Andreas Krumbein
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Andreas Krumbein German Aerospace Center Institute of Aerodynamics and Flow Technology, Numerical Methods Normann Krimmelbein Technical University of Braunschweig Institute of Fluid Mechanics, Aerodynamics of Aircraft.

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Outline

Andreas KrumbeinGerman Aerospace CenterInstitute of Aerodynamics and Flow Technology, Numerical MethodsNormann KrimmelbeinTechnical University of BraunschweigInstitute of Fluid Mechanics, Aerodynamics of Aircraft

Navier-Stokes High-Lift Airfoil Computations with Automatic Transition Prediction using the DLR TAU Code


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  • Introduction

  • Transition Prediction Coupling Structure

  • Test Case: 2D A310 take-off configuration

  • Computational Results

  • Conclusion

  • Outlook


Outline

Introduction

Introduction

  • Aircraft industry and research requirements:

    • RANS based CFD tool with transition prediction

    • Automatic, no intervention of the user

    • Reduction of modeling based uncertainties

      • Accuracy of results from fully turbulent flow or flow with prescribed transition often not satisfactory

    • Improved simulation of the interaction between transition locations and separation

  • Development of TAU transition prediction module by Institute of Fluid Mechanics, Technical University of Braunschweig in German research initiative MEGADESIGN


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Introduction

  • Different approaches:

    • RANS solver+ stability code + eN method

    • RANS solver+ boundary layer code + stability code + eN method

    • RANS solver+ boundary layer code+ eN database method(s)

    • RANS solver+ transition closure model or transition/turbulence model


Outline

Introduction

  • Different approaches:

    • RANS solver+ stability code + eN method

    • RANS solver+ boundary layer code + stability code + eN method

    • RANS solver+ boundary layer code+ eN database method(s)

    • RANS solver+ transition closure model or transition/turbulence model


Outline

Introduction

  • Different approaches:

    • RANS solver+ stability code + eN method

    • RANS solver+ boundary layer code + stability code + eN method

    • RANS solver+ boundary layer code + fully automated stability code+ eN method

    • RANS solver+ boundary layer code+ eN database method(s)

    • RANS solver+ transition closure model or transition/turbulence model


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Coupling Structure

cycle = kcyc

cycle = kcyc

Transition Prediction Coupling Structure


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Coupling Structure

  • Transition Prediction Module:

    • RANS infrastructure part: BL data from RANS grid (BL mode 2) transition inside separation bubble possible high mesh density necessary

    • External codes:

      • Laminar boundary-layer method COCO (G. Schrauf) for swept, tapered wings (BL mode 1)

         transition inside separation bubble NOT possible

         Laminar separation approximates transition if transition downstream of laminar separation point

      • eN database-methods for TS and CF instabilities (PD mode 1)

      • local, linear stability code LILO (G. Schrauf)(PD mode 2)

    • 2d, 2.5d (infinite swept) + 3d wings + 3d fuselages/nacelles (only BL mode 2)

    • Single + multi-element configurations

    • N factor integration along:

      • Line-in-Flight cuts

      • Inviscid streamlines

    • Attachment line transition & by-pass transition not yet covered


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Coupling Structure

  • Hybrid RANS solver TAU:

    • 3D RANS, compressible, steady/unsteady

    • Unstructured/hybrid grids: hexahedra, tetrahedra, pyramids, prisms

    • Finite volume formulation

    • Vertex-centered spatial scheme (edge-based dual-cell approach)

    • 2nd order central schemes, scalar or matrix artifical dissipation

    • Time integration:explicit Runge-Kutta with multi-grid acceleration orimplicit approximate factorization scheme (LU-SGS)

    • Turbulence models and approaches:

      • Linear and non-linear 1- and 2-equation eddy viscosity models (SA type, k-w type)

      • RSM  RST, EARSMs (full & linearized)

      • DES


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Coupling Structure

  • Transition Prescription:

    • Automatic partitioning intolaminar and turbulent zonesindividually for each element

    • Laminar points: St,p  0

PTupp(sec = 2)

PTupp(sec = 1)

PTupp(sec = 3)

PTupp(sec = 4)


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Coupling Structure

no

yes

STOP

  • Algorithm:

set stru and strl far downstream

compute flowfield

check for RANS laminar separation  set separation points as new stru,l

clconst. in cyclescall transition module

use outcome of prediction method (PD modes 1&2)

or

BL laminar separation point (BL mode 1)

set new stru,l underrelaxed  stru,l = stru,ld, 1.0 < d < 1.5

convergence checkDstru,l < e


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Test Case

  • 2d A310 take-off configuration

  • M = 0.221, Re = 6.11 x 106, a = 21.4°

  • grid 1: 22,000 pointsgrid 2: 122,000 points, noses refined

  • SAE turbulence model

  • prediction on upper sides, lower sides fully laminar, NTS 9 (F1 WT)

  • exp. Transition locations  slat: 15% & flap: 34.5%kink on main upper side  19%

  • different mode combinations:a) BL mode 1 & PD mode 1  BL code & TS database methodb) BL mode 1 & PD mode 2  BL code & stability codec) BL mode 2 & PD mode 2  BL in TAU & stability code

Test Case


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Results

Surface pressure

grid 1 grid 2

a.) & b.) results identical  all lam. seps. a.) & b.) results identical  all lam. seps.

c.) no convergence  grid too coarse c.) all from stability code


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Results

Skin friction

grid 1 grid 2

a.) & b.) no separation bubbles a.) & b.) very small sep. bubble on slat

c.) no convergence c.) much larger slat bubble & flap improved


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Results

Skin friction

grid 2

slat

very small bubble

transition locations:

error reduced by 40%

flap

large bubble


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Results

Transition locations and separation

grid 2 grid 2


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Conclusion/Outlook

  • TAU transition prediction module works fast and reliable for 2d multi-element configurations

  • Transition inside laminar separation bubbles can be detected with high accuracy when appropriate predcition approach is used

  • Therefor, high grid densities are required

  • much more testing necessary:

    • more test cases needed with TS transition (e.g. CAST 10, A310 landing)

    • full aircraft WB+HTP+VTP (wing with full-span flap without slit)

    • WB high-lift configuration with full-span slat and flap from EUROLIFT II

  • transition criteria:- transition in lam. sep. bubbles

    - attachment line transition

    - by-pass transition

  • development of a stream-line oriented bl code with transverse pressure gradientCOCO-3d → replaces COCO in 2007

  • unsteady transition prediction method based on eN method

  • alternative approaches based on transport equations in future DLR T&T-project RETTINA

done by TU-BS


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