Moving Vehicle

8/16/2019 Moving Vehicle

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GENERAL SIR JOHN KOTELAWALA

DEFENCE UNIVERSITY

CFD ANALYSIS OF A 2D MOVING CAR

SVC : 4121

RANK : OFFICER CADET

NAME : SKK DILSHAN

SUBJECT : COMPUTATIONAL FLUID DYNAMIC

INTAKE : 30

Department of Aeronautical Engineering


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Figure 1 dimensions of Ford Mustang 2

Figure 2 Ford Mustang side view 2

Figure 3 Sketching the car 2

Figure 4 Modelled car 3

Figure 5 Meshing process 4

Figure 6 Mesh in FLUENT 4 Figure 7 Converged solution 5

Figure 8 Static pressure 35ms-1 6

Figure 9 Velocity magnitude 35ms-1 6

Figure 10 Turbulent viscosity 35ms-1 7

Figure 11 Scaled residuals 7


Figure 13 Contours of static ressure 50ms-1 8

Figure 14 Modified turbulent viscosity 50ms-1 9


Figure 16 Static pressure 70ms-1 10

Figure 17 Modified turbulent viscosity 70ms-1 10

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1. Introduction

Moving car model was developed for a time-averaged vehicle wake investigation as well

as more aerodynamic information. By solving this problem we can identify the aerodynamicforces on the vehicle. CFD simulation of a moving car model is cheaper than designing a real

model. Through CFD flow visualization, lift and drag coefficients can be figured.

With the development number of vehicles used is getting increased. Due to this

environmental pollution occur by emission of CO2. So the automobile manufacturers are

much more considering on improving the vehicle efficiency and reduce the fuel consumption.

Drag is the most important factor that the researches are going on.

Aerodynamic is the study of motion of air, mostly when it interact with an object. By

understanding the motion of air around an object we can calculate the aerodynamic forces

and moments acting on the object. Typical properties calculated for a flow field include

velocity, pressure, density and temperature as a function of position and time. By defining a

control volume around the flow field, equations for the conservation of mass, momentum,

and energy can be defined and used to solve for the properties. Aerodynamic analysis can be

mainly divided into two categories, internal and external aerodynamics. External

aerodynamics is the study of flow through various shaped solid bodies. Study on airplane

wing, rockets, missiles and etc. are some external aerodynamic analysis. Internal

aerodynamics is the study of flow through passages in solid objects. Flow inside a turbine

engine, a pipe and etc. are examples for internal aerodynamic analysis.

The vehicle aerodynamic flow process is fall into three types

(i) Flow of air around the vehicle.

(ii) Flow of air through the vehicle body.

(iii) Flow of air within the vehicle machinery.


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Figure 2 Ford Mustang side view

2. MethodologyFirst I choose a car that is in the industry. I downloaded a picture of the car and picture with

dimensions. I choose Ford Mustang car to develop the 2-d model.

I used Solid Works for modelling the 2-d modelling. First I chose Right plane to sketch

the model. I used the Style Spline tool to model the car. I use the Corner Rectangle tool to

create the domain for the simulation. Wheels of the car was created with Circle tool.

Figure 1 dimensions of Ford Mustang

Figure 3 Sketching the car


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Then I selected Reference Geometry and chose Coordinate System. I selected the left

bottom point as the Origin. Model was created and saved as an iges file.

I selected width of the domain as 20m and height as 5m. Car was placed 3m from the

left edge of the domain.

Then I imported the iges file of the modelled car into GAMBIT software, which I used to

mesh the domain.

Then I used the Erase edges tool to erase so unnecessary edges that were used to design

the model in Solid Works. Then I used the create face tool. From that I created faces

Car

Front wheel

Rear wheel

Domain

Then I used subtract tool to subtract the Car, Front wheel and Rear wheel faces from

the face Domain.

Then I gave the boundaries for the simulation as below.

Car - WALL

Front wheel – WALL

Rear wheel – WALL

Inlet (Left edge) – VELOCITY INLET

Outlet (Right edge) – PRESSURE OUTLET

Bottom – WALL Top – SYMMETRY

Next I used Mesh Faces tool to mesh the Domain. There I selected Element as Quad and

type as pave. Then I gave the spacing of the mesh as 0.01. Then started meshing.

It went for about one hour to finish the meshing. The software got stuck for three or

four times while meshing was going on. Then I have to do the previous steps what I have done

in GAMBIT again.

Figure 4 Modelled car


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Then I exported the mesh file as msh file.

Then I used the FLUENT software to do the simulation. I opened the msh file in the

FLUUENT software and checked the grid. Then I open mesh in the display.

In the mesh there were 956632 quadrilateral cells were created.

I selected Energy equation and Spalart-Allmaras in the models. As the material I selected

air. Then I changed the Boundary Conditions. In Boundary Conditions inlet I gave the velocity.

Then I changed the velocity in the Reference Values. Then I initialize the conditions and start

the iterations. I used 1000 iterations.

I used three velocities for my simulation, 35, 50 and 70ms-1. Solution for every velocitygot converged near 150 iterations.

Figure 5 Meshing process

Figure 6 Mesh in FLUENT


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It took about two hours for the solution to get converged.

Figure 7 Converged solution


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3. Result of the simulation

Figure 9 Velocity magnitude 35ms-1

Figure 8 Static pressure 35ms-1


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From the above figures we can see that there many specialized area on the flow over

the car. There are reverse flows, high turbulent flows and flow separations we can see from

these figures. With these flow properties will change drastically.

As we can see in the backward of the car there is a reverse flow created. This is due to

the shape of the back of the car.

Figure 10 Turbulent viscosity 35ms-1

Figure 11 Scaled residuals


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Figure 12 Contours of static ressure 50ms-1



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Figure 14 Modified turbulent viscosity 50ms-1



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Figure 16 Static pressure 70ms-1

Figure 17 Modified turbulent viscosity 70ms-1


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3.1

FACTORS CONTRIBUTING TO FLOW FIELD AROUND VEHICLE

3.1.1 SEPPERATION

During the flow over the surface of the vehicle, there is a point when the change in

velocity comes to stall and the fluid starts flowing in reverse direction. This phenomenon is

called ‘Separation’ of the fluid flow. This is usually occurred at the rear part of the vehicle.This separation is highly dependent on the pressure distribution which is imposed by the outer

layer of the flow. The turbulent boundary layer can withstand much higher pressure without

separating as compared to laminar flow. This separation causes the flow to change its

behaviour behind the vehicle and thereby affect the flow field around the vehicle. This

phenomenon is the major factor to be considered while studying the wake of the vehicle.

3.1.2 PRESSURE DRAG

The blunt bodies like large size vehicle show different drag characteristics. On the rear part of

such vehicles, there is an extremely steep pressure gradient which leads to the separation of the flow

separation in viscous flow. The front part of the flow field shows high pressure value, whereas on the

rear part flow separates leading to a high suction in the area. As we integrate the force componentcreated by such high change in pressure, the resultant is called as ‘Pressure Drag’. This factor is

affected by the height of the vehicle as well as the separation of the flow field.

3.1.3 FRICTION DRAG

Each wall surface or material has a distinct friction which resists the flow of fluids. Due

to molecular friction, a stress acts on every surface of the vehicle. The integration of the

corresponding force component in the free stream direction leads to a friction drag. If the

separation does not occur, then friction drag is one of the main reasons to cause overall drag.


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4. CONCLUSION

Aerodynamics of a car is the most useful property to reduce the drag. There are other

aspects also other than drag to improve through aerodynamic analysis such as crosswind

stability, unsteadiness from passages of tunnels, platforms or other vehicles, ballast

projection for high speed trains, aero acoustics and soiling which require new improvedapproaches in flow predictions.

By using the results of the CFD analysis we can improve the aerodynamic shape of the

car and can reduce the drag. By reducing the drag we can increase the efficiency and reduce

the fuel consumption. Therefore it will be economical for the customer and due to reduction

of fuel consumption environmental pollution reduces.

The effects of different aerodynamic add-on devices on flow and its structure over a

generic passenger car may be analysed using CFD approach. The main objective is to reduce

aerodynamic drag acting on the vehicle and thus improve the fuel efficiency of passenger car.

Hence, the drag force can be reduced by using add on devices on vehicle and fuel economy,

stability of a passenger car can be improved

Documents

Moving Vehicle