Vorticity Confinement Applied to Induced Drag Prediction And
Total Page:16
File Type:pdf, Size:1020Kb
VORTICITY CONFINEMENT APPLIED TO INDUCED DRAG PREDICTION AND THE SIMULATION OF TURBULENT WINGTIP VORTICES FROM FIXED AND ROTATING WINGS A Thesis Presented to The Graduate Faculty of The University of Akron In Partial Fulfillment of the Requirements for the Degree Master of Science Kristopher C Pierson May, 2014 VORTICITY CONFINEMENT APPLIED TO INDUCED DRAG PREDICTION AND THE SIMULATION OF TURBULENT WINGTIP VORTICES FROM FIXED AND ROTATING WINGS Kristopher Pierson Thesis Approved: Accepted: _______________________________ _______________________________ Advisor Dean of the College Dr. Alex Povitsky Dr. George K. Haritos _______________________________ _______________________________ Faculty Reader Dean of the Graduate School Dr. Scott Sawyer Dr. George R. Newkome _______________________________ _______________________________ Faculty Reader Date Dr. Minel Braun _______________________________ Department Chair Dr. Sergio Felicelli ii ABSTRACT To improve accuracy of CFD simulations by combating numerical viscosity, the vorticity confinement (VC) approach was applied to tip vortices shed by stationary wings for the prediction of induced drag by far-field integration in Trefftz plane as well as the simulation of tip vortex evolution. Vorticity confinement schemes are first evaluated and compared using 2D Euler simulation of a Taylor vortex. The effect of numerical dissipation is shown to cause the vortex to spread and decay without the presence of physical dissipative terms; VC counteracts this effect and maintains the vortex strength. For 3D simulations, the coefficient of confinement was determined for various flight parameters. Dependence of vorticity confinement parameter on the flight Mach number and the angle of attack was evaluated. The aerodynamic drag results with VC are much closer to analytic lifting line theory compared to integration over surface of wing. To apply VC to viscous and turbulent flows, it is shown that in many cases VC does not affect the physical rate of dissipation of line vortices for temporal and viscous scales relevant to wingtip vortex simulations. VC was applied to the simulation turbulent wingtip vortices originating from fixed and rotating wings. The six Reynolds stress turbulence model combined with VC most accurately matched experimental results of tip vortex evolution. iii ACKNOWLEDGEMENTS I would like acknowledge the financial support and guidance from DAGSI/AFRL Ohio Student-Faculty Research Fellowship in 2011-2013 sponsored by Drs. J. Camberos and L. Downie, Army Research Office in 2012-2013 by ARO program director Dr. F Ferguson, and ASEE/AFRL summer faculty grant (AFRL/WPAFB, Dayton, OH) in 2012. Also, DAGSI/AFRL was the sponsor of previous work at the University of Akron from 2009-2011 that was built upon in the present research. Finally, I would like to thank Dr. Povitsky for his guidance and support that made this research possible. iv TABLE OF CONTENTS Page LIST OF TABLES ............................................................................................................ vii LIST OF FIGURES ......................................................................................................... viii NOMENCLATURE ........................................................................................................... x CHAPTER I. INTRODUCTION .......................................................................................................... 1 1.1 Background ................................................................................................... 1 1.2 Finite Volume Method .................................................................................. 4 1.3 Upwind Schemes and TVD .......................................................................... 5 1.4 Vorticity Confinement .................................................................................. 8 1.5 Wake-Integral Method ................................................................................ 12 1.6 Turbulence Modeling .................................................................................. 13 1.7 Research Goals............................................................................................ 15 II. SIMULATION OF 2-D VORTICES USING VORTICITY CONFINEMENT AND TVD ................................................................................................................................ 17 2.1 2D Code Description................................................................................... 17 2.2 Vorticity Confinement implementation and Formulations ......................... 20 2.3 Results of Second Order Euler Simulations without TVD ......................... 21 2.4 Comparison of TVD Schemes .................................................................... 26 2.5 Decay of Line Vortex, Viscous Test Case .................................................. 29 v III. INDUCED DRAG PREDICTION SIMULATIONS USING WAKE- INTEGRAL COMBINED WITH VORTICITY CONFINEMENT.................................................... 33 3.1 Implementation of VC in ANSYS FLUENT .............................................. 33 3.2 Lifting Line Theory..................................................................................... 34 3.3 Problem Description ................................................................................... 35 3.4 Results ......................................................................................................... 37 3.5 Differentiable Limiter ................................................................................. 42 IV. TURBULENT SIMULATIONS OF FIXED WINGTIP VORTICES USING VORTICITY CONFINEMENT ..................................................................................... 46 4.1 Evaluation of Viscous Drag ...................................................................... 46 4.2 Results of Turbulent 3D Fixed Wing Simulations .................................... 48 4.3 Comparison to Experiements .................................................................... 51 V. TURBULENT SIMULATIONS OF WINGTIP VORTEX COMBINED WITH VORTICITY CONFINEMENT ..................................................................................... 57 5.1 Grid and Setup .......................................................................................... 57 5.2 Inviscid Results ......................................................................................... 59 5.3 Turbulence Model Results ........................................................................ 61 5.4 Winglet Modeling and Results.................................................................. 63 VI. CONCLUSIONS AND RECOMMENDATIONS FOR FUTURE WORK ............. 66 BIBLIOGRAPHY ............................................................................................................. 69 APPENDICES .................................................................................................................. 72 APPENDIX A. FLUX SPLITING FOR STEGER WARMING SCHEME ....... 73 APPENDIX B. 2D MATLAB CODE ................................................................. 75 APPENDIX C. ANSYS FLUENT UDF .............................................................. 82 vi LIST OF TABLES Table Page 3.1 Solver settings, model settings and solution methods for inviscid simulations .. 37 3.2 Drag coefficients by surface integration and wake-integral method compared to lifting line theory ................................................................................................. 39 3.3 Tuned confinement parameters ........................................................................... 39 4.1 Flow characteristics for boundary layer in flat plate .......................................... 47 4.2 Results for near-field and far-field integration for viscous flow about flat plate, error is determined by comparison to Blasius solution for shear stress .............. 48 4.3 Shear force and induced drag results for turbulent simulations ......................... 51 5.1 Physical problem specifications for rotating wing simulations .......................... 58 5.2 Computational parameters for rotating wing simulations ................................... 58 vii LIST OF FIGURES Figure Page 2.1 Positive flux extrapolation used in second order scheme .................................. 19 2.2 First order simulation of Taylor vortex .............................................................. 23 2.3 Upwind second-order simulation of Taylor vortex with and without VC .......... 24 2.4 Results of second order calculations using VCF2 with c=4.5 ............................ 26 2.5 Comparison of TVD Schemes, c=4.0 ................................................................. 28 2.6 Comparison of TVD Schemes, c=5.0 ................................................................. 29 2.7 Decay of line vortex, second-order minmod TVD simulation comparison of epsilon values; Δx=Δy=0.1 ................................................................................. 32 2.8 Decay of line vortex, second-order minmod TVD simulations comparison of epsilon values; Δx=Δy=0.016 ............................................................................ 32 3.1 Computational domain for 3D fixed wing .......................................................... 35 3.2 Residual convergence for AoA = 4 Degrees and Mach = 0.3 ............................ 37 3.3 Induced drag coefficient for various Trefftz plane locations for inviscid simulations .........................................................................................................