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Computational Fluid Dynamics (CFD)
Hashim Hasnain Hadi(13ME36)M. Hanzla Tahir(13ME37)Sardar Gulshan
Lal(13ME39)AND ALL CLASMATESBatch 2013-14Dept. of Mechanical
EngineeringBalochistan University of Engineering &
Technology,Khuzdar..
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*Outline
What is CFD?Why use CFD?Where is CFD
used?PhysicsModelingNumericsCFD processResources
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*What is CFD?What is CFD and its objective?
Computational Fluid DynamicsHistorically Analytical Fluid
Dynamics (AFD) and EFD (Experimental Fluid Dynamics) was used. CFD
has become feasible due to the advent of high speed digital
computers.Computer simulation for prediction of fluid-flow
phenomena. The objective of CFD is to model the continuous fluids
with Partial Differential Equations (PDEs) and discretize PDEs into
an algebra problem , solve it, validate it and achieve simulation
based design.
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What is CFD?The field in which computers and numerical analysis
are combined to solve fluid problems/Energy prblems is termed as
Computational fluid dynamics
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*Why use CFD?Why use CFD?
Analysis and DesignSimulation-based design instead of build
& testMore cost effectively and more rapidly than with
experimentsCFD solution provides high-fidelity database for
interrogation of flow fieldSimulation of physical fluid phenomena
that are difficult to be measured by experimentsScale 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
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*Where is CFD used? (Aerospace)Where is CFD
used?AerospaceAppliancesAutomotiveBiomedicalChemical
ProcessingHVAC&RHydraulicsMarineOil & GasPower
GenerationSports
F18 Store Separation Wing-Body Interaction Hypersonic Launch
Vehicle
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*Where is CFD used? (Appliances)Where is CFD
used?AerospaceAppliancesAutomotiveBiomedicalChemical
ProcessingHVAC&RHydraulicsMarineOil & GasPower
GenerationSports
Surface-heat-flux plots of the No-Frost refrigerator and freezer
compartments helped BOSCH-SIEMENS engineers to optimize the
location of air inlets.
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*Where is CFD used? (Automotive)Where is CFD
used?AerospaceAppliancesAutomotiveBiomedicalChemical
ProcessingHVAC&RHydraulicsMarineOil & GasPower
GenerationSports
External Aerodynamics Undercarriage Aerodynamics Interior
Ventilation Engine Cooling
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*Where is CFD used? (Biomedical)Where is CFD
used?AerospaceAppliancesAutomotiveBiomedicalChemical
ProcessingHVAC&RHydraulicsMarineOil & GasPower
GenerationSports
Temperature and natural convection currents in the eye following
laser heating. Medtronic Blood Pump
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*Where is CFD used? (Chemical Processing)Where is CFD
used?AerospaceAppliancesAutomotiveBiomedicalChemical
ProcessingHVAC&RHydraulicsMarineOil & GasPower
GenerationSports
Polymerization reactor vessel - prediction of flow separation
and residence time effects.
Shear rate distribution in twin-screw extruder
simulationTwin-screw extruder modeling
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*Where is CFD used? (HVAC&R)Where is CFD
used?AerospaceAppliancesAutomotiveBiomedicalChemical
ProcessingHVAC&RHydraulicsMarineOil & GasPower
GenerationSports
Flow pathlines colored by pressure quantify head loss in
ductwork
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*Where is CFD used? (Hydraulics)Where is CFD
used?AerospaceAppliancesAutomotiveBiomedicalChemical
ProcessingHVAC&RHydraulicsMarineOil & GasPower
GenerationSports
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*Where is CFD used? (Marine)Where is CFD
used?AerospaceAppliancesAutomotiveBiomedicalChemical
ProcessingHVAC&RHydraulicsMarineOil & GasPower
GenerationSports
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Where is CFD used? (Oil & Gas)Where is CFD
used?AerospaceAppliancesAutomotiveBiomedicalChemical
ProcessingHVAC&RHydraulicsMarineOil & GasPower
GenerationSports
Flow vectors and pressure distribution on an offshore oil
rig
Flow of lubricating mud over drill bit
Volume fraction of water Volume fraction of oilVolume fraction
of gasAnalysis of multiphase separator
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*Where is CFD used? (Power Generation)Where is CFD
used?AerospaceAppliancesAutomotiveBiomedicalChemical
ProcessingHVAC&RHydraulicsMarineOil & GasPower
GenerationSportsFlow pattern through a water turbine.
Flow around cooling towers
Pathlines from the inlet colored by temperature during standard
operating conditions
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*Where is CFD used? (Sports)Where is CFD
used?AerospaceAppliancesAutomotiveBiomedicalChemical
ProcessingHVAC&RHydraulicsMarineOil & GasPower
GenerationSports
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*PhysicsCFD 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 (Ca)Thermal/density effects and energy equation (Pr, g,
Gr, Ec)Free-surface flow and surface tension (Fr, We)Chemical
reactions, mass transferetc
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PhysicsFluid
MechanicsInviscidViscousLaminarTurbulenceInternal(pipe,valve)External(airfoil,
ship)Compressible(air, acoustic)Incompressible(water)Components of
Fluid Mechanics
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*Claude-Louis NavierGeorge Gabriel StokesNavier-Stokes
Equation
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*ModelingMathematical representation of the physical problem
Some problems are exact (e.g., laminar pipe flow)Exact solutions
only exist for some simple cases. In these cases nonlinear terms
can be dropped from the N-S equations which allow analytical
solution.Most cases require models for flow behavior [e.g.,
Reynolds Averaged Navier Stokes equations (RANS) or Large Eddy
Simulation (LES) for turbulent flow]Initial Boundary Value Problem
(IBVP), include: governing Partial Differential Equations (PDEs),
Initial Conditions (ICs) and Boundary Conditions (BCs)
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*Governing Equations Continuityx - Equation of motion(Equations
based on average velocity)
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*Numerics / DiscretizationComputational solution of the
IBVPMethod dependent upon the model equations and physicsSeveral
components to formulation
Discretization and linearizationAssembly of system of algebraic
equationsSolve the system and get approximate solutions
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*Finite DifferencesMethods of SolutionDirect methodsIterative
methodsCramers Rule, Gauss eliminationLU decompositionJacobi
method, Gauss-SeidelMethod, SOR methodFinite
differencerepresentationTruncation error
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*Numeric Solution (Finite Differences)ox
ii+1i-1j+1jj-1imaxjmaxTaylors Series Expansion u i,j = velocity
of fluidDiscrete Grid Points
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*CFD processGeometry descriptionSpecification of flow conditions
and propertiesSelection of modelsSpecification of initial and
boundary conditionsGrid generation and transformationSpecification
of numerical parametersFlow solutionPost processing: Analysis, and
visualization
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*Domain for bottle filling problem.Filling NozzleBottleCFD - how
it worksAnalysis begins with a mathematical model of a physical
problem.Conservation of matter, momentum, and energy must be
satisfied throughout the region of interest.Fluid properties are
modeled empirically.Simplifying assumptions are made in order to
make the problem tractable (e.g., steady-state, incompressible,
inviscid, two-dimensional).Provide appropriate initial and boundary
conditions for the problem.
2673.bin
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*Geometry descriptionTypical approaches
Make assumptions and simplificationsCAD/CAE
integrationEngineering drawingsCoordinates include Cartesian system
(x,y,z), cylindrical system (r, , z), and spherical system(r, ,
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*Selection of models for flow fieldDirect Numerical Simulations
(DNS) is to solve the N-S equations directly without any modeling.
Grid must be fine enough to resolve all flow scales. Applied for
laminar flow and rare be used in turbulent flow.Reynolds Averaged
Navier-Stokes (NS) equations (RANS) is to perform averaging of NS
equations and establishing turbulent models for the eddy viscosity.
Too many averaging might damping vortical structures in turbulent
flowsLarge Eddy Simulation (LES), Smagorinsky constant model and
dynamic model. Provide more instantaneous information than RANS
did. Instability in complex geometriesDetached Eddy Simulation
(DES) is to use one single formulation to combine the advantages of
RANS and LES.
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*Mesh for bottle filling problem.CFD - how it works (2)CFD
applies numerical methods (called discretization) to develop
approximations of the governing equations of fluid mechanics in the
fluid region of interest.
Governing differential equations: algebraic.The collection of
cells is called the grid. The set of algebraic equations are solved
numerically (on a computer) for the flow field variables at each
node or cell.System of equations are solved simultaneously to
provide solution.The solution is post-processed to extract
quantities of interest (e.g. lift, drag, torque, heat transfer,
separation, pressure loss, etc.).
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*DiscretizationDomain is discretized into a finite set of
control volumes or cells. The discretized domain is called the grid
or the mesh.General conservation (transport) equations for mass,
momentum, energy, etc., are discretized into algebraic
equations.All equations are solved to render flow field.
control volume
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*Design and create the gridShould you use a quad/hex grid, a
tri/tet grid, a hybrid grid, or a non-conformal grid?What degree of
grid resolution is required in each region of the domain?How many
cells are required for the problem?Will you use adaption to add
resolution?Do you have sufficient computer memory?
arbitrary polyhedron
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*Tri/tet vs. quad/hex meshesFor simple geometries, quad/hex
meshes can provide high-quality solutions with fewer cells than a
comparable tri/tet mesh.
For complex geometries, quad/hex meshes show no numerical
advantage, and you can save meshing effort by using a tri/tet
mesh.
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*Set up the numerical modelFor 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.
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*Initial and boundary conditionsFor steady/unsteady flow
IC should not affect final solution, only convergence path, i.e.
iteration numbers needed to get the converged solution.Robust codes
should start most problems from very crude IC, . 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.
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*Compute the solutionThe discretized conservation equations are
solved iteratively. A number of iterations are usually required to
reach a converged solution.Convergence is reached when:
Changes in solution variables from one iteration to the next are
negligible.Residuals provide a mechanism to help monitor this
trend.Overall property conservation is achieved.The accuracy of a
converged solution is dependent upon:
Appropriateness and accuracy of the physical models.Grid
resolution and independence.Problem setup.
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*Numerical parameters & flow solution Typical time history
of residuals The closer the flow field to the converged solution,
the smaller the speed of the residuals decreasing.
Solution converged, residuals do not change after more
iterations
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*Post-processingAnalysis, and visualization
Calculation of derived variablesVorticityWall shear stress
Calculation of integral parameters: forces, momentsVisualization
(usually with commercial software)Simple X-Y plotsSimple 2D
contours3D contour carpet plotsVector plots and streamlines
(streamlines are the lines whose tangent direction is the same as
the velocity vectors)Animations (dozens of sample pictures in a
series of time were shown continuously)
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*Examine the resultsVisualization can be used to answer such
questions as:
What is the overall flow pattern?Is there separation?Where do
shocks, shear layers, etc. form?Are key flow features being
resolved?Are physical models and boundary conditions
appropriate?Numerical reporting tools can be used to calculate
quantitative results, e.g:Lift, drag, and torque.Average heat
transfer coefficients.Surface-averaged quantities.
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*Velocity vectors around a dinosaur
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*Velocity magnitude (0-6 m/s) on a dinosaur
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*Pressure field on a dinosaur
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*Advantages of CFDRelatively 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.
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*Limitations of CFDPhysical 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.
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*Limitations of CFD (2)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.
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*Software and resourcesCFD software was built upon physics,
modeling, numerics.Two types of available software
Commercial (e.g., FLUENT, CFX, Star-CD)Research (e.g.,
CFDSHIP-IOWA, U2RANS)More information on CFD can be got on the
following website:
CFD Online: http://www.cfd-online.com/CFD softwareFLUENT:
http://www.fluent.com/CFDRC: http://www.cfdrc.com/Computational
Dynamics: http://www.cd.co.uk/CFX/AEA:
http://www.software.aeat.com/cfx/Grid generation softwareGridgen:
http://www.pointwise.comGridPro:
http://www.gridpro.com/HypermeshVisualization softwareTecplot:
http://www.amtec.com/Fieldview: http://www.ilight.com/
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Software Used1. Matlab 2. Ansys3. Pro-Engineer4. Autodesk
Inventor professional. CATIA6. Fluent7. Maple
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Sofware UsedTecplotIcemCFDFemlab
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