Aerodynamics of Trucks and Trailers

This assignment delves into the relationship between trailer height, gap between truck and trailer, and the resulting drag force. It analyzes how these factors influence aerodynamic performance and proposes solutions for drag minimization, such as streamlining body designs and closing the gap between the cab and trailer. The document emphasizes that a smaller gap and optimized trailer height contribute to reduced drag coefficients and overall fuel efficiency.

University of Huddersfield
School of Computing and Engineering
Aerodynamics of ground vehicles
Abstract
The pressure distribution occurring on a truck when it is traveling with trailer is investigated using
three dimensional (3D) Computational Fluid Dynamics (CFD). The height of the trailer is varied
keeping the gap between cab and trailer constant and the gap among cab and the trailer is varied
keeping the height constant. Assuming that the comparative velocity of the vehicles has an
insignificant effect on the flow distribution over the both vehicles. The analysis is done using the
steady state with non-compressible condition. Numerical flow around the passenger car because of the
trailer variations, is the subject of present work. C. In this report, the truck is studied with the vehicles
at different variables, with steady flow conditions. Simulation outcomes have been examined and
related in terms of detected values of aerodynamic drag coefficients.
Introduction
In the conventional meaning, reducing the aerodynamic drag is associated with appropriate shaping of
the front and rear of a vehicle, in conjunction with change of the bluff bodied shapes to streamlined
ones. In case of a truck, where a cargo box is located directly behind the cab, there is only one
overpressure zone covering the front face of the cab and an upper part (above the cab) of the cargo
box, and only one zone of underpressure located behind the truck rear face. One of the important
forces that us acting on the vehicle is drag force. This Aerodynamic drag force grow into an important
obstruction at greater speeds and the performance of the vehicle and fuel consumption are greatly
influenced. Since, the aerodynamic drag force always increases proportional to the square of the
speed. The weighty vehicles do cruising at high speed in intercity and take way too much over the
years. The vehicle producers invest too much in aerodynamic studies in order to escalate vehicle
performance. This important step makes the decreasing aerodynamics drag force more important
matter for heavy vehicles which do a large portion of the transportation in and out of the city. In case
of an articulated vehicle there is an additional free clearance gap between the rear face of tractor and
the front face of semi-trailer, resulting in underpressure at rear face of the tractor, and overpressure at
front face of the trailer. In these simulations the main focus was put on determining the effect of
aerodynamic interference processes taking place between tractor, semi-trailer and a passenger car, and
on the aerodynamic drag level. In this study, only primary, rectilinear geometric forms of the tractor
and trailer were considered. The models did not have any roundings or aerodynamic drag reduction
devices. A numerical simulation of flow around vehicles in which truck having different height and
different gap between trailer and cab is achieved using commercial fluid dynamic software ANSYS
FLUENT. The study concentrates on CFD based drag calculation on the car and truck body. A three-
dimensional CAD model of a car and truck is used as the basic model in this report which is created in
the commercial software ANSYS WORKBENCH.
Problem Definition
A CAD geometry of the Truck (cab) with a trailer is need to be analysed for 3 different conditions of
trailer height and calculate the effect of varying height on the drag and lift forces. The design velocity
should be 56mph. The second case is to vary the distance between the cab and the trailer and see how
they reflect on the aerodynamics of the truck. The body of the cab is made separately and then

combined with the geometry of the trailer. Both were assembled using different mating features in
Solidworks.
Methodology
1. Open Ansys Workbench 18.1 from the program window
2. In the Analysis system window choose fluent as the system
3. Double click on design modeller in Fluent System to create the geometry
4. Go to the tools in Modeller and see that the measurement units are set to Metre

5. Now right click on the XY plane option and then click on look at
6. Go to sketching option in the tree outline window and start drawing the following geometry.

The dimensions are given below
7. Now go to extrude option in the tool bar select sketch 1 as a sketch to extrude
Choose the details in the extrude option as below and then click generate to create the extruded
part
8. The extruded truck will look like below

9. To create the feel of artificial atmosphere around truck we use encloser option. Go to tools and
select encloser option
Fill the details in the encloser as below and click generate

The geometry created is as below
10. To subtract the Truck from the encloser we will use Boolean operation. Go to Create and choose
Boolean option

Now in the detail view choose the body as following
11. Now close the design modeller and open mesh sub system in Fluent.
12. Expand the sizing option the mesh and write the max and min face size as below. The type of
meshing used is tetrahedral with relevance centre as medium and Fine span angle centre. Because
there are too many curves are present in the geometry, it is best to use the tetrahedral elements for
the meshing. The relevance centre is chosen as Medium, because Fine will create a lot of elements
and the computational time will increase exponentially

13. Now go to the front face and select it, right click on it and choose name selection
Name the front face as Inlet, similarly the back face as Outlet and 4the sides of the encloser as
Wall. They are named because it’s easy for fluent to identify them and apply boundaries
conditions on them.
14. Click on mesh and then click on update in tool bar to update and generate the mesh.
Meshed geometry will look like below and then close the mesh system

15. Now, open the setup part in the fluent analysis system. In the general bar leave everything as it is.
As we are doing non-compressible analysis the type of solver will be pressure based solver and this
time the analysis is done using Steady state condition. There is no relative velocity formation here and
also we are neglecting the gravity condition.
The type of model used is k-omega SST. This type model has a blending function which uses k-ϵ in
the free stream and k-ω near the wall and . The wall functions are not used here. It also gives the
benefits of k-ω model and accounts for turbulent shear stress. It also gives accurate prediction of
transition and separation, and also decent free stream as well as boundary layer results. It also

provides better separation, transition and works even for the adverse pressure grades. It will provide
superior skin friction drag. At the equal time, it works good away from the walls, which gives good
pressure drag and hence the parasitic drag. Also gives better flow visualization.
Change the unit for velocity from m/s to mph by going into user defined units. Now open the Intel in
boundary conditions and give the inlet velocity as 56 mph, do not change anything else

Now in the reference value bar, the area we need to put is the frontal area of the model. Only on the
area referred here the drag analysis will be done. This will look like below for the baseline case.
We are not changing anything in the solution methods.
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methods We are not changing anything in the solution methods We are not changing anything in the
solution methods We are not changing anything in the solution methods We are not changing
anything in the solution methods We are not changing anything in the solution methods We are not
changing anything in the solution methods We are not changing anything in the solution methods We
are not changing anything in the solution methods We are not changing anything in the solution
methods We are not changing anything in the solution methods We are not changing anything in the
solution methods We are not changing anything in the solution methods We are not changing
anything in the solution methods

In the monitors we have to create plot for the drag analysis for both car and truck. Go to report plots
and create new plot, choose force report and in that choose drag and now choose the body on which
you need to find the drag and check report file and report plot as below:
Now go to the Initialization tab and initialize using Hybrid Initialization. It is a collection of recipe
and boundary interpolation methods. It uses the Laplace equation to create a velocity field that
conforms to multiple domain geometries, and a pressure field which efficiently joins low and high
pressure values in the computational domain.

To start calculating the results, go to calculation tab and type the number of iteration as 250 and click
on calculate to start the calculation.
Results and Discussions
Analysis 1: Variation of drag due to increase in the height of the trailer
Case 1.
Cab and container gap = 1 m
Trailer height = 3.2 m
For this analysis we have to follow the above procedure and we can see the results as below
For getting the different Contour go to results in fluent
In the contour window, to see the pressure distribution on the Truck-trailer select the option as below

As a result of above selection you will get the following result
1. Pressure Contour
Now to make a middle plane and to see the pressure contour on it make a new plate. Go to the new
surface option in the

Now to create a surface in the middle of the truck put the following values of x, y, z.
The above step creates a plane in the middle of the truck now to see the Pressure and velocity contour
you can choose velocity or pressure in the dropdown option in Contour plots
For pressure plot on Plane 1
2. Pressure contour on the Plane

3.Velocity Contour on the middle Plane
Now to see the flow of pressure or velocity over the body of the truck we can plot the path lines on
the middle plane. Select Pathlines in the result option.
A window will pop up and to get the pathlines on the middle plan select the option in the window as
below

After putting all the values, click Save/Display in the bottom of the window to get the following result
4. Pressure Pathlines on the truck
5. Velocity Pathlines on the truck

6. Drag coefficient graph for Truck
To see the value of forces and the drag coefficient go to tool bar and in Post-processing choose Forces
A window will pop up which allow to calculate the forces in different direction, in this case put the
values as below
This gives the drag force value in this case in the console
The values of the drag and lift forces and coefficients on car and truck are given below

Vehicle Truck
Drag coefficient 1.2638302
Total drag force 5312.2433 N
To get the values of lift force change the values in the force window as below
The values in console are
Vehicle Truck
Lift coefficient 0.26826276
Total lift force 1127.5859 N
Case 2
Cab and container gap = 1 m
Trailer height 3.68 m (15% increase in height of the trailer)
The procedure is same as above, but there is a slight change in the height of the trailer. For changing
the height of trailer, go to Design Modeler and make the height of the trailer as 3.68 m, where will be
exactly the trailer’s height after 15% increase.
Just change the Frontal Area in the Reference in Fluent

All the contour that were presented above can be presented for this case also,
Vehicle Truck
Drag coefficient 1.4209812
Lift coefficient -0.20153406
Total drag force 6637.1596 N
Total lift force -941.33106 N
Here, there is increase in the drag for truck because of the increase in the height of the trailer which
increases the frontal area of the truck and in result more resistance will be offered by the air.
1. Pressure Contour
2. Drag coefficient graph for truck

3. Pressure Pathlines on middle plane
4. Velocity Pathlines on middle plane
5. Velocity Contour on middle plane

6. Pressure Contour on middle Plane
Case 3
Cab and container gap = 1 m
Trailer height 4.16 m (30% increase in height of the trailer)
The procedure is same as above, but there is a slight change in the height of the trailer. For changing
the height of trailer, go to Design Modeller and make the height of the trailer as 4.16 m, where will be
exactly the trailer’s height after 30% increase.
Just change values in Reference because of change in Frontal Area

Vehicle Truck
Drag coefficient 1.4429644
Lift coefficient -0.14205487
Total drag force 7431.1064 N
Total lift force -731.56681 N
Here also, there is increase in the drag for truck because of the increase in the height of the trailer
which increases the frontal area of the truck.
1. Drag coefficient graph for truck
2. Pressure contour

3. Pressure Contour on middle plane
4. Pressure Contour on middle plane
Case 4
Cab and container gap = 1 m
Trailer height 4.64 m (45% increase in height of the trailer)
The procedure is same as above, but there is a slight change in the height of the trailer. For changing
the height of trailer, go to Solidworks and make the height of the trailer as 4.64mm, where will be
exactly the trailer’s height after 45% increase.
Vehicle Truck
Drag coefficient 1.3999055
Lift coefficient -0.14246049
Total drag force 39430.565 N
Total lift force -4012.6264 N

1. Drag coefficient graph for truck
2. Pressure Contour on middle Plane
3. Pressure Contour on middle Plane

Here, there is decrease in the drag for truck because of the turbulence.
Analysis 2: Variation of gap between cab and trailer (Comparing with case 1 which has
1000mm gap)
Case 5
Cab and container gap = 1.25 m
Trailer height = 3.2 m
The procedure is same as above, but there is a slight change in the distance between the cab and
trailer. Open the geometry in Design Modeler and move the trailer behind keeping the gap as 1.25 m.
Vehicle Truck
Drag coefficient 1.3500894
Lift coefficient -0.30097262
Total drag force 5674.8093 N
Total lift force -1265.0734 N
Here also, there is increase in the drag for truck because of the more eddies formation due to greater
gap. Same as above the decrease in the drag of the car caused by increase in the pressure at the aft of
the car
1. Pressure Contour on Middle plane
2. Drag coefficient graph for truck

3. Pressure Pathlines on the truck
4. Velocity Pathlines for truck

Case 6
Cab and container gap = 1.5 m
Trailer height = 3.2 m
The procedure is same as above, but there is a slight change in the distance between the cab and
trailer. Open the cad geometry in Design Modeler and move the trailer behind keeping the gap as 1.5
mm
Vehicle Truck
Drag coefficient 1.4666419
Lift coefficient -0.20499894
Total drag force 6164.7194 N
Total lift force -861.66979 N
1. Drag coefficient graph for truck
2. Pressure contour on Middle plane

For truck the drag coefficient for different cases can be tabled below
S.No. Truck Drag Coefficient Drag force
1. Baseline truck
3.2 m height, 1m gap
1.2638302 5312.2433 N
2. 15% increase in height
3.64 m height, 1 m gap
1.4209812 6637.1596 N
3. 30% increase in height
4.16 m height, 1 m gap
1.4429644 7431.1064 N
4. 45% increase in height
4.64 m height, 1 m gap
1.3999055 39430.565 N
5. 3.2 m height, 1.25 m gap 1.3500894 5674.8093 N
6. 3.2 mm height, 1.5 m gap 1.4666419 6164.7194 N
From the above table we can say that the baseline truck was the most efficient in case of drag and the
truck with trailer height increase by 1.5 m gap was the least efficient. The increase in the frontal area
was the reason for the increase in drag. In other cases, by increasing the gap between the truck and
trailer the formation of eddies causes increase in drag force and the coefficient. Hence, because of the
aerodynamic intervention between the trailer and the truck, various combinations of the trailer length
and height can cause specific (positive or negative) result for the magnitude of the drag coefficient.
For such type of cases it is highly recommended to do the 3-dimensional simulations, which can
compute the aerodynamic consequences and define the aerodynamic drag coefficient in further
reliable way.
As in the above case the model of truck and the container are blunt body and the drag can be reduced
by streamlining the truck and trailer sections. Here the geometry is roughly made so no streamlining
is present. In the real case the truck bodies are very much streamlined to reduce the drag coefficient.
Even by decreasing the length of the trailer has significant effect on the reduction of drag.
Optimisation of the trailer

To optimise the trailer, condition the height of the trailer should be equal to the cab and the distance
between the cab and trailer should be minimum as possible. The result here clearly shows that the
decrease in the gap results in less drag coefficient and less force too. If we need to use trailer of much
height, a smooth surface stretching from cab front to trailer front will also help to reduce drag or else
we can close the gap between the cab and trailer from both the sides, this will also help to decrease the
drag. Even putting the caps in the wheel rims reduces the turbulence created in the vehicle and
increases fuel efficiency.
The result here clearly shows that the decrease in the gap results in less drag coefficient and less force
too. If we need to use trailer of much height, a smooth surface stretching from cab front to trailer front
will also help to reduce drag or else we can close the gap between the cab and trailer from both the
sides, this will also help to decrease the drag.
The
Conclusion
Based on the case above we can conclude the following
1. There is a relation between the trailer height and the drag force induced in the truck. The greater
the height, the greater the frontal area which create a large drag coefficient and large drag force
also unless there is generation of turbulence.
2. The gap between the truck and the trailer also plays a significant role in drag calculations. The
larger the difference between the cab and the trailer the greater the drag coefficient.
3. To optimise and to reduce the drag in truck and trailer, we have to choose some specific values of
trailer height and gap between truck and trailer. Streamlining the body will reduce the drag
forces.

References
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p to decrease the drag.