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RamMohanRao Pamoti Raval Chetan
Deputy Manager -CAE. Senior Manager -CAE
Mahindra Navistar Automotives Ltd Mahindra Navistar Automotives Ltd.
Pune - 411019, India Pune - 411019, India
Abstract
The aerodynamic characteristics of heavy commercial vehicles have received substantial interest recently, because of its
immediate impact on fuel efficiency at high speed cruising. In this study, a numerical simulation has been carried out for three-
dimensional turbulent flows around a MNAL truck carrying a container body. Particularly, the effect of a roof fairing attached on the
roof of the truck cabin was investigated. Spalart-allmaras turbulence model available in CFD solver ACUSOLVE was used for
evaluating aerodynamic forces, velocity and pressure distribution. The result shows the complex wake formation in the top front
edge of container and high pressure built up at exposed front face of the container. Roof fairing reduces the wake formation and
decreases aerodynamic drag, which in turn help reduce the fuel consumption of the truck.
Based on simulation, the height of roof fairing was further fine tuned, according to container height to get the best aero
dynamic drag reduction of 23%. Also, the pressure data on roof fairing was used for structural durability prediction under wind load
conditions using RADIOSS.
Keywords: Aerodynamics, truck, roof fairing, drag force.
Introduction:
Many studies of Heavy Commercial Vehicles (HCV) for long distant high way applications have shown
that significant proportion of fuel losses were due to aerodynamic drag [1]. Cost down tests have shown
that at speed above 60 mph, the aerodynamic drag account for 60% and rolling resistance causes 40% of
total drag. Therefore external aerodynamics study and simulation of HCV to reduce the aerodynamic drag
assumes importance in this era of high fuel costs.
Heavy commercial vehicles such as trucks and buses generally are large bluff bodies without overall
aerodynamic shape which causes strong wakes and trailing vortexes resulting in serious aerodynamic
drag at high speed cruising. Shinsuke, Jongsoo and Shuya etc.[2] investigated 3-dimentional turbulence
flow around bluff body, the effect of underbody slant and rear flaps on bluff body aerodynamics. Kim [3]
investigated the effect of rear spoiler on commercial bus body and reduced aerodynamic forces. Many
international OEMs have taken sustained measures to reduce air drag using aerodynamic front end, roof
fairings, cab extender, side skirts etc. The Department of Energy, USA, has launched Super Truck
program and set ambitious targets for HCV fuel economy improvements which include aerodynamic
improvements among other measures.
This paper presents interesting case of aerodynamic drag reduction of Mahindra Navistar’s 25T truck as
used in Indian scenario. MN25 straight truck is used with container load carriers which are of different
heights and built independently by road side body work. Therefore, an appropriate sized roof fairing with
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2. adjustable height is developed and assessed using CFD simulation of 3D turbulent flow around the truck,
cab and container. In addition, the wind pressure loads on air fairing were mapped on structural finite
element model and the design was evaluated for durability.
Process Methodology:
A. Truck and Roof Fairing Model
The aerodynamic drag can be divided in to two components: viscous drag and pressure drag. For flow
over heavy vehicles at highway speed the Reynolds number is large enough such that viscous forces can
be safely ignored. Consequently drag experienced by a truck is primarily due to pressure drag which is
comprises of pressure force that exist on front and rear of the vehicle.
The truck front end is modeled accurately which consist of cab, bumper, wheels and wheel arcs along
with container body. Even though container body is corrugated but for the model ease and size it is
considered as plain box. The container body height varies so in order to get better aerodynamic
performance on all containers adjustable roof deflector is considered. Since this is a comparative study
with or without air deflector, for simplicity other parts of the vehicle like mirrors, few underbody
components are not considered.
Figure 1: MN25 truck without and with roof deflector
Grid development
Flow around a vehicle body is usually not symmetrical about the vehicle centerline. Accordingly, a full
model should be used. However, grid numbers increase excessively and a very high performance
computer is needed to analyze such a complicated configuration, especially with the inclusion of the roof
deflector. Because of limited computer capacity, a half model without side mirrors was used to analyze
the aerodynamic characteristics. In complete volume, unstructured grid was generated using Acuconsole.
Multi block grid topology was used to generate volume mesh as shown in figure 2.
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3. Figure 2: Volume mesh with near truck refinement zone
The dimensions of computations space is shown in figure 3. To capture boundary layer phenomena
accurately, 15 layers were generated on truck surface with first layer height as 0.3 mm and growth rate as
1.3. fine and irregular grid at front and rear of the truck was used for accurate commutation. Total mesh
size was 16 Mn. tetrahedral elements.
Figure 3: Fluid domain around Truck
C. Boundary Condition
The ground plane was modeled as a no-slip surface, with a constant translational velocity matching the
forward speed of the truck. The truck cabin, container, roof deflector and wheels were modeled as no-slip
surfaces with zero relative velocity. The velocity vector direction was chosen to match 00 yaw angle. A
constant initial eddy viscosity condition was specified to be 0.00001 m2/s. A pressure outlet condition was
applied to the rear face boundary of the model domain. Fluid domain outer boundary was modeled as slip
surface & vehicle centerline surface is modeled as symmetry. Velocity is imposed at the inlet according to
vehicle speeds of 40, 60 and 80 kmph.
D. Numerical Methodology
In this work, the Navier-Stokes equations were solved using AcuSolve, a commercially available flow
solver based on the Galerkin/Least-Squares (GLS) finite element method. AcuSolve is a general purpose
CFD flow solver that is used in a wide variety of applications and industries. The flow solver provides fast
and accurate transient and steady state solutions for standard unstructured element topologies. AcuSolve
ensures local conservation for individual elements. Equal-order nodal interpolation is used for all working
variables, including pressure and turbulence equations. The resultant system of equations is solved as a
fully coupled pressure/velocity matrix system using a preconditioned iterative linear solver. The iterative
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4. solver yields robustness and rapid convergence on large unstructured meshes even when high aspect
ratio and badly distorted elements are present.
The following form of the Navier-Stokes equations were solved by AcuSolve to simulate the flow around
the Truck:
∂ρ
+ ∇ • ρu = 0
∂t (1)
∂u
ρ + ρu • ∇u + ∇P = ∇τ + ρb
∂t (2)
where ρ=density, u=velocity vector, P=pressure, τ=viscous stress tensor, b=momentum source vector.
Due to low mach number involved in these simulation, the flow was assumed to be incompressible, and
the density time derivative in Eq. (1) was set to zero. the three dimensional steady flow is simulated using
RANS single equation Spalart-Allmaras turbulence model. The turbulence equation is solved using GLS
formulation. The model equation is as follows:
{ }
~
∂v ~ 2 1
v 
+ u • ∇v = C b1 S v − C w1 f w   + ∇ • [(v + v )∇v ] + C b2 (∇v )
~ ~~ ~ ~ ~2
∂t d  σ (3)
~ ~
v χ χ3 ~
v
S=S + fv f v2 = 1 − f v1 = χ= S = 2 2S ij S ij
k 2d 2 2 1 + χf v
1
χ 3 + Cv 3
1
v
1/ 6
 1 + Cw 6  1  ∂ui ∂u j  ~
v
fw = g 6 3
 g = r + C w2 (r 6 − r ) S ij =  +  r= ~ 2 2
 g + Cw3 
6
2  ∂x j ∂xi 
  Sk d
 
~
where v is Spalart-Allmaras auxiliary variable, d=length scale , C b1 =0.1355, σ =2/3 , k=0.41, Cw3=2 ,
Cv1=7.1, Cb2 =0.622, Cw1 =(Cb1/k2)+((1+Cb2)/σ)
The eddy viscosity is then defined by ~
v1 = v f w1
For the steady state solutions presented in this work, a first order time integration approach with infinite
time step size was used to iterate the solution to convergence. Steady state convergence was typically
reached within 100 time steps.
Results & Discussions:
Base line vehicle:
Velocity and Pressure distribution on surface of front and rear of the truck is presented in fig. 4.
and fig.5 respectively. Flow comes from the upstream end forms a stagnation area at middle of the front
fascia and container top surface which are directly exposed to air. The divergent of flow has increased
with increase in pressure value as a result of stagnation phenomenon and then at each curvature flow is
radically faster. The stagnation area at the front body is the main cause of drag force with high pressure.
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5. However formation of stagnant area is inevitable in design. But how this is reduced is the key point to
reduce drag force of the truck.
Figure 4: Velocity distribution on centerline plane at 80 kmph vehicle speed
Figure 5: Pressure distribution at 80 kmph vehicle speed
Effect of roof fairing:
Figure 6: Aero deflector
Velocity and pressure distribution on surface of front and rear of the truck with roof deflector is
shown in fig. 7 and fig. 8 respectively. With roof deflector, stagnation area at the top front face of the
container was reduced because of that pressure on container reduced that leads to reduction in drag
significantly. Roof deflector angle is optimized to reduce flow separation zone, which is formed at tip of
the container, and for better drag reduction compared to base vehicle.
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6. Figure 7: Velocity distribution at center plane at 80 kmph vehicle speed
Figure 8: Pressure distribution at 80 kmph vehicle speed
Aero deflector position is optimized to reduce drag force on the vehicle. In case1, where roof
deflector is placed at the front roof edge, as the deflector angle reduces, stagnation area reduces that
leads to reduction in drag coefficient, but if its angle reduces further then stagnation area at container
front face is increases. Similar effect was observed for case 2 where roof deflector is placed 0.4 m away
from roof front edge. Even though case 1 gives better Cd compared to Case2 but it requires more material
cost.
Effect of roof fairing for two different cases as shown in fig. 6 are presented in the below table.
Table 1: Variation of drag coefficient according to position of roof deflector at 80 kmph
Angle, deg Drag Force, N % reduction with base
Base vehicle 2054.27
Case 1(roof 40 1650.20 19.3
deflector at 37 1583.19 22.9
cabin front edge)
35 1552.60 24.5
Case 2 (roof
44 1581.60 23.1
deflector at 0.4
m away from
43.5 1565.40 23.4
front edge
Variation of drag force with respect to vehicle speed is shown in figure 9. as the vehicle speed increases
Pressure force on front fascia increases leads to increase in drag force.
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7. Figure 9: Variation of drag force with vehicle speed
Later wind pressure load on aero deflector is extracted and used for structural durability with
inertia load conditions. Comparison of stress contours with and without wind loads are shown in figure 10.
Figure 10: Comparison of Stress contours with and without wind load
Conclusions:
The three dimensional turbulent flow around truck and the change in aerodynamic
characteristics caused by roof fairing were numerically investigated. The result and conclusions obtained
by the present simulation can be summarized as follows.
• It was conformed that Stagnation region is formed at front of the container, because of that flow
at container edges moves faster and sudden diverged flow leads to flow separation in turn
creates drag force.
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8. • Keeping roof deflector over MNAL flat roof cabin with container reduces aerodynamic drag by
22 % which in turn lead reduction in fuel consumption by 1.5 to 2 %.
• It was conformed that Wind pressure effect the structural durability of air deflector mounting
bracket.
Benefits Summary:
With the help of AcuSolve, Leading commercial finite element CFD code, we at MNAL product
development team quickly take a decision, whether we go for roof fairing or not, without doing wind tunnel
test which is expensive. And it was very useful for us to do parametric study by changing roof fairing
angle without spending much time as it was in physical test.
Challenges:
When we do parametric study in this project, every time when we modify roof fairing angle we
should generate volume mesh again and again which is time consuming. It would be useful for Acusolve
users, if automatic volume mesh updating option is there.
ACKNOWLEDGEMENTS
The authors would like to thank Mr. Sanjeev Bedekar, Altair technical support and Mr. Uday Srinivas, Sr.
Manger, MNAL cabin team for their valuable support and contributions during this project.
We also would like to thank Mr.Shekar Paranjape, General Manager, MNAL for allowing us to
publish this paper.
REFERENCES
1. Subrata Roy and Pradeep Srinivasan, "External flow analysis of truck for drag reduction", SAE International, 2000-01-
3500.
2. Shiksuke Kowata, Jong soo Ha, Shuya Yoshioka, Takuma kato and yasuaki kohama, "Drag force reduction of a bluff
body with an underbody slant and rear flaps", SAE International, 2008-01-2599.
3. Min-Ho Kim, "Numerical study on wake flow and rear-spoiler effect of a commercial bus body", SAE International, 2003-
01-1253.
4. K.P.Garrey, "Development of container-mounted devices for reducing the aerodynamic drag of commercial vehicles",
Journal of Wind Engineering and Industrial Aerodynamics, 1981.
5. Wolf heinrich Hucho (2001), "Aerodynamics of road vehicles ", 4th edition, SAE International, Vol.1, PP. 11-88.
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