UWE Bristol Engineering showcase 2015

Steve Tiley
Aerospace Design Engineering
Unmanned Combat Air Vehicle: Vortex Control Analysis
Project Supervisor
Eur. Ing. Dr. C.hrisToomer
Project summary
This study involved the CFD analysis of an Unmanned
Combat Air Vehicle designated Stability And Control
CONfiguration (SACCON) produced by the NATO
Applied Vehicle Technology board.
Wings of a high aspect ratio will begin to exhibit
separated flow at high angles of attack stalling parts of
the wing and causing a reduction in lift.
On delta wings (or wings of low aspect ratio) this
separation tends to occur near the leading edge and
unlike the case for low sweep or high aspect ratio wings
this separation does not cause a reduction in lift. Instead
the flow rolls up into a vortex which sits on the wing’s
upper surface.
Both models of the SACCCON geometry used for
this experiment were made by Jana-Sabrina
Stucke as part of a PhD using SolidWorks.
The mesh created for the half model geometry
features 2.27 million elements on the advice of Dr.
Raj Nangia, with greater density around the model
surface in order to better capture the flow around
it.
Instead of reducing lift as might be expected from
separated flow these vortices actually produce lift. They
do this by causing air to flow up and over them before
plunging down again. This increases the speed of the
flow and hence causes a lower pressure above the wing .
The literature study shows that
there has been a great amount of
various work conducted on the
SACCON. Other papers (e.g. Luckring
and Boelens, 2012) have mentioned
the existence of these vortices and it
is the behaviour of these vortices
that is of interest to this study.
The code used is CFX, and is used in
steady state viscous Euler and turbulent
RANS mode at medium turbulence
intensity using the Shear Stress
Transport (SST) turbulence model, and
is run to the default convergence below
1.0E-4 variable values, which was found
to be sufficient for all cases.
Of key interest were the formation and structure of
leading edge vortices and their effect on the
aerodynamic behavior of the SACCON model.
A number of CFD analyses were conducted using
ANSYS CFX at sea level conditions with a velocity of
Mach 0.14 with varying angles of attack from -10 to
20 degrees.
Post-processing of the simulations aimed to visualise
the vortices and quantify their effect on the model’s
lift and drag characteristics, and chord-wise pressure
distributions.
Project Objectives
• Set up and run a number of CFD simulations to
model the leading edge vortices indicative of swept
wing models of the SACCON type.
• Assess the vortices, quantifying their size and
strength in order to judge their effect on the
aerodynamic behaviour of the model.
• Calculate the key aerodynamic behaviours of the
model i.e. lift, drag, and pressure.
Vortex growth from 8 degrees (left), 12 degrees
(middle), and 16 degrees (right).
Comparison of skin fiction streamlines
from Tomac and Stenfelt (2014) (left)
and the vortex visualisation from this
study (right) indicating that the vortex is
indeed the cause of the separation line
near the leading edge.
The effect of vortices of the section lift
coefficient can be seen. As the vortex
moves inboard with increasing angle of
attack the peak section lift also moves
inboard and increases in magnitude.
Streamline plots indicate laminar flow at 4 degrees
angle of attack (left) and turbulent flow at 14
degrees angle of attack (right)
Pressure distributions show the effect of vortices on
increasing the lift, between 28 % span (left) and 42%
span (right) an increase in pressure difference is
seen.
Project Conclusion
ANSYS CFX and the methods presented in this study
are viable for the analysis of leading edge vortices on
the SACCON model.
It is clear how the leading edge vortices react to
changing angle of attack and how the vortex changes
the aerodynamic behaviour of the model.
The methods used are however not viable for angles
of attack greater than 15 degrees and at far outboard
span-wise locations.