Alternative approaches to rear end drag reduction

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Introduction

Aerodynamic drag of a commercial vehicle is a large part of the vehicles fuel consumption,
according to Hucho [1] it can contribute to as much as 60 % of the vehicles fuel consumption.
So far aerodynamic design of commercial vehicles has concentrated on the front end of the
vehicle. Since it produces most drag it has been the most urgent part to optimise. This
optimisation can easily be spotted on trucks and tourist coaches. The rear end configuration
has up until recently been neglected. Gilhaus [2] acknowledge the fact that on tourist coaches
the rear end can contribute to as much as 27 % of the over all drag. This is the reason to why
the author has chosen to take a closer look at the different technologies available to reduce
rear end drag. Much of this technology has its offspring in aeroplane aerodynamics and the
design of diffusers.

Different technologies

Boat-tailing
The most common and natural way of reducing rear end drag is boat-tailing, also called rear
end tapering. It offers a technology commonly known, widely used and with recognised effect.
But the practical application of it is limited due to the fact that it greatly reduces the comfort
for the passengers and loading capability, see figure 1. Taking tapering to its drag reduction
possibility limits is not a realistic possibility of practical reasons but it is still interesting to
study the results of such research since it could be used as a benchmark and goal for other
studies.

Controlled boundary layers

Active separation control system of rear end flow can be performed with tangential blowing as
suggested by Englar [6], [7] among others. The technology is also called the Coanda Effect
named by Henri Coanda. The idea is that a slow airflow that generally would separate over a
surface is energized with a high-velocity flow and thus the flow becomes attached to a curved
surface as shown by figure 7.

Conclusions

Mentionable is that there are measures that are more important to take than changing the
airflow around the rear of a vehicle. Those measures are the rounding of front corners which
can contribute to as much as 52% drag reduction [6] and a full-length underbody seal can
contribute to as much as 15% drag reduction [6]. These measures needs to be taken care of
before it is interesting to take a look at rear end flow. But when at least the first measure has
been taken care of it can be beneficial to take a look at altering rear end airflow.