This presentation explains basics of CFD with example case studies.

1.  Prepared By: Anubhav Singh

2. What is CFD?
Computational Fluid Dynamics (CFD) provides a qualitative
(and sometimes even quantitative) prediction of fluid flows
by means of
• mathematical modeling (partial differential equations)
• numerical methods (discretization and solution techniques)
• software tools (solvers, pre- and post processing utilities)
CFD enables scientists and engineers to perform ‘numerical
experiments’(i.e. computer simulations) in a ‘virtual flow
laboratory’.

3. The need for CFD
Applying the fundamental laws of mechanics to a fluid gives the governing equations
for a fluid. The conservation of mass equation is
and the conservation of momentum equation is
These equations along with the conservation of energy equation form a set of
coupled, nonlinear partial differential equations. It is not possible to solve these
equations analytically for most engineering problems.
However, it is possible to obtain approximate computer-based solutions to the
governing equations for a variety of engineering problems. This is the subject matter
of Computational Fluid Dynamics (CFD).

4. Ansys Fluent Applications
•Used to create fast,Accurate results,Flexible meshes.
•Used to model simulations of acoustics problems
•Used to model difficult combustion processes
•Generate meaningful graphics,Animations & reports & transfer data to
other applications
•Used to simulate conditions of convection,Conduction & radiation.
•Used to model multiphase flows.

5. Key Companies using Ansys fluent
•Cyient ltd.,U.K.
•Mellanox technologies ltd.,Israel
•Dyson Inc., U.K.
•National renewable energy laboratory,U.S.A.
•In India 193 companies use Ansys fluent
•Sectors in which Ansys fluent is used:
•Education
•Automotive
•Machinery
•Aerospace
•Research
•Industrial engineering
Source:www.enylft.com

6. Problem Identification
1. Define goals
2. Identify domain
Pre-Processing
3. Geometry
4. Mesh
5. Physics
6. Solver Settings
Solve
7. Compute solution
Post Processing
8. Examine results
9.UpdateModel
• Problem Identification
1. Define your modeling goals
2. Identify the domain you will model
• Pre-Processing and Solver Execution
3. Create a solid model to represent the
domain
4. Design and create the mesh (grid)
5. Set up the physics
• Physical models, domain properties,
boundary conditions, …
6. Define solver settings
• numerical schemes, convergence
controls, …
7. Compute and monitor the solution
• Post-Processing
8. Examine the results
9. Consider revisions to the model

7. CAR AERODYNAMICS

8. •According to first law, Any object continues to move with a specific velocity
until it is acted upon by a external force to come to rest. When a car moving
with a particular high velocity hits a wall it suddenly comes to rest & the
passengers in it are also moving with same velocity as that of car, So, their
bodies comes to zero velocity.
The collision between a stationary wall & a moving car is an inelastic collision.
The collision between two moving cars(colliding & bouncing back) is an
elastic collision.
•According to third law, When a object exerts a force on another one, It
experiences a equal & opposite force. When a car hits a wall, It exerts a force
on wall & wall also exerts a equal & opposite force on car which is severe
enough to damage car.
•According to second law,F=m*a.When a car speeds up,It’s velocity increases & so is
acceleration(it’s=rate of change in velocity).So,the force which car applies on wall also
increases due to increase in acceleration. That’s why, the car experiences such a high
reaction force from wall & so is the passengers which are moving with the same
acceleration as that of car. Also, Collision of car with more mass is severe as compared
to car with less mass.
For safety always wear seatbelts & airbags as they absorb part of reaction forces
resulting in less injury to passengers.

9. CAR AERODYNAMICS
When a vehicle moves it must displace the air around it, This air then attempts
to fill in the void that would otherwise be left behind the vehicle. This movement
of air on vehicle’s body produces forces that slows it down, Lift it up, Or push it
down.
•Drag- is the term when the air tries to slow down our vehicle.
•Lift-is the term given when it tries to lift up the vehicle.
•Downforce-is the term given when it tries to push our vehicle into the ground.
•For a car drag & lift are bad. Downforce is good because it provides more grip
with which to negotiate the corners of road.
As per calculations,The airfoil(or aerodynamic) shape of a car results in very
reduced coefficient of drag(=0.04) as compared to any other shape & to
know the effect of drag & lift we multiply the respective coefficients with
frontal area of car which comes least in aerodynamic shape. Hence,This
shape is preferred most.
Also to improve downward force designs of diffusers are optimized in
cars.

10. FRONT DIFFUSER(SPLITTER)
REAR DIFFUSER
The purpose of conducting CFD analysis is to study flow pattern of air flowing across
Car & to find important quantities like drag coefficient which in turn are used to
Calculate drag force.

11. •Pressure drag is caused by the air particles being more compressed
(pushed together) on the front-facing surfaces and more spaced out on the
back surfaces.
•A body moving through a fluid experiences an opposing force similar to a box
sliding over the ground experiences friction. Viscous drag corresponds to this
in particular.
•Working Of DIFFUSER:
A diffuser works by providing space for the air flowing under the car to
decelerate & expand (in area, As, Density is assumed to be constant at the
speeds that cars travel)so that it does not cause excessive drag. The diffuser
itself accelerates the flow in front of it, Which helps generate downforce.

13. •Why a Car front end has lower ground clearance than rear:
Looking at the profiles of most of the cars, The front bumper has the lowest
ground clearance & the rear bumper has the highest one. Due to this, Air
flowing under front bumper will be restricted to a lower cross-sectional area &
thus achieve a lower pressure & as the underside gap widens from front to
lower air flows from this low pressure zone to high pressure zone & thus more
efficient downwards air pressure or force is generated. Additional downforce
comes from the rake(or angle) of the vehicle’s body, Which directs the the
underside air up & creates a down force opposite to lift force on top & hence
due to this lift force on top decreases. Hence, Front underside of a car is
always lower than the rear under side.
As a vehicle is driven down the road,High-speed air is passing underneath
the vehicle at low pressure.As the air passes to the rear of the vehicle
through the diffuser,The air expands through an expansion chamber.Through
this expansion,Air speed is reduced & pressure is increased. This pressure
difference between the low-pressure air under the car & the high pressure at
the rear creates a vaccum sucking air out from under the car. With high
pressure air above the car & low pressure under the car,High downforce is
created.
In the same way a front splitter fitted at bottom of bumper allows high
pressure air to go up & low pressure air to flow underneath car resulting in
efficient downforce.

14. •Bernoulli’s Principle explained:
The answer to increase of pressure when passing through area of bigger
cross-section lies in equation of continuity which states that area & velocity
have inverse relationship.
Then, According to Bernoulli’s principle, Pressure & velocity too have inverse
relationship. It follows law of conservation of energy.

15. The RNG - model was derived using a statistical technique called renormalization group
theory. It is similar in form to the standard - model, but includes the following refinements:
•The RNG model has an additional term in its equation that improves the accuracy for rapidly
strained flows.
•The effect of swirl on turbulence is included in the RNG model, enhancing accuracy for swirling
flows.
•The RNG theory provides an analytical formula for turbulent Prandtl numbers, while the
standard - model uses user-specified, constant values.
•While the standard - model is a high-Reynolds number model, the RNG theory provides an
analytically-derived differential formula for effective viscosity that accounts for low-Reynolds
number effects. Effective use of this feature does, however, depend on an appropriate
treatment of the near-wall region.
These features make the RNG - model more accurate and reliable for a wider class of flows
than the standard - model.

16. The realizable - model differs from the standard - model in two important ways:
•The realizable - model contains an alternative formulation for the turbulent viscosity.
•A modified transport equation for the dissipation rate, , has been derived from an exact
equation for the transport of the mean-square vorticity fluctuation.
The term “realizable” means that the model satisfies certain mathematical constraints on the
Reynolds stresses, consistent with the physics of turbulent flows. Neither the standard -
model nor the RNG - model is realizable.

17. A shell and tube heat exchanger is the most common type of heat exchanger
in oil refineries and other large chemical processes, and is suited for higher-
pressure applications. As its name implies, this type of heat exchanger consists
of a shell (a large pressure vessel) with a bundle of tubes inside it. One fluid runs
through the tubes, and another fluid flows over the tubes (through the shell) to
transfer heat between the two fluids.
In order to transfer heat efficiently,
a large heat transfer area should
be used, leading to the use of
many tubes.

18. The fluids can be single or two
phase and can flow in a parallel or
a cross/counter flow arrangement.
The shell and tube exchanger consists of four major parts:
•Front Header—this is where the fluid enters the tubeside of the exchanger. It
is sometimes referred to as the Stationary Header.
•Rear Header—this is where the tubeside fluid leaves the exchanger or where
it is returned to the front header in exchangers with multiple tubeside passes.
•Tube bundle—this comprises of the tubes, tube sheets, baffles and tie rods
etc. to hold the bundle together.
•Shell—this contains the tube bundle.

19. The heat exchanger here is designed to transfer heat from the hot coolant that
flows through it to the water flowing inside the shell.The material of heat
exchanger is Aluminum.
The amount of heat transferred to the tubes from the fluid running through
them depends on the difference in temperature between the tube and the fluid
touching it.

20. The heat exchanger showed here is used in power plants where exhaust
gases often contain heat that's heading uselessly away into the open air.
That's a waste of energy and something a heat exchanger can certainly
reduce (though not eliminate entirely—some heat is always going to be lost).
The way to solve this problem is with heat exchangers positioned inside the
exhaust tail pipes or smokestacks. As the hot exhaust gases drift upward,
they brush past copper fins with water flowing through them. The water
carries the heat away, back into the plant. There, it might be recycled directly,
maybe warming the cold gases that feed into the engine or furnace, saving
the energy that would otherwise be needed to heat them up.
Types:

21. 1.The Counterflow Heat Exchanger
A counterflow heat exchanger has the hot
fluid entering at one end of the heat
exchanger flow path and the cold fluid
entering at the other end of the flow path.
2.The Parallel Flow Heat Exchanger
A shell and tube heat exchanger can be
operated in approximately parallel flow by
having both fluids enter at one end and exit
at the other end. With parallel flow the
temperature difference between the two
fluids is large at the entrance end, but it
becomes small at the exit end as the two
fluid temperatures approach each other.

22. 3.The Crossflow Heat Exchanger
The fluids are in cross flow (perpendicular
to each other) here.
CFD helps to design the heat exchanger by varying the different
variables very easily otherwise it is very difficult if done
practically. CFD models or packages provides the contours and
data which predict the performance of the heat exchanger design
and are effectively used because it has ability to obtain optimal
solutions and has work in difficult and hazardous conditions.

23. Temperature Plot For Heat Exchanger in Ansys Fluent