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Demonstrating the Potential of Transitional CFD for Sailplane Design

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Demonstrating the Potential of Transitional CFD for Sailplane Design The Pennsylvania State University The Graduate School Department of Aerospace Engineering DEMONSTRATING THE POTENTIAL OF TRANSITIONAL CFD FOR SAILPLANE DESIGN A Thesis in Aerospace Engineering by Christopher J. Axten Submitted in Partial Fulfillment of the Requirements for the Degree of Master of Science December 2019 ii The thesis of Christopher J. Axten was reviewed and approved* by the following: Mark Maughmer Professor of Aerospace Engineering Thesis Advisor Sven Schmitz Associate Professor of Aerospace Engineering Amy Pritchett Professor of Aerospace Engineering Head of the Department of Aerospace Engineering *Signatures are on file in the Graduate School iii ABSTRACT Traditional computational fluid dynamics solvers either model the flow as laminar or with assuming the presence of turbulence. If the flow is modeled with turbulence the initial influence of turbulence is minimal, and the flow can be considered laminar-like, but as the flow develops the amount of turbulence grows until it acts as a fully turbulent boundary layer. Neither approach properly models flow dynamics for the flight regime of a sailplane. To demonstrate the potential of using computational fluid dynamics for sailplane design a racing sailplane is analyzed with computational fluid dynamics using a recently developed transition model to accurately model viscous effects. The results of the analysis are validated against a conventional sailplane analysis program and are found to agree well. Regions with complex flows, such as the wing-fuselage juncture and the empennage juncture, are examined to highlight the potential for utilizing computational fluid dynamics to refine junctures in ways not possible with conventional design methods. Practical uses for computational fluid dynamics in sailplane analysis, such as investigating the stall characteristics and evaluating the tailwheel and pushrod fairing drags, are also discussed along with notable gains in aircraft performance. Two computational fluid dynamics transition models are compared and found to predict similar lift and drag characteristics but determine conflicting transition locations at high-speed, with the recently developed model more closely matching the predictions of the conventional analysis program. iv TABLE OF CONTENTS List of Figures .......................................................................................................................... v Nomenclature ........................................................................................................................... vi Acknowledgements .................................................................................................................. vii Chapter 1 Introduction ............................................................................................................. 1 Motivation ........................................................................................................................ 1 Fundamentals of Sailplane Performance .......................................................................... 2 The Flight Regime of a Sailplane..................................................................................... 3 Conventional Analysis Methods ...................................................................................... 4 Previous Studies ............................................................................................................... 6 The Schempp-Hirth Ventus 3........................................................................................... 9 Research Objectives ......................................................................................................... 9 Contributions of this Work .............................................................................................. 10 Chapter 2 Modeling Tools ....................................................................................................... 11 Conventional Tools for Problem Setup ............................................................................ 11 Boundary-Layer Transition in CFD ................................................................................. 14 Amplification Factor Transport Model ............................................................................ 15 CFD Solver ...................................................................................................................... 17 Validation of Tools .......................................................................................................... 17 Chapter 3 Mesh Generation ..................................................................................................... 23 Trimmed Cell Mesher Model ........................................................................................... 23 Adjustments for Transition Modeling .............................................................................. 24 Volume Mesh Generation ................................................................................................ 26 Chapter 4 Discussion of Results .............................................................................................. 28 Amplification Factor Flow Visualizations ....................................................................... 30 Skin Friction Coefficient Flow Visualizations ................................................................. 32 Stall Characteristics .......................................................................................................... 40 Component Drag .............................................................................................................. 42 Transition Model Comparison ......................................................................................... 43 Chapter 5 Conclusion ............................................................................................................... 49 Future Work ..................................................................................................................... 49 References ................................................................................................................................ 51 v LIST OF FIGURES Fig. 1 Typical sailplane drag breakdown (Thomas [2]) ........................................................... 5 Fig. 2 Standard Cirrus speed polar comparison (Hansen [3]) .................................................. 7 Fig. 3 Trimmed hexahedral mesh (Hansen [3]) ....................................................................... 8 Fig. 4 Schempp-Hirth Ventus-3 [8] ......................................................................................... 9 Fig. 5 Ventus 3 drag polars for all flap settings ....................................................................... 13 Fig. 6 PSU 94-097 mesh for Re=1,000,000 ............................................................................. 19 Fig. 7 PSU 94-097 lift curves .................................................................................................. 20 Fig. 8 PSU 94-097 drag polar for high-speed flight ................................................................ 21 Fig. 9 PSU 94-097 drag polar for low-speed flight .................................................................. 21 Fig. 10: Trimmed cell mesher process ..................................................................................... 24 Fig. 11: Fuselage and wing surface meshes ............................................................................. 25 Fig. 12: Domain volume mesh ................................................................................................. 26 Fig. 13: Vortical structure volumetric refinement ................................................................... 27 Fig. 14 Ventus 3 drag polar comparison .................................................................................. 28 Fig. 15 Ventus 3 speed polar comparison ................................................................................ 29 Fig. 16 Amplification factor predictions on top (right) and bottom (left) of the Ventus 3 at 퐶퐿 = 0.62 ........................................................................................................................ 31 Fig. 17 Amplification factor predictions on left (top) and right (bottom) of the Ventus 3 at 퐶퐿 = 0.62 ........................................................................................................................ 32 Fig. 18 Skin-friction coefficient predictions for the top (right) and bottom (left) of the Ventus 3 at 퐶퐿 = 0.62 using the AFT model .................................................................. 33 Fig. 19 Skin-friction coefficient predictions for the left (top) and right (bottom) of the Ventus 3 at 퐶퐿 = 0.62 using the AFT model .................................................................. 35 Fig. 20 Skin-friction coefficient predictions for the top (right) and bottom (left) of the Ventus 3 at 퐶퐿 = 0.23 using the AFT model .................................................................. 36 Fig. 21 Skin-friction coefficient predictions for the left (top) and right (bottom) of the Ventus 3 at 퐶퐿 = 0.23 using the AFT model .................................................................. 37 vi Fig. 22 Skin-friction coefficient predictions for the left (top) and right (bottom) of the Ventus 3 at 퐶퐿 = 1.5 using the AFT model .................................................................... 38 Fig. 23 Skin-friction coefficient predictions
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