Abstract
Because the aerodynamic loads, which are acting on a car, play a significant
part concerning the dynamic behaviour of the latter as regards stability, handling, crosswind
sensitivity, wind noise and not lastly, upon the fuel consumption, the aerodynamics becomes
an important design consideration. Till recently, the exterior shape of the vehicles was
represented the main preoccupation of the engineers, the underbody geometry playing a
secondary role, or being completely neglected for some type of vehicles, as for off-road ones.
In the last decade, in order to optimise the flow around cars, the underhood flow management
becomes a major problem of the designers, too [1].
The aerodynamic performances of the vehicles are characterised using specific coefficients,
dimensionless, as drag and lift coefficients. Using of these as the measure of the state of the
art in the vehicle aerodynamics, the continuously progress is possible in this field. In this
context, because the decomposition of the aerodynamic forces into measurable components
would facilitate the optimisation design process of the carriage body, in the previous study [3]
was presented a theoretical method for computing of the drag due to the underhood airflow. In
this sense, was proposed the decomposition of the global drag, D , into two components, ext D
and ub D . The first one is the drag due to the flow upon the external surface of the vehicle,
having the rate flow ext Q . The second term represents the drag due to the flow under the body
of vehicle, in the space determined by the lower surface of the vehicle and the road, treated as
a convergent-divergent air nozzle with the flow rate ub Q . Also, dimensionless indicators were
defined to characterise the underhood airflow as following: ub D c , drag coefficient of the
underbody, ub D K , coefficient what represent the ratio between underbody drag and total drag
and ub Q K , coefficient what characterise the participation of the underbody flow rate on total
flow rate Q .
In this paper are emphasised the main factors which having importance on the underbody drag
and which may lead to optimising steps with respect to the aerodynamics of the vehicles. The
study is illustrated by means of a numerical exemplification and experiments performed with
a model having the underbody geometry modelled as a Venturi tunnel. The parametric study
of the underbody drag coefficient permits the plotting of a diagram of this as function of the
dimensionless indicators, which reveal the minimum value of ub D c . Also, the tests reveal that
an underbody geometry which conducive the Venturi effect is essential for the development
of a deportante vertical aerodynamic load what increase adhesion of the wheels, and improve
the dynamic behaviour of the car. This contribution is a companion paper of Huminic &
Benche 2005.
Keywords - Aerodynamic loads, underhood airflow, underbody drag of vehicle,
dimensionless characteristic indicators, optimising methods