Computational fluid dynamics (cfd)

Computational Fluid Dynamics
(CFD)

Outline
● What is CFD?
● History of CFD
● Why to use CFD?
● Purpose and AIM
● Applications
● Physics
● Governing Equations
● Basics of Fluids
● How It Works
● Limitations
● Software

What is CFD?
Computational Fluid dynamics(CFD) is the
science of predicting fluid flow, heat transfer,
mass transfer, chemical reactions, and related
phenomena by solving the mathematical
equations.
We are interested in the forces (pressure, viscous forces, stress etc.) , velocity
field, temperature distribution

History of CFD
Historically Analytic Fluid Dynamics(AFD) and Experimental
Fluid Dynamics(EFD) was used.
Since 1940s analytical solution to most fluid dynamics
problems was available for idealized solutions. Methods for
solution of ODEs or PDEs were conceived only on paper due
to absence of personal computers.
CFD has become feasible due to the advantage of high
speed digital computers

Why to use CFD?
● Relatively low cost.
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.
● Ability to simulate real conditions.
CFD provides the ability to theoretically simulate any physical condition.
● Comprehensive information
CFD allows the analyst to examine a large number of locations.

Purpose and Aim
-Analysis and Design
● Simulation-based design instead of “build and 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)

Computational fluid dynamics (cfd)

Applications
2. Appliances
Axial Fan CFD Study Radial Fan CFD Study

Applications
3. Automotive
Full Car Simulation Air conditioning in Vehicle

Applications
4. Biomedical
Blood flow

Applications
9. Power Generation
Distribution of pressure Contour plots for pressure in the fluid

Applications
10. Sports
Air flow around bicyclists Air flow around racing car

Applications
11. Chemical processing
Chimney Cooling Tower

Governing Equations
Equation of motion
In x direction
Equation of continuity

Basic Fluid Motion
1) Translation
Motion of the centre of mass
2) Dilatation
Volume change
3)Rotation
About one, two or 3 axes
4)Shear Strain

Newtonian and Non Newtonian Fluids
A newtonian fluids viscosity remains constant, no
matter the amount of shear applied for a constant
temperature. These fluids have a linear
relationship between viscosity and shear stress.
In non-newtonian fluid viscosity changes with
respect to the amount of shear or stress applied
to the fluid.

Compressible and Incompressible flow
A fluid flow is said to be compressible when
the pressure variation in the flow field is large
enough to cause substantial changes in the
density of fluid.

Viscous and Inviscid flow
In a viscous flow the fluid friction has significant
effects on the solution where the viscous forces
are more significant than inertial forces.

Laminar Flow: The flow of a fluid when each
particle of the fluid follows a smooth path, paths
which never interfere with one another. One result of
laminar flow is that the velocity of the fluid is constant
at any point in the fluid.
Turbulent Flow: The irregular flow that is characterized
By tiny whirlpool regions. The velocity of this fluid
is definitely not constant at every point.
Laminar and Turbulent flow

How CFD Works?
● Appropriate initial and boundary conditions are provided for
the problem.
● CFD applies numerical method called discretization to
develop approximations of the governing equations of fluid
mechanics in the fluid region of interest.
● The solution is post-processed to extract quantities of
interest (e.g. lift, drag, torque, heat transfer, separation,
pressure loss, etc.).

Initial or Boundary Conditions
● Initial condition involves knowing the state of pressure and
initial velocity at all points in the flow.
● Boundary conditions such as walls, inlets and largely
specified what the solution will be.

Discretization
● Domain 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 provide flow field.

Discretization Method
● Finite volume method
● Finite element method
● Finite difference method

Types of Meshes
● Tri/tet vs quad/hex meshes
● Hybrid mesh

Finite Volume Method
● The finite volume method is a common approach used in CFD codes, as it
has an advantage in memory usage and solution speed, especially for large
problems,high reynolds number turbulent flows, and source term dominated
flows (like combustion).
● In this method the governing partial differential equations are recast in the
conservative form and then solved over a discrete control volumes and thus
guarantees the conservation of fluxes through a particular control volume.
● Here Q is the vector of conserved variables, F is the vector of fluxes V is the
volume of the control volume element, and A is the surface area of the control
volume element. The finite volume equation yields governing equations in the
form:

Finite element method
● The finite element method (FEM) is used in structural
analysis of solids, but is also applicable to fluids.
● It is much more stable than the finite volume approach.
However, it can require more memory and has slower
solution than the FVM.
● It subdivides a large system into smaller, simpler parts
that are called finite elements. The simple equations that
model these finite elements are then assembled into a
larger system of equations that models the entire problem.

Finite difference method
● The finite difference method (FDM) has historical
importance and is simple to program.
● The reduction of the differential equation to a system of
algebraic equations makes the problem of finding the
solution to a given ODE ideally suited to modern computers,
hence the widespread use of FDMs in modern numerical
analysis
● It is currently only used in few specialized codes, which
handle complex geometry with high accuracy and efficiency
by using embedded boundaries or overlapping grids.

Numerical Model Setup
For a given problem, you will need to:
-Select appropriate physical models.
-Define material properties.
1)Fluid
2)Solid
3)Mixture
-Prescribe operating conditions.
-Prescribe boundary conditions at all boundary zones.
-Set up solver controls.
-Set up convergence monitors.

Calculation of Coefficient of Drag over the Dinosaur
We calculate the theoretical values for the various parameters
required for calculation of the coefficient of drag on the
dinosaur:
Drag force: 17.4 N
Lift force: 5.5 N
Wind velocity: 5m/s
The dinosaur is 3.2 m tall.
It has a projected frontal area of A = 2.91m2
Air density: 1.225 kg/m3

The drag coefficient is:
Cd
= F/(0.5*ρv2
A) = 17.4/(0.5*1.225*25*2.91)
= 0.11
This is pretty good compared to the average car. The
streamlined back of the dinosaur resulted in a flow pattern
with very little separation.
Some drag coefficients :
1)Ferrari-> 0.36
2)Typical truck-> 0.6
3)Honda Odyssey-> 0.39

Pressure field on dinosaurs Velocity vectors on dinosaurs

Limitations
● The CFD solutions can only be as accurate as the physical models on which
they are based.
● 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.
● 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.

Software and resources
● Commercial (Fluent, CFX, Star-CD)
● Opensource( Openfoam)
● Grid generation software
Gridgen: http://www.pointwise.com
GridPro: http://www.gridpro.com/
Hypermesh
● Visualization software
Tecplot
Paraview

Thank you!
PRESENTED BY -BHAVANA KANWAR RAO

Computational fluid dynamics (cfd)

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