Introduction to CFD

Sachin Pande

Introduction to CFD

Slide 2

Content

■ What is CFD

■ Why CFD

■ Application of CFD

■ CFD for oil & gas

■ CFD analysis process

■ Related software

■ Issue related with software selection

■ Pre-processing

■ Solver

■ Post-processing

■ Typical CFD problem

■ Grid Independence study

■ Validation

■ Sample problem

Sachin Pande

Introduction to CFD

Slide 3

What is CFD ?

Computational Fluid Dynamics (CFD) is the use of computers and

numerical techniques to solve problems involving “Fluid flow”

● Previously only Theoretical Fluid Dynamics and Experimental Fluid

dynamics exist

● But advent of computers and development in it made CFD possible

Now, this is just a simulation of what the blocks will look like once they are assembled

Sachin Pande

Introduction to CFD

Slide 4

What is CFD? Brief History

■ Foundation of Experimental Fluid dynamics – In France and England in 17th

century

■ Development of Theoretical Fluid Dynamics – In Europe in 18th and 19th century

■ 20th century witnessed the use of pure theory on one side and pure experiments

on other side

● Observe the experimental results, formulate the theory, or modify the

existing one guided by the experimental results.

● Look at the theoretical results, set up experiments to verify the theoretical

results and later use the experience

■ Suppose for a real life problem in Fluid Dynamics, no theoretical studies are

available and experimental studies can not be made due to inherent

impracticability, cost, time design problem etc.

■ What shall we do?

Sachin Pande

Introduction to CFD

Slide 5

Why CFD ?

Scaled Model creation

Measurement Probe insertion Visualization or recording of data

Most Accurate, but

• Big setup required

• Huge investment

• Time-consuming

• Short duration run

• Single purpose

Wind Tunnel- ONERA S1MA Working concept Aircraft ~ 900 km/hr

■ To appreciate CFD, first we should know what is experimental study

Sachin Pande

Introduction to CFD

Slide 6

Why CFD ?

■ Qualitative tool to discard various designs / ideas

■ Observation of flow properties without disturbing the flow itself

■ Observation of flow properties at a locations which are not accessible or can

be harmful to measuring equipments

Combustion chamber, between turbine blade…etc

■ Cost effective, fast, parallel, and multi-purpose compared to experiments

■ Simulation of physical fluid phenomenon that are difficult for experiments

Full aircraft simulation

Environmental effects

Sachin Pande

Introduction to CFD

Slide 7

Why CFD ?

■ Simulation of physical fluid phenomenon that are difficult for experiments

Tacoma Narrows Bridge, Washington

Collapsed on Nov, 01, 1940

Aeroelastic flutter

Sachin Pande

Introduction to CFD

Why CFD ?

In brief, CFD enables scientists and engineers to perform “numerical

experiments” (in other word computer simulations) in a “virtual flow

laboratory”

Real Experiment Numerical Experiment

[CFD Simulation]

Slide 8

Sachin Pande

Introduction to CFD

Slide 9

Application of CFD

■ Aerodynamics of cars and aircrafts

■ Hydrodynamics of ships

■ Turbomachinery - Pumps and turbines

■ Combustion - IC engine, jet engine

■ Heat transfer – heating and cooling

■ Process engineering – mixing, reacting

■ Electronic cooling

■ HVAC - Building ventilation

■ Fire and explosion hazards

Sachin Pande

Introduction to CFD

Slide 10

Application of CFD

■ Wind loading - force and dynamics of

structure

■ Hydrology - flow in river and aquifers

■ Oceanography - tidal flow, ocean current

■ Meteorology - weather forecasting

■ Biomedical engineering - blood flow

■ Wind power

■ Oil & gas

■ Chemical engineering

■ Dispersion of pollutant

Sachin Pande

Introduction to CFD

Slide 11

CFD for Oil & Gas

■ Drill bit coolant analysis

■ Natural ventilation

■ Fire and smoke propagation

■ Process equipments

■ Platform aerodynamics

■ Heat transfer

■ Hydraulic, pneumatic valve

■ Blower, fan, pumps, compressor

Sachin Pande

Introduction to CFD

Slide 12

Part II

Sachin Pande

Introduction to CFD

Slide 13

CFD Analysis Process

■ Formulate the flow problems

■ Model the geometry or simplify

■ Establish the boundary condition

■ Generate Mesh

■ Establish the simulation strategy

■ Perform simulation

■ Monitor the simulation for completion

■ Post-process the simulation to get the results

■ Repeat the process to examine sensitivity

■ Report preparation

Optional

Tentative time required

Sachin Pande

Introduction to CFD

Slide 14

CFD Analysis Process

Pre-Processing

Geometry cleanup

Mesh generation

Boundary conditions

Solving

Steady/Transient state

Single/Multi-phase

Inviscid/Turbulent

Incompressible/compressible

Post-Processing

Visualization of data using

Contour Vector plot

Streamlines XY plot

Grouping into

Major three process

Sachin Pande

Introduction to CFD

Slide 15

CFD Analysis Process

Pre-Processing

Solving Post-Processing

Grouping into

Major three process

(Graphical Presentation)

Geometry

Creation

Meshing

Sachin Pande

Introduction to CFD

Slide 16

Related software

Pre-Processing Software

Ansys - ICEM / Gambit / T-grid

Pro-star / Star-CCM+

Gridgen

Numeca

Solver

Ansys - Fluent / CFX

Star-CD / Star-CCM+

OpenFOAM

Post-Processing Software

Ansys - Fluent / CFX

Star-CD / Star-CCM+

Techplot

Advanced Visual Systems

Commercial Software

which cater need of these

process

Sachin Pande

Introduction to CFD

Slide 17

Issue related with software selection

■ Application

● Specific code vs. general code

■ Commercial aspects

● Cost

● Service and maintenance

■ Technical constraint in switching to another software

● Difficult to change ‘a process’ set from long time

● Huge data which is not compatible with new software

● Cost and time involved in conversion of data

■ Compatibility Issues in data exchange

● Workbench concept - CAD, CFD, FEA is integrated

Can we afford to have two software

Sachin Pande

Introduction to CFD

Slide 18

Part III

Sachin Pande

Introduction to CFD

Slide 19

Pre-processing: Geometry

■ Geometry generation from scratch (for Simple problems, 2D to 3D)

● Extrusion from 2d drawing

● NACA aerofoil generation

■ Geometry cleanup or simplification

● Large domain – Car parking, environmental

● Flow distribution is aim and not accuracy – fire simulation, pollution

● Example - Flow over a building, car parking, environmental

■ Extraction of fluid domain – Negative of solid domain

Sachin Pande

Introduction to CFD

Slide 20

Preprocessing: Mesh Generation

Quadrilateral – Hexahedral mesh

Most accurate solution

Time consuming for complicated profile

Suitable for – critical problems, Aerospace

Triangular – Tetrahedral mesh

Less accuracy

Time saver for complicated profile

Suitable for – Automotive, environmental

Combining advantages - hybrid mesh

“Good mesh is Half CFD done”

Sachin Pande

Introduction to CFD

Slide 21

Preprocessing: Mesh Generation

Automated cartesian mesh

Moderate accurate solution

Time saver for complicated profile

Effect on accuracy is in testing phase

Automated polyhedral mesh

Moderate accurate solution

Time saver for complicated profile

Effect on accuracy is in testing phase

Sachin Pande

Introduction to CFD

Slide 22

Solver: Governing Equation

■ Flow is governed by few equations which are based on three conservation

laws of physics,

■ Conservation Laws,

● Conservation of mass General

● Conservation of momentum Newton’s second law

● Conservation of energy First law of thermodynamics

■ Conservation laws derived by considering a given quantity of matter (or

control mass (CM)) and its extensive properties, mass, momentum and

energy

■ This approach is used to study the dynamics of solid bodies, where CM is

easily identified.

■ For Fluid flow, this is not appropriate and need to transform these laws into

a Control Volume form

0dt

dm

f

dt

mvd

Sachin Pande

Introduction to CFD

Slide 23

Solver: Integral form of Navier-Stoke Equations

General conservation equation in integral form, called as Navier-Stokes equation

Rate of change

of in a control

volume

Convective flux

across the

surface

Diffusive flux

across the

surface

Source or sink

term

Rate of change

of density Net flow rate due

to velocity

Net flow rate due

to concentration

Mass produced

due to reaction

When = 1 Mass is conserved Unit of density is

u, v, w Momentum is conserved

E Energy is conserved

kgmm

kg 3

3

dVSdAgraddAdvt

)(n)V(n

Applying Gauss divergence theorem, we can transform surface integral into volume integral.

This lead to differential form of equation

Sachin Pande

Introduction to CFD

0

z

w

y

v

x

u

t

zyxx

p

z

uw

y

uv

x

u

t

u xzxyxx

r

Re

12

zyxy

p

z

vw

y

v

x

uv

t

v yzyyxy

r

Re

12

zyxz

p

z

w

y

vw

x

uw

t

w zzyzxz

r

Re

12

zzyzxzyxyyxyxzxyxx

r

zyx

rr

TTTT

wvuz

wvuy

wvux

z

q

y

q

x

q

z

wp

y

vp

x

up

z

wE

y

vE

x

uE

t

E

Re

1

PrRe

1

Solver: Differential form of Navier-Stoke Equations

Continuity

X-momentum

Y-momentum

Z-momentum

Energy

Three-dimensional unsteady form of Navier-Stokes equations

Jz

H

y

G

x

F

t

U

Slide 24

Sachin Pande

Introduction to CFD

Slide 25

■ Divide the domain into a number of control volumes (cell, elements) where

the variable is located at the centroid of the control volume.

● Mesh Generation

■ “Integrate” the differential form of governing equation over each control

volume.

■ Discretization involves the substitution of a variety of “finite-difference-type

approximations” for the terms in the integrated equation representing flow

processes such as convection, diffusion and sources. This convert the

integral equation into a system of algebraic equations

■ Result is a set of linear algebraic equations: one for each control volume.

■ Solve iteratively or simultaneously with some initial guess

Solver: Finite volume Method

Sachin Pande

Introduction to CFD

...!4!3!2 4

44

3

33

2

22

1

iiii

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ux

x

ux

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ux

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Forward difference Backward difference

Taylor Series expansion

Central difference

In short, more the number of points in calculation,

more is the accuracy

Solver: Numerical Scheme

Eq. 1

Eq. 2

iu 1iu1iu

Example

Slide 26

Sachin Pande

Introduction to CFD

0

y

v

x

u

2

2

y

u

e

p

xy

uv

x

uu

■ 2D incompressible laminar flow boundary layer

(L,m-1)

(L,m)

(L,m+1)

(L-1,m)

1l

l lmm m

uuu u u

x x

1

ll lmm m

vuv u u

y y

2

1 12 22l l l

m m m

uu u u

y y

Solver: Numerical Scheme

Backward Difference

Forward Difference

Central Difference

Slide 27

Sachin Pande

Introduction to CFD

Slide 28

■ The iterative process is repeated until the “change” in the variable from one

iteration to the next becomes so small that the solution can be considered

converged.

■ At convergence:

● All discrete conservation equations (momentum, energy, etc.) are

obeyed in all cells to a specified limit.

● The solution no longer changes with additional iterations.

● Mass, momentum, energy and scalar balances are obtained.

■ Residuals measure imbalance (or error) in conservation equations.

Solver: Convergence

Sachin Pande

Introduction to CFD

Slide 29

Post processing

■ The Purpose of Computing is insight, along with numbers

■ A picture is worth a million word

■ Various post processing methods,

● Contour plot

● Vector plot

● Streamline plot

● Isolines plot

● XY plot

● Animation

Sachin Pande

Introduction to CFD

Slide 30

Typical CFD Problem

Effect of domain discretization (mesh density) and higher-order scheme (solver)

Courtesy : http://www.ltnt.ethz.ch/teaching/IntrotoCFD/cfd_class_April_13_2010.pdf

Cold Fluid

T = 0 °C

Hot Fluid

T = 100 °C

Expected result Boundary conditions Problem definition

Inle

t B

C

T =

100 °

C

Outlet BC

T = 100 °C

Ou

tlet

BC

T =

0 °

C

Inlet BC

T = 0 °C

Sachin Pande

Introduction to CFD

Slide 31

Typical CFD Problem

For same boundary conditions, I have four solutions!

Courtesy : http://www.ltnt.ethz.ch/teaching/IntrotoCFD/cfd_class_April_13_2010.pdf

First order Upwind Second order Upwind

Effect of domain discretization (mesh density) and higher-order scheme (solver)

Sachin Pande

Introduction to CFD

Slide 32

Grid Independence Study

60 x 60 grid can produce reasonable

accurate solution in less time

How much I should do grid refinement? Grid independence Study

dSmin is the minimum grid spacing along

the airfoil surface

Sachin Pande

Introduction to CFD

Slide 33

Validation : Comparison with experimental results

Are these results correct? How to verify?

Sachin Pande

Introduction to CFD

Slide 34

Sample Problem: High Lift Configuration

Mach Number 0.15

Reynolds number 14.3 million

Total Pressure 4.3 atm

Geometry Domain Mesh

Boundary conditions

Post processing

Comparison with experimental results

Sachin Pande

Introduction to CFD

Slide 35

Sample Problem: Slender body aerodynamics

Angle of Attack 45

Reynolds number 1.07 e+05

Geometry Domain Mesh

Boundary conditions

Post processing

Comparison with experimental results

Diameter d = 8 cm (0.08 m)

Wind Tunnel Dimension – 1.02 x 0.76 meters

2.65% to -0.28%~3.4-3.53.49Fine45

-5.3% to -8%~3.4-3.53.22Coarse45

Percentage DiffExp CLCFD CLGridAoA

2.65% to -0.28%~3.4-3.53.49Fine45

-5.3% to -8%~3.4-3.53.22Coarse45

Percentage DiffExp CLCFD CLGridAoA

Sachin Pande

Introduction to CFD

Slide 36

Books

■ An introduction of Computational Fluid Dynamics, The Finite Volume Method,

H.K. Versteeg and W. Malalasekera

■ Computational Fluid Dynamics, The Basics with Applications,

John D. Anderson, Jr.

■ Computational Fluid Dynamics,

T.J. Chung

■ Computational Methods for Fluid Dynamics,

Joel H. Ferziger and Milovan Peric

Courtesy : I donwloaded images from various we site to make this presentaion more understandable I would like to thank to authors whose

presentation or research data is available on internet

Sachin Pande

Introduction to CFD

Slide 37

Future Study

Turbulence modeling :

A vast area to cover and has significant influence on CFD results

Sachin Pande

Introduction to CFD

Slide 38

Thank you !