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Turbulent boundary layer structure
Shear velocity, u*u* = √τo/ρ
0 hR Sτ γ=
= cross-sectional area/wetted perimeter
Shear velocity, u*u* = √τo/ρ
Turbulent boundary layer structure over a FLAT bed
Classic research by the groups of Kline (Stanford) and Grass (UCL)
Tony Grass (UCL)JFM 1971
Used H2 bubbles over different bed roughness
Burst and sweeps
Grass, 1971
+U’-U’
+v’
-v’
If U’ and v’ are deviation of downstream and vertical velocity from their mean (+ve v = upwards)
12
3 4
2 = bursts4 = sweeps1 = outward interactions3 = inward interactions
Define a ‘hole’ size to exclude small
events Quadrant Analysis
Tb=fU/Y~5
Jackson, 1975, 1976
Burst period
Smith and Metzler, 1983
Planform characteristics…Smith and Metzler, 1983
Smith and Metzler, 1983
H2 bubblewire
low-speedstreaks
flow
time
Looking down onto the channel bed
Low speed streak spacing, λ+:
λ+ = λ.u*/ν
where λ = streak spacingu* = shear velocity
ν = kinematic viscosity
λ+ = λ.u*/ν ≅ 100
Smith and Metzler, 1983
Smith and Metzler, 1983
Smith and Metzler, 1983
Smith and Metzler, 1983
The burst-sweep cycle (from Allen, 1984) - hairpins
The earlier work of Kline and
colleagues
Smith et al., 1991
Generation of secondary hairpin vortices (Smith et al., 1991)
Turbulent Boundary Layer Structure (Robinson, 1991)
Robinson, 1991
Turbulent Boundary Layer Structure (Robinson, 1991)
Wall-attached hairpin
Adrian et al., 2000
Adrian, 2007
More complex packets of vortices
Adrian, 2007
A model of vortex packets
Spanwise merger of hairpins by vortex reconnection of legs
Adrian, 2007
Adrian, 2007
Movie 1
Movie 2
transitions to turbulence
Links to Large-Scale-Motions (Falco, 1977)
streaks
entrainment patch
movie
Adrian, 2007
Long streaks of low-momentum fluid in streamwise-spanwise planes of boundary layer flow at y/δ = 0.2 and Reθ = 2216. The red (blue) contours represent regions of swirling strength.
Adrian, 2007
Summary sketch of the organization of hairpins and packets in a boundary layer.
Adrian, 2007
References***Adrian, R.L. (2007) Hairpin vortex organization in wall turbulence, Physics of Fluids, 19, 041301.****Marusic, I. (2009) Unravelling turbulence near walls, Journal of Fluid Mechanics, 630, 1-4.*
Grass, A.J. (1971) Structural features of turbulent flow over smooth & rough boundaries, J. Fluid Mechanics, 50, 233-255.Kline, S. J., Reynolds, W. C., Schraub, F. A. & Runstadler, P. W. (1967) The structure of turbulent boundary layers. Journal of Fluid Mechanics, 30, 741-773. Robinson, S. K. (1991) Coherent motion in the turbulent boundary layer. Ann. Rev. Fluid Mech. 3, 601-639.Smith, C.R. and Metzler, S.P. (1983) The characteristics of low-speed streaks in the near-wall region of a turbulent boundary layer, Journal of Fluid Mechanics, 129, 27-54.Smith, C.R. (1996), Coherent flow structures in smooth-wall turbulent boundary layers: Facts, mechanisms and speculation. in Coherent Flow Structures in open channels edited by P.J. Ashworth, S.J. Bennett, J.L. Best, and S.J. McLelland, pp. 1-39, John Wiley and Sons.
Next weeks seminars
Alex: Head, M.R., and P. Bandyopadhyay (1981), New aspects of turbulent boundary layer structure, Journal of Fluid Mechanics, 107, 297-338.Justin: Adrian, R. J., C. D. Meinhart and C. D. Tomkins (2000), Vortex organization in the outer region of the turbulent boundary layer, Journal of Fluid Mechanics, 422, 1-54. Ron: 2 papers in Science!Marusic et al., (2010) Predictive model for wall-bounded turbulent flow, Science, 329, 9 July 2010.Adrian, R.J. (2010) Closing in on models of wall turbulence, Science, 329, 9 July 2010.
NOTE: These can be large papers!: start with Intro, Conclusions and Discussion: reviewers can then focus in on
papers