Saab AB 2008 www.saabgroup.com 1 Transonic store separation
studies on the SAAB Gripen aircraft using CFD Ingemar Persson and
Anders Lindberg Stockholm, October 18, 2010 Slide 2 Saab AB 2008
www.saabgroup.com 2 Outline of presentation Introduction Models
Computations (CFD and 6-DOF) Results Conclusions Slide 3 Saab AB
2008 www.saabgroup.com 3 Introduction External stores must be
released in a safe and well predicted manner 6-DOF simulation model
to predict store trajectories SSM includes free-flying store
aerodynamics as well as interference aerodynamics Data collected
from complicated and extensive WT-tests using a two- sting-rig Time
and budget limitations lead to alternative ways in achieving store
release predictions Advances in CFD and low cost computer power
make computational aerodynamics an interesting alternative Slide 4
Saab AB 2008 www.saabgroup.com 4 Introduction SAAB Gripen Flight
condition M=0.9, AoA=1.9 degrees Payload: Four 227 kg Mk82LD at
2L/2R and 3L/3R WT: centerline DT300 droptank FT: centerline FUNK
camera pod Slide 5 Saab AB 2008 www.saabgroup.com 5 Models CAD
models CATIA v.4 models, imported to ICEM CFD using direct CAD
interface Slight modifications of surface geometry to improve
prismatic boundary layer grid Discrepancy: Pylon 4 not present in
CAD/CFD models Different levels of modelling complexity tried on
pylons Complexity of sway braces Suspension lugs present or not Gap
distance between payload and pylon uncertain, used 8.7 mm Both WT
and FT configurations modelled Slide 6 Saab AB 2008
www.saabgroup.com 6 Models Discrete CFD models Grid generation
using ICEM Tetra/Prism Captive position and 6 subsequent vertical
positions Tetrahedral grids approximately 4.5-6.5 Mnodes Mixed
tetrahedral / prismatic grids approximately 19-22 Mnodes Boundary
layer grid holding 40 prismatic layers y + =1 grid, Initial cell
height 2e-5 m, expansion factor 1.2 Far field positioned 10 a/c
lengths away from aircraft Slide 7 Saab AB 2008 www.saabgroup.com 7
Grid generation Detail of mixed tetrahedral / prismatic grid around
payload Mk82LD in captive position Detail of grid around the bomb
fins Slide 8 Saab AB 2008 www.saabgroup.com 8 Computations -
General CFD solution obtained using the EDGE v.4.1.0 fluid flow
solver Inviscid computations utilising a central scheme using JST
art.visc. Viscous computations performed with both central and
upwind scheme Thin shear layer NS equations solved Turbulence model
is Menter SST k-w RK time integration with agglomorated FAS
multigrid conv.acc. Upwind computations always initiated with 1st
order scheme and later switch to 2nd order using a Roe flux
difference splitting employing a minmod limiter Slide 9 Saab AB
2008 www.saabgroup.com 9 Computations Boundary conditions Farfield
Riemann invariants Solid surface slip / no slip Engine inlet /
outlet flow through surfaces with prescribed flow ECS inlet /
outlet flow through surfaces with prescribed flow Example of RANS
computation Slide 10 Saab AB 2008 www.saabgroup.com 10 Computations
6-DOF simulation Store relative motion depends on Store free flight
aerodynamics Mass and inertial data Aircraft interference
aerodynamics Aircraft motion during separation ERU force on the
store ERU module consists of a gas dynamic model ODE system solved
by RK-Merson Solution visualised in SAAB system ICARUS Example from
ICARUS Slide 11 Saab AB 2008 www.saabgroup.com 11 Flight test
Separation as seen from chase a/c Slide 12 Saab AB 2008
www.saabgroup.com 12 The first simulation based on WT data from a
configuration with a DT300 attached to pylon 5 Slide 13 Saab AB
2008 www.saabgroup.com 13 Surface pressure field, Euler simulation
Note the difference in shock strength. The drop tank results in a
large under pressure on the bomb fins which gives a large yawing
moment. Slide 14 Saab AB 2008 www.saabgroup.com 14 Results
Corrective techniques Slide 15 Saab AB 2008 www.saabgroup.com 15
Results Corrective techniques Slide 16 Saab AB 2008
www.saabgroup.com 16 Results Corrective techniques Slide 17 Saab AB
2008 www.saabgroup.com 17 Results Simulation with captive
corrections Slide 18 Saab AB 2008 www.saabgroup.com 18 Slide 19
Saab AB 2008 www.saabgroup.com 19 Results Based on CFD alone Grid
based approach Captive and subsequent vertical positions computed
(0.0625, 0.125, 0.25, 0.5, 1.0, 2.0 m) Different complexity of
pylon attachment investigated (sway bracer realisation, suspension
lugs etc.) Euler and Navier-Stokes Different numerical schemes
(central and upwind) Captive absolute condition hard to capture
Slide 20 Saab AB 2008 www.saabgroup.com 20 Example of RANS
computation with 0.25 m vertical drop of RHS Mk82LD Slide 21 Saab
AB 2008 www.saabgroup.com 21 Results Based on CFD alone Slide 22
Saab AB 2008 www.saabgroup.com 22 Results Based on CFD alone Slide
23 Saab AB 2008 www.saabgroup.com 23 Results Based on CFD alone
Slide 24 Saab AB 2008 www.saabgroup.com 24 Slide 25 Saab AB 2008
www.saabgroup.com 25 Comparison between simulations based on the
different settings Slide 26 Saab AB 2008 www.saabgroup.com 26
Conclusions Computational aerodynamics is a useful tool when used
in a corrective manner Achieving the correct captive aerodynamic
load is of vital importance Computational aerodynamics as a sole
contributor of aero data was not as accurate but can be used as an
indicative method For this case, inviscid physics was accurate
enough. Viscous computations did not improve results to motivate
the increased work load Slide 27 Saab AB 2008 www.saabgroup.com 27
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