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By Evangelos K. Koutsavdis, Fluent Inc.
One of the most challenging problems in aerospace engineering, especially
for military vehicles, is the analysis of a store (a weapon, fuel tank,
or electronic countermeasures device, for example) that is released from
a high-speed aircraft. Store separation analysis typically includes such
things as a calculation of the trajectory, the identification of safe
separation zones, an assessment of aerodynamic interference, and making
sure that collisions are avoided. For multiple separations, typical of
cluster bombs for example, the analysis could also include the dispersion
characteristics of the weapon, so that the munitions cover the biggest
possible area upon impact with the ground.
The evolution of the grid for a 2D store separation simulation
For many years, physical testing
using the actual aircraft and device
has been the only method for performing
store separation analysis. The
cost and risks associated with such
tests can be high, however, especially
during parametric studies. The
dynamic mesh model in FLUENT now
provides a safer, more cost-effective
solution to the analysis needs of aerospace
companies involved in this kind
of application.
The basic characteristic of store
separation analysis is the presence
of a body that moves in the computational
domain as a result of its
interaction with the computed
flow field. This means that in addition
to the need for a dynamic mesh,
tools are also required that determine
the body movement based on
the local flow conditions. These tools
need to accurately compute the aerodynamic
forces on the body, and
determine the dynamic response of
the body to these forces. A trajectory
calculation is performed to integrate
the forces and moments on
the body, and provide an accurate
position of the body as a function
of time.
The most challenging of these tasks, by far, is the mesh handling. The
geometric complexity of modern aircraft and the stores, which may be outfitted
with fins, guidance devices or release mechanisms, necessitates the use
of complex meshes, comprised mostly of tetrahedral elements. The remeshing
schemes need to be robust and deliver high quality meshes that can be
relied upon for accurate aerodynamic load predictions at each time step.
Since thousands of time steps may be needed for an accurate analysis,
depending on such factors as the release speed or aircraft speed, the
mesh handling also needs to be done in a time-efficient manner.
Pressure contours and pathlines on a generic store being released from
an aircraft bay at a Mach number of 0.7 at three times
For the store separation simulation
shown at left, a user-defined function
(UDF) is used to compute the aerodynamic load and trajectory of the store
at each time step, based
on the local flow conditions. The UDF
is a full force and moment calculation
that allows for six degrees of freedom.
User inputs include the basic
characteristics of the store, such as
the location of the center of gravity,
the store mass, and components
of the moment of inertia tensor. Once
the new location and orientation of
the store is computed, a new mesh
is constructed using a combination
of the spring smoothing and local
remeshing algorithms.
Sizing functions, introduced in the
latest version of GAMBIT, are used
in these algorithms to produce an
optimum mesh distribution. Other
quality controls include user-specified
limits on the mesh skewness and
cell volume. When complemented
with the full suite of postprocessing
tools in FLUENT, including animations,
the dynamic mesh model can
offer a clear picture of the store trajectory
and identify potential problems
before or even without an actual
flight test.
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