ability to theoretically simulate any
physical condition.

6. 6
Advantages of CFD
 Ability to simulate ideal conditions.
– CFD allows great control over the physical process, and
provides the ability to isolate specific phenomena for study.
– Example: a heat transfer process can be idealized with
adiabatic, constant heat flux, or constant temperature
boundaries.
 Comprehensive information.
– Experiments only permit data to be extracted at a limited
number of locations in the system (e.g. pressure and
temperature probes, heat flux gauges, LDV, etc.).
– CFD allows the analyst to examine a large number of
locations in the region of interest, and yields a
comprehensive set of flow parameters for examination.

7. 7
Limitations of CFD
 Physical models.
– CFD solutions rely upon physical models of real world
processes (e.g. turbulence, compressibility, chemistry,
multiphase flow, etc.).
– The CFD solutions can only be as accurate as the physical
models on which they are based.
 Numerical errors.
– Solving equations on a computer invariably introduces
numerical errors.
– Round-off error: due to finite word size available on the
computer. Round-off errors will always exist (though they can
be small in most cases).
– Truncation error: due to approximations in the numerical
models. Truncation errors will go to zero as the grid is refined.
Mesh refinement is one way to deal with truncation error.

8. 8
poor better
Fully Developed Inlet
Profile
Computational Domain
Computational Domain
Uniform Inlet
Profile
Limitations of CFD
 Boundary conditions.
– As with physical models, the accuracy of the CFD solution
is only as good as the initial/boundary conditions provided to
the numerical model.
– Example: flow in a duct with sudden expansion. If flow is
supplied to domain by a pipe, you should use a fully-
developed profile for velocity rather than assume uniform
conditions.

9. How does a CFD code work?
All codes contain three main elements:
A pre-processor
A solver and
A post-processor

10. Discretization for a domain
Discretization for equations
Solution of the algebraic equations
Analysis of results
Basic idea for numerical solution

11. Three basic methods and their relations
Analytical
Numerical
Experimental

12. 12
Where is CFD used? (Aerospace)
• Where is CFD used?
– Aerospace
– Appliances
– Automotive
– Biomedical
– Chemical Processing
– HVAC&R
– Hydraulics
– Marine
– Oil & Gas
– Power Generation
– Sports
F18 Store Separation
Wing-Body Interaction Hypersonic Launch
Vehicle

13. 13
Where is CFD used? (Appliances)
• Where is CFD used?
– Aerospace
– Appliances
– Automotive
– Biomedical
– Chemical Processing
– HVAC&R
– Hydraulics
– Marine
– Oil & Gas
– Power Generation
– Sports
Surface-heat-flux plots of the No-Frost
refrigerator and freezer compartments helped
BOSCH-SIEMENS engineers to optimize the
location of air inlets.

14. 14
Where is CFD used? (Automotive)
• Where is CFD used?
– Aerospace
– Appliances
– Automotive
– Biomedical
– Chemical Processing
– HVAC&R
– Hydraulics
– Marine
– Oil & Gas
– Power Generation
– Sports
External Aerodynamics Undercarriage
Aerodynamics
Interior Ventilation
Engine Cooling

15. 15
Where is CFD used? (Biomedical)
• Where is CFD used?
– Aerospace
– Appliances
– Automotive
– Biomedical
– Chemical Processing
– HVAC&R
– Hydraulics
– Marine
– Oil & Gas
– Power Generation
– Sports
Temperature and natural
convection currents in the eye
following laser heating.
Spinal Catheter
Medtronic Blood Pump

16. 16
Where is CFD used? (Chemical Processing)
• Where is CFD used?
– Aerospace
– Appliances
– Automotive
– Biomedical
– Chemical Processing
– HVAC&R
– Hydraulics
– Marine
– Oil & Gas
– Power Generation
– Sports
Polymerization reactor vessel - prediction
of flow separation and residence time
effects.
Shear rate distribution in twin-
screw extruder simulation
Twin-screw extruder
modeling

17. 17
Where is CFD used? (HVAC&R)
• Where is CFD used?
– Aerospace
– Appliances
– Automotive
– Biomedical
– Chemical Processing
– HVAC&R
– Hydraulics
– Marine
– Oil & Gas
– Power Generation
– Sports
Particle traces of copier VOC emissions
colored by concentration level fall
behind the copier and then circulate
through the room before exiting the
exhaust.
Mean age of air contours indicate
location of fresh supply air
Streamlines for workstation
ventilation
Flow pathlines colored by
pressure quantify head loss
in ductwork

18. 18
Where is CFD used? (Hydraulics)
• Where is CFD used?
– Aerospace
– Appliances
– Automotive
– Biomedical
– Chemical Processing
– HVAC&R
– Hydraulics
– Marine
– Oil & Gas
– Power Generation
– Sports

19. 19
Where is CFD used? (Marine)
• Where is CFD used?
– Aerospace
– Appliances
– Automotive
– Biomedical
– Chemical Processing
– HVAC&R
– Hydraulics
– Marine
– Oil & Gas
– Power Generation
– Sports

20. 20
Where is CFD used? (Oil & Gas)
• Where is CFD used?
– Aerospace
– Appliances
– Automotive
– Biomedical
– Chemical Processing
– HVAC&R
– Hydraulics
– Marine
– Oil & Gas
– Power Generation
– Sports
Flow vectors and pressure
distribution on an offshore oil rig
Flow of lubricating
mud over drill bit
Volume fraction of water
Volume fraction of oil
Volume fraction of gas
Analysis of multiphase
separator

21. 21
Where is CFD used? (Power Generation)
• Where is CFD used?
– Aerospace
– Appliances
– Automotive
– Biomedical
– Chemical Processing
– HVAC&R
– Hydraulics
– Marine
– Oil & Gas
– Power Generation
– Sports
Flow pattern through a water
turbine.
Flow in a
burner
Flow around cooling
towers
Pathlines from the inlet
colored by temperature
during standard
operating conditions

22. 22
Where is CFD used? (Sports)
• Where is CFD used?
– Aerospace
– Appliances
– Automotive
– Biomedical
– Chemical Processing
– HVAC&R
– Hydraulics
– Marine
– Oil & Gas
– Power Generation
– Sports

23. 23
Physics
 CFD codes typically designed for representation
of specific flow phenomenon
– Viscous vs. inviscid (no viscous forces) (Re)
– Turbulent vs. laminar (Re)
– Incompressible vs. compressible (Ma)
– Single- vs. multi-phase (Co)
– Thermal/density effects and energy equation (Pr, g, Gr,
Ec)
– Free-surface flow and surface tension (Fr, We)
– Chemical reactions, mass transfer
– etc…

24. 24
Physics
Fluid Mechanics
Inviscid Viscous
Laminar Turbulence
Internal
(pipe,valve)
External
(airfoil, ship)
Compressible
(air, acoustic)
Incompressible
(water)
Components of Fluid Mechanics

25. 25
Governing Equations
Continuity
Equation of motion
(Equations based on “average” velocity)
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26. Claude-Louis Navier George Gabriel Stokes
Navier-Stokes Equations

27. 27
Navier-Stokes Equations
(constant  and m)
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