1. © 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

2. © Saab AB 2008 www.saabgroup.com 2 Outline of presentation  Introduction  Models  Computations (CFD and 6-DOF)  Results  Conclusions

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

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

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

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

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

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

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

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

11. © Saab AB 2008 www.saabgroup.com 11 Flight test – Separation as seen from chase a/c

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

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.

14. © Saab AB 2008 www.saabgroup.com 14 Results – Corrective techniques

15. © Saab AB 2008 www.saabgroup.com 15 Results – Corrective techniques

16. © Saab AB 2008 www.saabgroup.com 16 Results – Corrective techniques

17. © Saab AB 2008 www.saabgroup.com 17 Results – Simulation with captive corrections

18. © Saab AB 2008 www.saabgroup.com 18

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

20. © Saab AB 2008 www.saabgroup.com 20 Example of RANS computation with 0.25 m vertical drop of RHS Mk82LD

21. © Saab AB 2008 www.saabgroup.com 21 Results – Based on CFD alone

22. © Saab AB 2008 www.saabgroup.com 22 Results – Based on CFD alone

23. © Saab AB 2008 www.saabgroup.com 23 Results – Based on CFD alone

24. © Saab AB 2008 www.saabgroup.com 24

25. © Saab AB 2008 www.saabgroup.com 25 Comparison between simulations based on the different settings

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

27. © Saab AB 2008 www.saabgroup.com 27 www.saabgroup.com