Research on Aerodynamic Drag Reduction by Vortex Generators


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Introduction

To save energy and to protect the global environment,
fuel consumption reduction is primary concern of
automotive development. In vehicle body development,
reduction of drag is essential for improving fuel
consumption and driving performance, and if an aerodynamically
refined body is also aesthetically attractive,
it will contribute much to increase the vehicle’s appeal
to potential customers.


Finding the optimum VGs

To select appropriate shape and size of the VG
which generates streamwise vortex the most efficiently
(with the least drag by itself) is important to achieve
objectives.
In connection with the size, the thickness of the
boundary layer is measured based on the assumption
that the optimum height of the VG would be nearly
equal to the boundary layer thickness. Fig. 4 shows the
velocity profile on the sedan’s roof. From this figure,
the boundary layer thickness at the roof end immediately
in front of the separation point is about 30 mm.
Consequently, the optimum height for the VG is estimated
to be up to approximately 30 mm.


Verification of VG’s mechanism

In Section 2 above, the effect of VGs is estimated
that the separation point is shifted to downstream,
which in turn narrows the flow separation region. The
flow field was thus investigated in order to verify the
correctness of this estimation.


Conclusions
The conclusions of this research can be summarized
into the following points:
Fig. 11 Pressure distribution of vehicle (CFD)
Fig. 12 Vorticity distribution behind
vortex generators (CFD)

(1) Vortex generators (VGs) were studied to install
immediately upstream of the flow separation point
in order to control separation of airflow above the
sedan’s rear window and improve the aerodynamic
characteristics. It was found that the optimum
height of the VGs is almost equivalent to the thickness
of the boundary layer (15 to 25 mm) and the
optimum method of placement is to arrange them
in a row in the lateral direction 100 mm upstream of
the roof end at intervals of 100 mm.