The US Air Force Academy, in conjunction with the NATO RTO
(Research Technology Organization) AVT-201 Task Group, "Extended
Assessment of Reliable Stability and Control Prediction Methods
for NATO Air Vehicles", contributes to evaluating the
state-of-the-art of computational aerodynamics by applying
computational fluid dynamics to a generic UCAV configuration
known as SACCON (Stability And Control CONfiguration). The
objective of the task group is to determine an overall
strategy for creating stability and control databases for vehicle
simulation at full-scale conditions, including the deflection of
control surfaces, throughout the operational envelope of
the vehicle. The aim of these simulations was to
validate the CFD results against wind tunnel data obtained by
DLR, the German Aerospace Center. Available experimental
data included force and moment coefficients, and surface
pressure coefficients at various cuts through the fuselage and
The SACCON model has a reference chord length of 0.479m and a
wingspan of 1.538m. It is a swept wing flying wing configuration
typical of the next generation UCAVs currently under development.
The tests were conducted at approximately 50 m/s giving a Reynolds
number of 1.57 million. The flowfield for this configuration at
even moderate angles of attack is dominated by the vortices shed
from the leading edge.
The full-span grid has up to ~150 million cells, with very high
resolution close to the leading edge--in order to resolve the
initiation of the vortices, and downstream of the wing--in order to
resolve the off body vortices that arise at higher angles of attack.
The CFD codes applied to this problem include Cobalt and Kestrel.
The HPCMP resource used was the ERDC's Garnet machine. Typical
simulations of the full-span configuration run on 5120 nodes over 24
hours to generate 0.2 seconds of flight time.
Visualization is the key to understanding the phenomena that occurs
in the flowfield about the air vehicle. In particular, we are
interested in the distribution of pressure across the surface, but
also understanding how vortices form off the leading edge of the
wing and evolve and interact downstream, affecting the pressure
distribution and therefore the forces and moments experienced by the
Several visualization techniques have been used. One of the images
above shows an isosurface of vorticity-magnitude colored by
velocity-magnitude. The plan view images show contours of surface
pressure coefficient along with streamtraces seeded close to the
leading edge. This allows one to get a sense of the behavior of the
vortices. Such visualizations are repeated at different angles of
attack to help explain the observed changes in forces and moments.
This project has both large grids and a large number of flight
conditions to analyze: static angle of attack and angle of sideslip
sweeps, and dynamic motion about the pitch and yaw axes. All this
adds up to a large quantity of data to post-process. The secure
remote desktop (SRD) system supported by DAAC has been critical in
enabling efficient post-processing of this large volume of data.
Rather than downloading the data down to a local machine, it can be
passed to the utility servers and post-processed "in the cloud" via
the SRD interface. While there are many software options for
providing visualization available on SRD, we chose to use Tecplot
for the majority of our visualizations in order to be consistent
with visualizations provided by other institutions from partner