The Boundary layer
Added on 2022-09-01
15 Pages3443 Words26 Views
CFX ON TRUCK
Contents
ABSTRACT................................................................................................................................................ 3
INTRODUCTION ....................................................................................................................................... 3
Boundary layer. ................................................................................................................................... 3
Skin Friction......................................................................................................................................... 4
Air airstream. ...................................................................................................................................... 4
Aerodynamic drag. .............................................................................................................................. 4
Aerodynamic lift. ................................................................................................................................. 4
Vehicle lift. .......................................................................................................................................... 5
VARIOUS AERODYNAMIC MODIFICATIONS ON TRUCK .......................................................................... 5
Profile edge chamfering and filleting. ................................................................................................. 5
Windscreen rake. ................................................................................................................................ 5
Front bumper ...................................................................................................................................... 5
Front skirt. ........................................................................................................................................... 5
Side skirts ............................................................................................................................................ 5
Flat underbody. ................................................................................................................................... 5
Diffuser................................................................................................................................................ 5
Cab deflector ....................................................................................................................................... 6
Chassis Fairings ................................................................................................................................... 6
METHODOLOGY ...................................................................................................................................... 7
UNMODIFIED VAN............................................................................................................................... 7
PRE-MODELING. .............................................................................................................................. 7
Geometry ........................................................................................................................................ 7
Mesh ............................................................................................................................................... 7
SETUP .............................................................................................................................................. 8
Solution. ........................................................................................................................................ 11
MODIFICATIONS ................................................................................................................................ 11
RESULTS AND DISCUSSION.................................................................................................................... 13
ORIGINAL MODEL.............................................................................................................................. 13
Surface Plots ................................................................................................................................. 13
Streamlines ................................................................................................................................... 13
Contents
ABSTRACT................................................................................................................................................ 3
INTRODUCTION ....................................................................................................................................... 3
Boundary layer. ................................................................................................................................... 3
Skin Friction......................................................................................................................................... 4
Air airstream. ...................................................................................................................................... 4
Aerodynamic drag. .............................................................................................................................. 4
Aerodynamic lift. ................................................................................................................................. 4
Vehicle lift. .......................................................................................................................................... 5
VARIOUS AERODYNAMIC MODIFICATIONS ON TRUCK .......................................................................... 5
Profile edge chamfering and filleting. ................................................................................................. 5
Windscreen rake. ................................................................................................................................ 5
Front bumper ...................................................................................................................................... 5
Front skirt. ........................................................................................................................................... 5
Side skirts ............................................................................................................................................ 5
Flat underbody. ................................................................................................................................... 5
Diffuser................................................................................................................................................ 5
Cab deflector ....................................................................................................................................... 6
Chassis Fairings ................................................................................................................................... 6
METHODOLOGY ...................................................................................................................................... 7
UNMODIFIED VAN............................................................................................................................... 7
PRE-MODELING. .............................................................................................................................. 7
Geometry ........................................................................................................................................ 7
Mesh ............................................................................................................................................... 7
SETUP .............................................................................................................................................. 8
Solution. ........................................................................................................................................ 11
MODIFICATIONS ................................................................................................................................ 11
RESULTS AND DISCUSSION.................................................................................................................... 13
ORIGINAL MODEL.............................................................................................................................. 13
Surface Plots ................................................................................................................................. 13
Streamlines ................................................................................................................................... 13
MODIFIED TRUCK .............................................................................................................................. 14
Surface plots ................................................................................................................................. 14
COMPARISSON BETWEEN ORIGINAL MODEL AND MODIFIED MODEL ............................................ 14
CONCLUSION......................................................................................................................................... 15
RECOMMENDATION ............................................................................................................................. 15
REFERENCES .......................................................................................................................................... 15
Surface plots ................................................................................................................................. 14
COMPARISSON BETWEEN ORIGINAL MODEL AND MODIFIED MODEL ............................................ 14
CONCLUSION......................................................................................................................................... 15
RECOMMENDATION ............................................................................................................................. 15
REFERENCES .......................................................................................................................................... 15
ABSTRACT
This task aims at showcasing the use of CFD simulation in determining the drag and lift forces of a
truck. There are various elements or modifications which are made on the bodies of trucks to reduce
the drag and lift forces. The effect of putting various aerodynamic elements to the body of a truck is
also illustrated.
INTRODUCTION
Solid body aerodynamics is the study of flow of air around a solid model comparing it with the
surrounding air with its speed. A truck aerodynamics or any other car in this case is also considered
as solid body aerodynamics since it involves study of air around a model. There are several forces
offering resistance to the flow of air acting around a model moving in air. The common forces are
the lift and drag forces. Aerodynamic drag is usually insignificant at low velocities of the body but
increases when the speed of the car increases. The figure below shows the dag and lift forces at
different velocities. The drag forces of a poorly streamlined vehicle is very high. However, it is also
very small at low velocities. High drag forces in a truck hinders the acceleration of the truck but the
top speed stays unaffected. The fuel efficiency is also not hindered by the drag forces. To reduce the
drag forces, a body styling is done. The body styling has to accommodate the people and goods
inside, and other functional parts of the car such as the wheels, the power train and the engine and
the boot area. (Sherman, 2014).
Boundary layer.
The internal resistance to flow of fluid I called viscosity. Air has a viscosity of 1.225 kg/m/s. A thin
layer is formed whenever air flows over a solid surface. All the movement in comparison to the solid
surface of the airstream happens in this layer in a shearing process of the layers next to each other.
There is a parabolic rise in the velocity of air from when one moves from the outer layers to the
inner layers. The inner layers, which experience most of the shear stresses and friction flows at low
velocities while those at the interior flow at higher velocities since they have less viscosity and shear
stresses. (Hermann, 2004).
This task aims at showcasing the use of CFD simulation in determining the drag and lift forces of a
truck. There are various elements or modifications which are made on the bodies of trucks to reduce
the drag and lift forces. The effect of putting various aerodynamic elements to the body of a truck is
also illustrated.
INTRODUCTION
Solid body aerodynamics is the study of flow of air around a solid model comparing it with the
surrounding air with its speed. A truck aerodynamics or any other car in this case is also considered
as solid body aerodynamics since it involves study of air around a model. There are several forces
offering resistance to the flow of air acting around a model moving in air. The common forces are
the lift and drag forces. Aerodynamic drag is usually insignificant at low velocities of the body but
increases when the speed of the car increases. The figure below shows the dag and lift forces at
different velocities. The drag forces of a poorly streamlined vehicle is very high. However, it is also
very small at low velocities. High drag forces in a truck hinders the acceleration of the truck but the
top speed stays unaffected. The fuel efficiency is also not hindered by the drag forces. To reduce the
drag forces, a body styling is done. The body styling has to accommodate the people and goods
inside, and other functional parts of the car such as the wheels, the power train and the engine and
the boot area. (Sherman, 2014).
Boundary layer.
The internal resistance to flow of fluid I called viscosity. Air has a viscosity of 1.225 kg/m/s. A thin
layer is formed whenever air flows over a solid surface. All the movement in comparison to the solid
surface of the airstream happens in this layer in a shearing process of the layers next to each other.
There is a parabolic rise in the velocity of air from when one moves from the outer layers to the
inner layers. The inner layers, which experience most of the shear stresses and friction flows at low
velocities while those at the interior flow at higher velocities since they have less viscosity and shear
stresses. (Hermann, 2004).
(Hermann, 2004).
Skin Friction.
The friction which is generated as a result of a fluid flowing over the solid body is called skin friction.
The surface area of the body where the fluid flows over affects the kin friction. The surface
roughness or smoothness of these solid surfaces also affects the skin friction. (Sighard, 2012).
Air airstream.
A moving body moving through a fluid displaces the fluid ahead of it so that it can occupy that space.
To visualize the pattern of air around a moving object such a s a truck, streamlines are used.
Streamlines are imaginary lines of flow that follow the body’s contour and are deviated by any
sudden pressure or velocity drops in their floe trajectories leaving behind areas of stagnant air called
vortices. (Sighard, 2012).
Aerodynamic drag.
Pressure drag. The streamline of air flowing around a car have deviations at the rear of the car or
truck where are vortices are present. The atmospheric pressure is lower than the air pressure in
front of the car and higher after the car at the rear of the car where these vortices are present. The
low pressure system behind the vehicle causes tendencies of suction of the vehicle towards the
reverse direction. The skin drag also supports this tendencies. To reduce the pressure drag, the most
straight forward way is to reduce the pressure difference between the front and the rear of the car,
to do this, the vehicle need to be more streamlined.
The aerodynamic drag coefficient is given as;
𝐶𝑙 = 𝐷
𝜌×𝑣2×𝐴
2
Where D is the Drag force, 𝜌 is the density of air, v is the velocity of the body and A is the
surface area of the body. (Hucho, 2015).
Aerodynamic lift.
The aerodynamic lift is graded by a constant called aerodynamic lift coefficient. The coefficient
describes the difference in pressure between the region above the car to the region below the car or
any other body such as aerofoil moving in a viscous medium such as air. Lift forces are not desirable
in vehicle dynamics since they reduce the tire traction to the ground. Manufacturers build their cars
so that they can have more of negative lift forces (downward forces) than lift forces.
The aerodynamic lift coefficient is given as;
𝐶𝑙 = 𝐿
𝜌×𝑣2×𝐴
2
Skin Friction.
The friction which is generated as a result of a fluid flowing over the solid body is called skin friction.
The surface area of the body where the fluid flows over affects the kin friction. The surface
roughness or smoothness of these solid surfaces also affects the skin friction. (Sighard, 2012).
Air airstream.
A moving body moving through a fluid displaces the fluid ahead of it so that it can occupy that space.
To visualize the pattern of air around a moving object such a s a truck, streamlines are used.
Streamlines are imaginary lines of flow that follow the body’s contour and are deviated by any
sudden pressure or velocity drops in their floe trajectories leaving behind areas of stagnant air called
vortices. (Sighard, 2012).
Aerodynamic drag.
Pressure drag. The streamline of air flowing around a car have deviations at the rear of the car or
truck where are vortices are present. The atmospheric pressure is lower than the air pressure in
front of the car and higher after the car at the rear of the car where these vortices are present. The
low pressure system behind the vehicle causes tendencies of suction of the vehicle towards the
reverse direction. The skin drag also supports this tendencies. To reduce the pressure drag, the most
straight forward way is to reduce the pressure difference between the front and the rear of the car,
to do this, the vehicle need to be more streamlined.
The aerodynamic drag coefficient is given as;
𝐶𝑙 = 𝐷
𝜌×𝑣2×𝐴
2
Where D is the Drag force, 𝜌 is the density of air, v is the velocity of the body and A is the
surface area of the body. (Hucho, 2015).
Aerodynamic lift.
The aerodynamic lift is graded by a constant called aerodynamic lift coefficient. The coefficient
describes the difference in pressure between the region above the car to the region below the car or
any other body such as aerofoil moving in a viscous medium such as air. Lift forces are not desirable
in vehicle dynamics since they reduce the tire traction to the ground. Manufacturers build their cars
so that they can have more of negative lift forces (downward forces) than lift forces.
The aerodynamic lift coefficient is given as;
𝐶𝑙 = 𝐿
𝜌×𝑣2×𝐴
2
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