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Automatic Transition Prediction and Application to 3D Wing Configurations Current status of development and validation. Outline. Outline. Introduction Transition Prescription Transition Prediction Modeling of Transitional Flow Transition Prediction Strategy

guy-atkinson
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  1. Automatic Transition Prediction and Application to 3D Wing ConfigurationsCurrent status of development and validation

  2. Outline Outline • Introduction • Transition Prescription • Transition Prediction • Modeling of Transitional Flow • Transition Prediction Strategy • Preliminary Results: ONERA M6 wing • Outlook

  3. Introduction Introduction • aerospace industry requirement: RANS based CFD tool with transition handling→ • 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 • RANS solver + transition closure model or transition/turbulence model - prescription - prediction - transitional flow modeling - automatic, autonomous

  4. Introduction Introduction • aerospace industry requirement: RANS based CFD tool with transition handling→ • 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 • RANS solver + transition closure model or transition/turbulence model - prescription - prediction - transitional flow modeling - automatic, autonomous

  5. Introduction transition prediction module • Structured approach: FLOWer + laminar BL method for swept, tapered wings + + eN database methods for TS and CF instabilities • FLOWer • 3D RANS, compressible, steady/unsteady • structured body-fitted multi-block meshes • finite volume method, cell-vertex scheme • explicit Runge-Kutta time integration • multi-grid acceleration • mainly eddy viscosity models, Boussinesq

  6. Prescription Transition Prescription - automatic partitioning of flow field into laminar and turbulent regions - individual laminar zone for each element - different numerical treatment of laminar and turbulent grid points, e.g. mt = 0 in laminar zones

  7. Prescription • transition line on ONERA M6 wing, 4 points on upper and lower side PTupp(sec = 2) PTupp(sec = 1) PTupp(sec = 3) PTupp(sec = 4)

  8. Prediction Transition Prediction - RANS solver  shall predict transition points automatically! - stability database  shall yield accurate values of transition points! - eN database method  needs highly accurate BL data!  BL adaptation in NS grid  very time consuming, coupling with grid generator: NO!  laminar BL method  fast, cheap, easy to couple: YES! - restrictions:  linear stability theory  parallel flow assumption - independent of mesh topology, grid structure, 2D or 3D -integration paths: grid lines of the structured grid

  9. Modeling Modeling of transitional flow • - algebraic models for the transition lengthltr •  Reltr = 5.2 (Restr)3/4 downstream of RANS laminar separation point •  Reltr = 2.3 (Red*(str))3/2 downstream of BL laminar separation point •  Reltr = 4.6 (Red*(str))3/2downstream of TS instability • - intermittency function  g(s) = 1 – exp (-0.412 [3.36 (s-str)/ltr]2) • s: arc length starting at the stagnation point displacement thickness

  10. Strategy Transition prediction strategy - coupling structure

  11. Strategy 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 eN-databases or BL laminar separation point as new transition point set new stru,l underrelaxed  stru,l = stru,ld, 1.0 < d < 1.5 convergence check Dstru,l < e

  12. Results Preliminary Results - ONERA M6 wing: single-element semi-span: A = 3.8 swept: LLE = 30° LTE = 15.8° tapered: l = 0.562 - based on ONERA D airfoil (symmetric), perpendicular to 40%-line - “designed for studies of three-dimensional flows from low to transonic speeds at high Reynolds numbers“

  13. Results • feasibility: • 1 block-grid, 384,000 points • M = 0.84, Re = 2.0106,a = - 4.0° • turbulence model: Baldwin-Lomax • critical N-factors: NcrTS = 4.0, NcrCF = 2.0, arbitrariliy set

  14. Results • Validation, 1st test: • 1 block-grid, 800,000 points • M = 0.84, Re = 11.72106,a = 3.06° → classic CFD validation test case • Tu = 0.2% → N = 6.485 using Mack’s relationship WT: S2MA, Modane Center • turbulence model: Baldwin-Lomax, Spalart-Allmaras with Edwards mod. (SAE), Wilcox k-w • critical N-factors: NcrTS = NcrCF = 6.485 • transition prediction in 3 wing sections near h = z/b = 0.1, 0.5, 0.9

  15. Results • surface pressure and transition lines • influence of TMs extremely low • all transition points due to CF instabilities, except: • BL, h = 0.1, lower side • → lam. sep.

  16. Results • cp-distributions at h = 0.2, 0.44, 0.65, 0,9 • almost no difference to fully turbulent re-sults • accuracy of results comparable to those of others (e.g. lite-rature, TAU code)

  17. Results • Validation, 2st test: • 1 block-grid, 800,000 points • M = 0.262, Re = 3.5106,a = 0°, 5°, 10°, 15° • Tu = 0.2% → N = 6.485 using Mack’s relationship WT: S2Ch, Chalais-Meudon • transition detection in experiment: sublimation of acenaphtene • turbulence model: SAE • critical N-factors: NcrTS = NcrCF = 6.485 • transition prediction in 4 wing sections near h = 0.1, 0.44, 0.5, 0.9 upper side lower side

  18. Results • transition locations from experiment at h = 0.44 h = 0.44 h = 0.44 TS TS lower side exp. lower side upper side upper side ls ls TS ls

  19. Results • transition lines for a = 5° and exp. transition locations at h = 0.44 • Has acenaphtene triggered transition on the lower side? • Is NcrCF correct? TS outcome of the database methods h = 0.44 on lower side CF

  20. Results TS CF xTexp. • max. N-factor curves for a = 5° at h = 0.44 on lower side from a linear stability code (from H.W. Stock using COAST (?) code): NcrCF  3.2 In other cases, e.g. ONERA D infinite swept, NcrCF  6.0 was found.

  21. Results CF DL= 4°, 5°, 6° TS DL= 0° • What is wrong? • 1. Error in coding of the 3d coupling procedure? → compute infinite swept wing flow for ONERA D airfoil using sweep angle at xTlow(h = 0.44)  fails due to problems with BL code: BL code does not converge  another problem to be solved! • 2. Is sweep angle correct? → account for effective sweep angle Leff = L + DL = arcsin (UT/U)*) due to influence of changing absolute wing thickness ratio UT: velocity in the attachment line tested: 1°  DL 6°  cp around stagnation point must be reduced to prevent BL code crash  are database results affected?  another problem to be solved! *) G. Redecker, G. Wichmann, ‘Forward Sweep – A Favorable Concept for a Laminar Flow Wing‘, Journal of Aircraft, Vol. 28, No. 2, 1991, p. 97-103

  22. Results • What is wrong? • 3. Is CF database method erroneous? → ONERA D infinite swept successfully analyzed by ISM (TU-BS) with same program for M = 0.23, Re = 2.4106,an = 4°,L = 60° using BL data from TAU code  Results from CF database method are almost the same as those from linear stability code COAST.  Is the functioning of the CF database method case dependent? • 4. Are the grid lines of the structured grid a too bad approximation of the streamline? • 5. Is the selected test case a reliable validation test case?

  23. Outlook Outlook • clarification/solution of the problems: • convergence problems of BL code • automatic determination and consideration of Leff in the iteration loop • automatic reduction and adaption of cp around stagnation point • guarantee that CF databse results are do not depend on manipulation of cp • reproduction of the results of the ONERA D infinite swept case • coupling with linear stability code LILO (G. Schrauf) • empirical criteria for: - attachment line transition - bypass transition - transition in laminar separation bubbles

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