CFD pickup truck by ankur_iitr .pdf

@Zeus Numerix Pvt Ltd

PROJECT REPORT

ON

CFD Analysis of Pickup Truck

SUMMER INTERNSHIP

AT

Project by:

Ankur Bansal

Department of Mechanical Engineering

Indian Institute Of Technology, Roorkee

Summer Internship ZEUS Numerix Pvt Ltd

Flow through a Pickup Truck Page 2

Preface

The interconnection between theoretical study and the practical implementation is the very need of

engineering and this internship helps me in understanding the nuances of actual world application of the

theoretical knowledge of engineering.

The internship with Zeus Numerix Pvt Ltd gave me the opportunity to apply my engineering skills and it

was a scintillating experience to have successfully completed the project.

Knowledge of something useful can become redundant. The difference between a successful and an

unsuccessful professionals lies not only in the amount of knowledge one has, much he can actually use in

various situations.

In this report presented here, a sincere effort has been made by me to mention the knowledge attained by

me during the training.

The information is given as provided by the company.



Abstract

Nowadays the reduction of drag is becoming a very important challenge for all the car manufacturers as

they are competing intensely to produce powerful pickup trucks with better gas mileage in the market

regulated with law reinforcement on fuel emissions and consumers need for bigger size trucks with more

horse power and cargo capacity. Lower drag provides better performances such as higher top speed and

better stability. It also often lowers aerodynamic noise and greenhouse gas emission above all decreases in

fuel consumption. However, modern designs of pickup trucks tend to go higher and wider and thus they

have higher frontal areas due to the functional, economic and aesthetic requirements. Increasing frontal

area of the vehicle tend to increase the drag force acting on the vehicle which is proportional to the

dimensionless drag coefficient Cd and the projected area of the vehicle. Consequently, to hold or even

decrease the drag on a truck that has a larger frontal area, tremendous effort has to be made.

The purpose of this project is to carry out complete transient, three dimensional numerical simulations of

the flow field around a pickup truck. A simplified truck with smooth under body and no side mirrors is

considered in the study. CFD simulations were carried out using the CFD expert-lite software which is an

unstructured finite-volume based Navier-Stokes solver. In the simulations k- turbulence model with high

Re model was used. The results show all the flow structures around the truck. Through this research work,

our understanding of the complex flow field around a pickup truck could be improved



Acknowledgement

I would like to express my sincere gratitude to Mr. Irshad khan, COO, Zeus Numerix Pvt Ltd, Pune,

who is also my mentor, for their immense support and guidance throughout the project.

I would also like to thank Mr. Vijay P, Sr. CAE Engineer; Mr. Vivek Warade, CAE Engineer; Mr.

Manjunath, CAE Engineer, Zeus Numerix Pvt Ltd for their help and without whom it would have been

difficult for me to implement the project.

Lastly, I would like to thank all other office colleagues at for their support and cooperation which

immensely helped me in the completion of my project and my stay at Zeus Numerix.



About the Company

Introduction

Zeus Numerix is a computer aided engineering company which provides total solutions to engineering

problems in the field of Computational Fluid Dynamics, Computational Electromagnetics, Computational

Structural Mechanics, Dynamics and Controls.

Backed by a group of technically strong domain experts and developers, Zeus designs, develops,

customizes products and offers services to both national and international engineering clients in the field of

aeronautics, automobile, marine, nuclear and power. Company also provide solutions and take up R&D

work from the various departments of the Indian Government such as defense, nuclear and power.

Services

Consultancy

Zeus Numerix delivers solutions to a continually expanding number of applications and industries. Our

competencies in these technologies stem from the fact that we are engaged in development of engineering

analysis packages & implementation of latest state-of-the-art innovations in them. The key technologies

that Zeus Numerix masters are:

CFD

Electromagnetism

Structural Analysis

Dynamics

High Performance Computing

Computational Aerodynamics

Aerodynamic configurations recently addressed include:

Complete Low Subsonic Civil Aircraft

Miniaturized Air Vehicles (MAV) and Unmanned Air Vehicles (UAV)

Fighter Aircraft: Delta Wing, Multi-role & Trainer

Hypersonic Vehicle / Reusable Launch Vehicle

Missiles, Rockets and similar Projectiles



Design/Optimization

Zeus Numerix develops automated design environment by leveraging its automated pre/post capability,

HPC interfaces for multiple analysis tools and ably supported by its domain knowledge. The usage of such

environment speeds up the design cycles, keeping the costs low.

Products

CFD

In the field of Computational Fluid Dynamics, company professionals are providing services and solutions

for applications across the industry vertical. Zeus with its pressure based and density based solvers cover

the entire task of grid generation, solutions and visualization.

Software:

FlowZ

CFDExpert-Lite

ZNTutor-CFD

CEM

Zeus Numerix in the field of Computational Electromagnetics, createdEMWaveZ - a solver for solving

Maxwell's Equations and later on came out with a suite, CEMExpert, for solving problems involving

electromagnetics. Zeus now provides end-to-end services and solutions using its indigenous and

customized CEM software.

Pre- and Post-Processors

Every engineering problem which requires computational analysis requires a CAD import, grid generation

and finally visualization of the results. Keeping this in mind, a number of products have been developed by

the company which took external data in different forms, processed it to make them compatible with the

solvers and then a post processor to visualize the result.

Software:

GridZ

ViewZ



Introduction to CFD

Computational Fluid Dynamics (CFD) is a computer based mathematical modeling tool that can be

considered the amalgamation of theory and experimentation in the field of fluid flow and heat transfer. It is

now widely used and is acceptable as a valid engineering tool in industry. CFD calculations are based upon

the fundamental governing equations of fluid dynamics: the conservation of mass, momentum and energy.

These equations combine to form the Navier-Stokes equations, which are a set of partial differential

equations that cannot be solved analytically except in a limited number of cases.

Figure : Velocity profile for flow past a pickup van

However, an approximate solution can be obtained using a discretization method that approximates the

partial differential equations by a set of algebraic equations. There are a variety of techniques that may be

used to perform this discretization; the most often used are the finite volume method, the finite element

method and the finite difference method. The resulting algebraic equations relate to small sub-volumes

within the flow, at a finite number of discrete locations.



Pickup Truck

The continuing increase in fuel price coupled with uncertainty of future supply has created widespread

interest in vehicles with high efficiency including pickup trucks. Pickup trucks, vans and SUVs account for

48% of sales fraction of light duty vehicle in United States while light duty vehicles account for

approximately 40% of all US oil consumption. Therefore improving the fuel economy of pickup trucks will

have tremendous impact on energy security, emission of green house gas and cost of fueling when gasoline

price rises.

Today auto manufacturers are competing intensely to produce a powerful pickup truck with better gas

mileage in the market regulated with law reinforcement on fuel emissions and consumers need for bigger size truck with more horse powers and cargo capacity. Energy efficiency of vehicles can be improved by

reducing the total structural mass, using engine with higher thermally efficiency, or altering the exterior

body shape to reduce the aerodynamic drag. According to US department of energy, in urban driving

aerodynamic drag accounts for 2.6% of the 12.6% of fuel energy being used to propel the car as shown in

Fig. Since the aerodynamic drag increases at higher speeds, the aerodynamic drag on a highway driving

accounts for 11% of 20% fuel energy needed to propel the vehicle. Therefore improving vehicle

aerodynamics is one of the factors that play crucial role for getting better mileage and better performance

including the handling of the vehicle especially at high speeds.

Fig. Typical energy uses

and losses in a vehicle



Aerodynamics of trucks is also as important as that of passenger cars. Research on this topic will help to

improve the design of trucks to improve gas mileage and to understand the complex flow structures around

trucks which are quite different from passenger cars. This CFD work will overcome the shortcomings of

the conventional wind tunnel experiments such as wind tunnel blockage effects.

Model Design

Grabbing the geometry of the truck under study

The geometry to be studied was searched on the internet and was finally downloaded from

Grabcad.com. The downloaded CAD model closely resembles the truck

.

Geometry cleaning

Geometry cleaning is one of the most important and time consuming part of any CFD analysis. It involves-

1. Making the geometry air tight.

Edges stitched



2. Ensuring no Tripple coons are present. That is there is no junction which has more than two

shared surfaces. If present, it is treated as open boundary by the solver.

3. Some amount of geometry cleaning is sometimes also required after the generation of surface and

volume mesh to improve the quality of the mesh.

Multiple surfaces

removed

Step added to improve

quality of prisms and

tetra mesh



Initial

geometry

Final

geometry



Geometrical changes

Some CAD modeling has to be done and some geometrical changes has to be made in order to bring the

analysis model as close to the reality as possible. A domain is created, which acts as the field of study or

the control volume, for the problem. The dimensions of the domain should be such that all the phenomena

generating on the body diminishes before reaching the domain boundaries. In my case I have chosen the

domain boundaries to be of the following dimensions- Ahead of the car- a length of 4 times the length of

the car (432*4 = 1728cm), behind the car a length of 7 times the length of the car (432*7 = 3028cm), Span

wise a length of 1 times the length of the car on both sides (432*2 = 864cm) and the domain was given a

height of 1 times the length of the car (432*1 = 432cm). It is a general convention to give the dimensions of

the domain in terms of the length of the car.

*dimensions in cm

One geometrical change done for this study is the creation of the road and tire interaction zone.

Road for

interaction



Generating Surface mesh

Surface mesh is basically a 2-D mesh. Surface mesh elements is of two types based on the number of nodes

which it shares with its neighboring element. They are a. triangular elements

b. quad elements.

Surface mesh generating method are also varied depending upon the algorithm and the element type used.

Some of them are-

a. Unstructured mesh Consisting of only triangular elements.

b. Structured/blocked mesh- Consisting of only Quad elements.

Quad element

Tri element



c. Hybrid mesh Consisting of portions of structured and unstructured meshes.

For this project is have generated both unstructured and hybrid mesh due to the complexity of the geometry

and the time restraint, as generating a structured mesh requires more time than the other two meshes.

The guidelines followed while creating the surface mesh are- a. Measure the dimension of the CAD model. This gives us an idea of the maximum size

which can be used as a global input.

b. Measure the dimensions of the smallest detail to be captured; this gives us an idea of the smallest element size which can be assigned to the model.

c. Decide the physics to be captured and perform the meshing operation accordingly i.e. capture accurately only those parts of the body which might have a part to play in the

physics to be captured.

Tire -road junction



d. There must be a smooth transition (growth) of elements both in the same macro and from one macro to the other. As a rule of the thumb the size of adjacent elements of two macros must not be more than 1.2 time of each other.

e. The ultimate aim must be to make the best mesh, according to the problem statement, by using the least number of elements.

The result of following the above mentioned guidelines was a mesh good enough to capture all the physics

intended to be studied. The mesh thus obtained had approximately .2 million elements (both tri and quad).

Truck bed

Pickup truck surface

mesh



Triangular mesh -

Pickup Truck



Quad mesh (near tire-road juction)

Quality check and re-mesh

Improving the quality of the mesh is one of the most important and tedious work. A good quality mesh is

imperative for generating a good quality volume mesh. There are various parameters which determine the

quality of surface elements. Some of them are-

a. Aspect ratio: It is the ratio of longest to the shortest side in a cell. Ideally it should be equal to 1 to ensure best results. For multidimensional flow, it should be near to one.

Also local variations in cell size should be minimal, i.e. adjacent cell sizes should not

vary by more than 20%. Figure given below is the Aspect ratio of my surface mesh

elements :-

Minimum aspect ratio is b/w 0.1 and 0.2. So, need to improve the quality of surface mesh.

b. Skewness: The skewness of a grid is an apt indicator of the mesh quality and suitability. Large skewness compromises the accuracy of the interpolated regions.



Minimum skewness is 0.25. So, skewness of my grid is appropriate.

c. Determinant: The Determinant is the ratio of the smallest determinant of the Jacobian matrix divided by the largest determinant of the Jacobian matrix, where each

determinant is computed at each node of the element. A Determinant value of 1 would

indicate a perfectly regular mesh element, 0 would indicate an element degenerate in one

or more edges, and negative values would indicate inverted elements.

Determinant is close to 1 which is quite good.

Various methods are used to improve the quality of bad elements, depending upon the parameter to be

improved. Some of these methods are- a. Node movement.

Node moved to

capture the surface



b. Mesh refinement

c. Re-mesh bad elements

Truck bed refined

to improve quality



After applying above methods, Aspect ratio plot is:-

Thus minimum quality achieved is 0.29 which is suitable for generating volume mesh.

Generating volume mesh

Like surface mesh elements volume mesh elements are also of different types-

a. Tetrahedral b. Hexahedral c. Prism d. Pyramid

Each element has its own advantage and use-

- Tetrahedral elements are easy generate and are mostly used when neither viscous forces nor boundary layer is to be captured.

- Hexahedral and Prism layers are good to capture the boundary layer and the viscous forces.

- Pyramids are basically used as transition elements between one type of element to another.

For the current project I have used a combination of Prism layer and tetra along with the use of density

zone, which would have helped capture all the viscous forces and the boundary layer.



There are various parameters which govern the generation of Prism layer. They are-

a. Y+(plus) : A non-dimensional wall distance for a wall-bounded flow can be defined in the following way:

Where u* is the friction velocity at the nearest wall, y is the distance to the

nearest wall and v is the local kinematic viscosity of the fluid. y+ is often

referred to simply as y plus and is commonly used in boundary layer theory and

in defining the law of the wall.

For my project, y+ factor = 3 was taken (reference: CFD Online) to capture the

boundary layer.

b. Reynolds number: The Reynolds number characterises the relative importance of inertial and viscous forces in a flow. It is important in determining the state of the

flow, whether it is laminar or turbulent. At high Reynolds numbers flows

generally tend to be turbulent.

From CFD Online, Y+ wall estimation distance (first prism layer height which is very important) was

calculated:

which comes out to be 3.7e-5 m. Boundary layer height of .012 m was given in calculations.

From first layer height, Boundary layer height and exponential growth rate, 12 prism layers were formed on

the truck to capture viscous forces.



As a rule of the thumb the size of the topmost prism over a surface element should not be more than three

time of it and likewise the size of the tetra over the prism should not be more than three times of it.

Growth rate also plays an important role in determining the quality of the prism cells.

It is the volume mesh which ultimately captures the physics. So the size of the volume mesh and its growth

plays a vital role in generating a good result. The most important zone is the volume around the object.

There are various ways of generating good volume mesh around the object under study. Some of them are-

a. Refining the surface mesh in the concerned zone and giving a small growth rate for the volume mesh. But many a times this approach doesnt give satisfactory results.

b. Another approach is to generate a density zone around the object. A density zone is user defined volume in which a user can restrict the size and growth of the

volume elements. The commands given in the density zone supersedes those

given as a global input for volume meshing.

Growth rate = 1.2

for volume mesh

4 density zones defined

to improve quality of

volume mesh



The result of following the above mentioned guidelines was a mesh good enough to capture all the physics

intended to be studied. The size of the volume elements of the project was approximately 3.5 million cells.

12 Prism layers and tetra

meshing



Improving the quality of Volume mesh

Some of the tetra and prism elements thus generated have very poor quality either they are very skewed or

they have very poor aspect ratio. Various steps were taken to improve the quality of the mesh which

included geometrical changes, surface mesh refinement, De-featuring, and giving smaller growth rate for

the tetras to grow. For the refinement of prism layers, ortho weight, fillet ratio, Max height over base

parameters were changed to improve the quality. The result of all this effort was an overall improvement of

the quality of the tetras. The quality of the worst cell improved form 9.6e^-6 to .18. This stage was the most time consuming and tedious amongst all the stages of this project.

So, minimum volume mesh quality achieved is 0.18 which is suitable for CFD simulation.

Initial quality

Final quality



Giving proper solver setup This is the most technical part of the whole project as without a proper solver setup all the efforts made are

in vain. Without a proper solver setup one cannot one can never get the desired result. The solver used for

this project is 'znuns_incompressible_release' which is a copyright of Zeusnumerixpvt.ltd. It is a Density

Based incompressible solver.

The fluid properties given as an input were-

Simulation model



Simulation control -

Flow initialization



Boundary conditions

INLET

Velocity magnitude: 18 m/s,

Boundary condition type: BCInflow,

Velocity specification method: MAGNITUDE_NORMAL_TO_BOUNDARY

OUTLET

Outflow static pressure: 0 Pascal,

Boundary condition type: BCOutflow

VAN

Boundary condition type: BCWall Viscous

FARFIELD

Boundary condition type: SYMMETRY

GROUND

Boundary condition type: BCWall Viscous

INLET

(yellow)

FARFIELD

(red)

TRUCK

(ivory green)

GROUND

(blue)

OUTLET

(black)



Post Processing: Result visualization

The residual for each of the flow variables were monitored and the solutions convergence was achieved

when the residuals were down to approximately 4.72 e00. A residual convergence history for the mass flow is shown in Fig-

Figure given below shows the pressure coefficient plot on the symmetry plane from present simulation. The

pressure coefficient plot shows that the stagnation point was created on the front surface of the pickup

truck. The pressure coefficient also indicates that CFD simulations have a tendency to overshoot the Cp

value at stagnation point. The Maximum Cp value obtained in present simulation was Cp= 1.52



Figure given below show the pressure coefficient plot of the vehicle underbody on the symmetry plane

from present simulation.

Figure given below show pressure coefficient distribution on the tail-out surface of the vehicle on

symmetry plane.



Figure given below shows the u-velocity (u/V) at x = 400cm in the symmetry plane (y = 0).

Figure given below shows the u-velocity plots in at x = 450 in the horizontal plane (z = 73 cm).

The static pressure distribution on the symmetry plane and on the surface of the pickup truck is shown in

fig (below), indicating that pressure dooms were created in front of the vehicle and the maximum pressure

was created on the front vehicle surface near the bumper. The figure also shows that the low pressure was

created in the pickup box and also over the cab of the vehicle, which tends to increase the drag and lift

coefficient of the baseline truck. The total pressure distribution in the symmetry plane and over the surface



of the truck is also shown, indicating a high total pressure gradient region where the flow separates with the

flow recirculation created.

Static pressure distributions

over the symmetry plane

Total pressure distributions over

the baseline truck and symmetry

plane



Below fig shows the velocity magnitude vectors in the symmetry plane (y = 0). The vectors indicate the

flow separation occurring at the rear edge of the cab and the vortex created in the box of the truck. It also

indicates the downwash created at the outer edge of the tailgate behind the truck.

Streamlines in a vertical plane at y=33cm

Velocity magnitude

vectors in the symmetry

plane (y = 0)



The aerodynamic drag and lift coefficients computed from the simulation were Cd = 0.345 and Cl = 0.235

respectively.

The non dimension drag coefficient is defined as

Where:

Cd : Aerodynamic Drag Coefficient

D : Drag force = 125.2 N (calculated through CFD simulations)

A : Frontal area of truck = 1.8422 sq. m

: Air density = 1.225 kg/m3

V : Air velocity = 18 m/s

And the non dimension Lift coefficient is defined as

Where:

Cl : Aerodynamic Lift Coefficient

Streamline on

z=73 cm



L : Lift force = 85.85 N (calculated through CFD simulations)

A : Frontal area of truck = 1.8422 sq. m

: Air density = 1.225 kg/m3

V : Air velocity = 18 m/s

However, in the real world pickup trucks manufactured today have a drag coefficient at Cd = 0.463 ~

0.491. These might be due to the fact that the generic pickup model lacks accessories such as side mirror

and windshield wipers. Also in the case of the generic pickup model there were no exposed axles,

underbody, radiator cooling vents and many cavities on the surface of the vehicle that connects the inside

of the vehicle to the flow.

Conclusion

Flow over the generic pickup model was simulated using CFD and the results from the simulation were

validated against CFD results of flow over the same generic model from Yang and Khalighi[1]. The results

from present simulation were compared and found to be in agreement. The complex features in the flow

fields are well captured in the simulations and the validation with experimental data is very encouraging.

To avoid the over-prediction of the turbulent kinetic energy near stagnation points caused by standard k- model, high RE modifications were implemented into CFD analysis.



References

1. Yang, Z., and Khalighi, B., CFD Simulation for Flow Over Pickup Trucks, SAE Paper No. 2005-01-0547, 2005.

2. Al-Garni, A., Bernal, L., and Khalighi, B., Experimental investigation of the Near Wake of a Pick-up Truck, SAE Paper No. 2003-01-0651, 2003.

3. Cooper, K., Pickup trucks Aerodynamics - Keep your tailgate Up, SAE Paper No. 2004-01-1146, 2004.

4. Mokhtar, W., Britcher, C., Camp, R.,Further Analysis of Pickup trucks Aerodynamics, SAE Paper No. 2009-01-1161, 2009.

5. Aerodynamics of Road Vehicles, Edited by Wolf-Heinrich Hucho, SAE International, Warrendale, PA, 1998.

6. Barnard, R.H., Road Vehicle Aerodynamic Design-An Introduction, 2nd Edition, Longman ISBN 0-582-24522-2, 1996.

7. White, F.M., Fluid Mechanics 4th Ed. Boston. MA. WCB McGraw-Hill, 1999

8. Ahmed, S.R., Computational Fluid Dynamics, Chapter XV in Hucho, W.H (Ed.), Aerodynamics of Road Vehicles, 4th Edition, SAE International, Warrendale, PA, USA, 1998

9. Munson, Young, Okiishi Fundamentals of Fluid Mechanics 4th Edition John Wiley and Sons, 2002

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CFD pickup truck by ankur_iitr .pdf