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By Sandeep Sovani, Fluent Inc., and Bipin Lokhande, Fluent India Pvt. Ltd.
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Pressure contours on the surface of
the pickup and pathlines, colored by
velocity magnitude showing the flow
around the vehicle
For 22 years, America’s best-selling vehicle has been the pickup truck. Although pickup
trucks together account for just 13% of new vehicles sold in the United States,
familiar offerings from Ford, GM, and Chrysler held down the top three spots in
overall units sold in 2004 through the first half of the year. This statistic is due in part to
the country’s love of size and cargo utility. Whether they have short beds or long beds,
two doors or four doors, short cabs or long cabs, pickup trucks are in continuous demand
by the public.
From an operational point of view, a drawback of pickups is that they consume much
more fuel than a standard passenger car. At typical highway cruising speeds, the majority
of fuel consumed by an automobile is spent on overcoming aerodynamic drag. Because
trucks are aerodynamic bluff bodies, they experience particularly high drag forces, something
pickup designers are always interested in reducing.
CFD is an effective tool for helping designers understand the flow structure around a
truck and optimize its shape. The less-streamlined shape of a truck means that the flow
field around it is inherently transient. Unsteady CFD simulations therefore model the flow
field more accurately than steady-state computations. In the present study [1], the external
aerodynamics of a generic truck body are considered by making use of GAMBIT and
FLUENT. The truck being modeled is a 1:12 scale replica of an actual pickup with its side
mirrors and underbody components removed. Starting from a triangular surface mesh, a
fine prismatic boundary-layer mesh is created on all prominent surfaces of the truck. A
layer of tetrahedra is created immediately above the prism layers. The rest of the domain
is filled with hexahedra for an overall mesh size of about 3.5 million cells.
Using FLUENT’s large-eddy simulation (LES) and RNG k-ε turbulence models in separate
simulations, transient CFD calculations were conducted using a 0.3 ms time step and compared
to detailed wind tunnel experiments [2]. These comparisons include time-averaged
pressure distribution around the vehicle and wake velocity profiles. Pressure profiles predicted
on the vehicle body show excellent agreement with experimental data. A notable feature
of the air flow is a large vortex forming behind the truck’s cab. Additionally, there is a rapid
change of direction behind the tailgate as the flow passing above the vehicle turns downward
and mingles with the flow underneath. The pathlines predicted by FLUENT closely follow
those measured in the experiments.
Pressure coefficient distribution on the truck’s
upper body, plotted at the symmetry plane,
compared to experiment [2]
Time-averaged pathlines in the truck’s wake are
in very good agreement with experiment [2]
The goal of this study was to generate a set of data that can be used for validating
CFD simulations of pickup trucks in the long term. Now that fuel economy is joining safety
and versatility as a consideration when buying a truck, designers can be confident that
modern CFD techniques will accurately predict the complex bluff-body wake flows that
occur around pickups and give them insight into the challenge of drag reduction.
References:
- B. Lokhande, S. Sovani, and B. Khalighi, “Transient Simulation of the Flow Field Around a Generic
Pickup Truck,” SAE Transactions: Journal of Passenger Cars – Mechanical Systems, Paper no.
2003-01-1313, p. 1358-1376 (2003).
- A. Al-Garni, L. Bernal, and B. Khalighi, “Experimental Investigation of the Near Wake of a Pickup Truck,”
SAE Paper 2003-01-0651 (2003).
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