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Understanding Turbulent/Non-Turbulent Interfaces in Jet Flows

Investigating the sharpness of turbulent/non-turbulent interfaces in jet flows and their impact on turbulence modeling. Study explores engulfment vs. nibbling, interface representation, and interface-related phenomena. Experimental data supports discontinuity in axial velocity and shear layer dynamics. Implications for turbulence models and eddy viscosity highlighted.

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Understanding Turbulent/Non-Turbulent Interfaces in Jet Flows

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  1. Singular structure at the turbulent/non-turbulent interface of a jet J. Westerweel, J.C.R. Hunt, A. Petracci, R. Delfos Delft University of Technology, The Netherlands J.M. Pedersen Technical University of Denmark C. Fukushima Hiroshima Institute of Technology, Japan PRL 95 (2005) 174501

  2. Turbulent/non-turbulent interfaces • free turbulent flows: bounded by regions of irrotational flow • how does the interface remain sharp? • how does irrotational fluid become turbulent? • ‘engulfment’ vs. ‘nibbling’ • how related to outward propagation of interface? • how to represent the interface in turbulence model?

  3. Turbulence modeling problem – u'v'=T (U/y) (1) T = const (2) T ~ e2/ T 0 no growth U T • add small T to allow growth, or • growth due to numerical diffusion T  0, butno turbulence

  4. Combined PIV/LIF measurement of a self-similar turbulent jet

  5. 1. binarization y – yi 2. remove disconnected objects 3. detect boundary 4. save outmost points 5. superimpose vorticity 6. determine conditional flow statistics Detection of the interface Threshold level: Prasad & Sreenivasan, EiF 7 (1989) 259

  6. Conditional sampling with respect to the interface

  7. Results: vorticity

  8. Results: conditional statistics

  9. Super-layer jump condition I V T turbulent transport See e.g.: W.C. Reynolds, JFM 54 (1972) 481

  10. Results: velocity jump condition (Kovasznay et al.:U ~ 0) Turner:

  11. Results: mass flux - engulfment Total mass within boundary envelope Engulfed mass within boundary envelope See also: Mathew & Basu, PoF 14 (2002) 2065

  12. s Results: length scales

  13. Results: enstrophy transport

  14. Eddy viscosity in the outer jet region

  15. Conclusions • Experimental data support presence of a discontinuity in axial velocity; • Boundary propagation (Eb) given by superlayer jump condition matches entrainment velocity; • Interface associated with a shear layer with ‘constant’ strength; • Length scales of conditional velocity fluctuations increase proportional to distance from interface; • Zero turbulent transport of enstrophy across interface; • Kolmogorov-scale nibbling appears to be the dominant process; • Implications for turbulence modeling: eddy viscosity (νT) retains a small constant value in outer jet region: Prandtl was right !!!

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