Transonic store separation studies on the SAAB Gripen aircraft using CFD

www.saabgroup.com

1© Saab AB 2008

Transonic store separation studies on the SAAB Gripen aircraft using CFD

Ingemar Persson and Anders Lindberg

Stockholm, October 18, 2010

www.saabgroup.com2© Saab AB 2008

Outline of presentation

Introduction Models Computations (CFD and 6-DOF) Results Conclusions

WeaponWeaponWeaponWeapon


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


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


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


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


Grid generation – Detail of mixed tetrahedral / prismatic grid around payload

Mk82LD in captive position Detail of grid around the bomb fins


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


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


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


Flight test – Separation as seen from chase a/c


The first simulation based on WT data from a configuration with a DT300 attached to pylon 5


Surface pressure field, Euler simulationNote the difference in shock strength. The drop tank results in a large under pressure on the bomb fins which gives a large yawing moment.


Results – Corrective techniques


Results – Simulation with captive corrections


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


Example of RANS computation with 0.25 m vertical drop of RHS Mk82LD


Comparison between simulations based on the different settings


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


www.saabgroup.com

Transonic store separation studies on the SAAB Gripen aircraft using CFD

Documents

Transcript of Transonic store separation studies on the SAAB Gripen aircraft using CFD