14.6 Prospects for the Aerodynamic Design
295
14.6 Prospects for the Aerodynamic Design
As mentioned at the beginning of this book, the place of CFD in the aerodynamic
design has become increasingly important. This is due to the exponential progress in
computing powers and the techniques of numerical analysis allowing the resolution
of increasingly sophisticated theoretical models. In parallel, wind tunnels, where
real operating conditions can be approached, are being developed and the existing
facilities are being equipped with more sophisticated means of positioning the models, measurements and analysis of the flow which has considerably increase their
productivity. Thus, for several decades now the aerodynamic design of a vehicle
(automobile or aircraft) has consisted of overlapping cycles.
CFD allows for computation of the flow field around the configuration of a vehicle
in a few minutes and it is often more time consuming to define the geometry and
generate the mesh than the actual computation. An aerodynamic design office can
thus compute several configurations per day. There are even techniques for automatic
shape optimisation which can reproduce the best shape according to the criterion
defined by the user in few hours. This makes it possible to evaluate a very large number
of variables within the space of possible configurations and a multi-disciplinary
optimisation can be performed based on other constraints imposed by aeroelasticity
or flight mechanics, for instance.
However, CFD is hindered by their lack of representativeness of complex flow
phenomena. Competition in the aviation sector is driving more unconventional architectures that cannot be handled by the current tool set developed and validated from
semi-empirical database; therefore this is proving to be a real issue right from the
initial design phase. The wind tunnel tests are the means of studying a flow mechanism in a real environment, even if the conditions are not completely identical to the
open, infinite conditions. The major disadvantages of the wind tunnel test for design
and optimisation are the test durations, the costs of both model manufacturing and
operation, including instrumentations. The duration of a campaign, including both
model manufacturing and wind tunnel tests is typically of several months. If together
with a baseline model another set of alternative configurations are manufactured, the
test duration for the additional configurations can be reduced, since the test protocol
has already been established. These test cases can be very useful, especially at conditions close to the edge of the flight envelope, where complex flow phenomena are
encountered for which the representativeness of CFD is still questionable.
Wind tunnels can also be distinguished by their operational cost. The large wind
tunnels in which the conditions closer to flight can be reproduced have significant
operational costs. The models adapted for these tunnels are also very expensive. For
these reasons, aircraft manufacturers often use smaller wind tunnels, less representative but cheaper, for the development tests. These wind tunnels are complementary
to CFD since they provide test conditions for which CFD is least validated.
The more flight representative, large wind tunnels are then used to characterise
more accurately the aerodynamic performance at the later stage of design or to explore
295
14.6 Prospects for the Aerodynamic Design
As mentioned at the beginning of this book, the place of CFD in the aerodynamic
design has become increasingly important. This is due to the exponential progress in
computing powers and the techniques of numerical analysis allowing the resolution
of increasingly sophisticated theoretical models. In parallel, wind tunnels, where
real operating conditions can be approached, are being developed and the existing
facilities are being equipped with more sophisticated means of positioning the models, measurements and analysis of the flow which has considerably increase their
productivity. Thus, for several decades now the aerodynamic design of a vehicle
(automobile or aircraft) has consisted of overlapping cycles.
CFD allows for computation of the flow field around the configuration of a vehicle
in a few minutes and it is often more time consuming to define the geometry and
generate the mesh than the actual computation. An aerodynamic design office can
thus compute several configurations per day. There are even techniques for automatic
shape optimisation which can reproduce the best shape according to the criterion
defined by the user in few hours. This makes it possible to evaluate a very large number
of variables within the space of possible configurations and a multi-disciplinary
optimisation can be performed based on other constraints imposed by aeroelasticity
or flight mechanics, for instance.
However, CFD is hindered by their lack of representativeness of complex flow
phenomena. Competition in the aviation sector is driving more unconventional architectures that cannot be handled by the current tool set developed and validated from
semi-empirical database; therefore this is proving to be a real issue right from the
initial design phase. The wind tunnel tests are the means of studying a flow mechanism in a real environment, even if the conditions are not completely identical to the
open, infinite conditions. The major disadvantages of the wind tunnel test for design
and optimisation are the test durations, the costs of both model manufacturing and
operation, including instrumentations. The duration of a campaign, including both
model manufacturing and wind tunnel tests is typically of several months. If together
with a baseline model another set of alternative configurations are manufactured, the
test duration for the additional configurations can be reduced, since the test protocol
has already been established. These test cases can be very useful, especially at conditions close to the edge of the flight envelope, where complex flow phenomena are
encountered for which the representativeness of CFD is still questionable.
Wind tunnels can also be distinguished by their operational cost. The large wind
tunnels in which the conditions closer to flight can be reproduced have significant
operational costs. The models adapted for these tunnels are also very expensive. For
these reasons, aircraft manufacturers often use smaller wind tunnels, less representative but cheaper, for the development tests. These wind tunnels are complementary
to CFD since they provide test conditions for which CFD is least validated.
The more flight representative, large wind tunnels are then used to characterise
more accurately the aerodynamic performance at the later stage of design or to explore