Turbulent Rayleigh-Benard Convection A Progress Report Work done in collaboration with Eric Brown,...
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Transcript of Turbulent Rayleigh-Benard Convection A Progress Report Work done in collaboration with Eric Brown,...
Turbulent Rayleigh-Benard Convection A Progress Report
Work done in collaboration withEric Brown, Denis FunfschillingAnd Alexey Nikolaenko,supported by the US Department of Energy
Guenter AhlersDepartment of Physics and iQUEST UCSB
QuickTime™ and aYUV420 codec decompressor
are needed to see this picture.
Rayleigh-Benard Convection Cell
D
LT
= D/L = 1.0
( H2O, 40 oC )Prandtl No. = 4.4
Three samples:
Small: D = L = 9 cm
Medium: D = L = 25 cm
Large: D = L = 50 cmT = 20
oC : R = 1011
Q = 1500 W
Fluid: H2OWall: Plexigl.
R = g L3 T / Q
Q
Important process in the
Atmosphere: Weather
Mantle: Continental Drift
Outer core: Magnetic field
Sun: Surface temperature
etc.: ???
Interesting Physics
Why is it interesting?
Models
A central prediction: Heat transport.
Define Nusselt number N = eff / effQ / ( T / L )
Various models were proposed:
Malkus (1954), Priestly (1959): N ~ R1/3
Kraichnan (1962): N ~ R1/2
Castaing et al. (1989) Shraiman + Siggia (1990): N ~ R2/7
Grossmann and Lohse (2000): No single power law; Crossover between two power laws
Assume that the heat-current density Q is determined by the BLs and does not depend on the distance between them. Then
Define N = eff / effQ / ( T / L ) Q = N T / L = heat-current density
Assume that there are power laws and that theR- and -dependence of N separates:
N = f() RR = g T ) L3
Q = f() RT / LQ = f() g T ) T L(3
3 - 1 = 0; = 1/3; N ~ R1/3
C.H.B. Priestley[Quarterly Journal of the Royal MeteorologicalSociety 85, 415 (1959)]
R. Krishnamurty and L.N. Howard, Proc. Nat. Acad. Sci. 78, 1981 (1981):
Large Scale Circulation (“Wind of Turbulence”)
QuickTime™ and aYUV420 codec decompressor
are needed to see this picture.
connects the BLs and invalidates the simple models.
Movie from the group of K.-Q Xia, Chinese Univ., Hong Kong
R = 6.8x108
= 596
= 1cylindricalslightly tiltedin real time
See, e.g., X.-L. Qiu and P. Tong, Phys. Rev. E 66, 026308 (2002)
hot plumes
cold plumes
“wind” orlarge scalecirculation
T1
T2 > T1
thermal boundary layer
schematic drawingof the flow structure
thermal boundary layer
( T1 + T2 ) / 2
Q
Q
Assume laminar BLs with conductivity and T/2 across each:
Q = N T / L = T/2) / ll = BL “thickness”l = L / 2N
Assume laminar BLs are marginally stable:R = Rc = g l 3 Tc / O(103); Tc = T/2l ~ (T/2)-~ R; = 1/3; N = L/2 l; N ~ R1/3
S.-L. Lui and K.-Q. Xia, Phys. Rev. E 57, 5494 (1998).
z / l
However, assuming no interaction between BLs is not needed to get 1/3 !W.V.R. Malkus, Proc. Roy. Soc. (London) A 225, 196 (1954)
Experiment: not = 1/3 and depends on horizontal location !
x
Winddirection
1975: D.C. Threlfall, J. Fluid Mech. 67, 17 (1975).
1987: F. Heslot, B. Castaing, + A. Libchaber (Chicago), Phys. Rev. A 36, 5870 (1987).
N ~ R0.282
Mixing layer model (bulk, BL, and plume region between them)of the Chicago group[Castaing, Gunaratne, Heslot, Kadanoff, Libchaber, Thome, Wu, Zaleski, & Zanetti,J. Fluid Mech. 204, 1 (1989)]
and of B.I. Shraiman and E. Siggia, Phys. Rev. A 42, 3650 (1990):
N ~ -1/7 R2/7; 2/7 = 0.2857… .
S. Grossmann and D. Lohse, J. Fluid Mech. 407, 27 (2000) (GL) start with the kinetic and thermal dissipation rates
Their volume averages follow from the Boussinesq equations and are given by
GL set each equal to a sum of a BL and a bulk contribution:
They assume that the separate contributions can be modeled using approximations to the length, temperature, and time scales, e.g.
(assumes laminar BLs, uniform in thex-y plane, with conductivity )
(based on Blasius BL model)
etc.
log( R )
log(
)
1. No simple power laws, but rather cross-overs from a small-R to a large-Rasymptotic region.
2. Various regions in the R -plane, depending on which dissipativeterm dominates, etc. For > 1 and large R, IVu pertains. There u and are both bulk dominated.
At large R
else
yield and
D. Funfschilling, E. Brown, A. Nikolaenko, and G. A., J. Fluid Mech., submitted.
X. Xu, K.M.S. Bajaj, and G. A., Phys. Rev. Lett. 84, 4357 (2000);G. A. + X. Xu, Phys. Rev. Lett. 86, 3320 (2001)
1.) No power law
2.) 4 parameters of theGL model were determined from a fit to these data
A.) The important components have been identified:1.) top and bottom boundary layers2.) “plumes”3.) large-scale circulation
B.) The nature of the interactions between boundary layers, plumes, and large scale circulation is not so clear.
C.) The GL model can be fitted to existing Nusselt data by adjusting its four undetermined coefficients
D.) Adjustment of a fifth parameter gives reasonably good agreement with the measured Reynolds numbers of the LSC.
New Nusselt-Number Measurements
X. Xu, K.M.S. Bajaj, and G. A., Phys. Rev. Lett. 84, 4357 (2000);G. A. + X. Xu, Phys. Rev. Lett. 86, 3320 (2001)
K.-Q. Xia, S. Lam, and S.-Q. Zhou, Phys. Rev. Lett. 88, 064501 (2002).
S. Grossmann and D. Lohse, Phys. Rev. Lett. 86, 3316 (2001).
N /
R1/
4
Prandtl Number
R = 1.8x109
R = 1.8x107-1/7
Adiabatic bottom-plate shield
Joule heated Cu bottom plate
Plexiglas side wall
adiabatic side shield
Water cooled Cu top plate
Foam inside of here
Leveling and support plate
Catch basis
E. Brown, A. Nikolaenko, D. Dunfschiling, and G.A., Phys. Fluids, submitted.
D. Funfschilling, E. Brown, A. Nikolaenko, and G. A., J. Fluid Mech., submitted.
2 %
D. Funfschilling, E. Brown, A. Nikolaenko, and G. A., J. Fluid Mech., submitted.
The GL model can not reproduce the effective exponent= 0.333 ~ 1/3 derived from the Nusselt number datafor R > 1010.
Reynolds-Number Measurements
0.63 cm
15 6 071
2
3
4
5
6
7
0
C00
-C04
Medium SampleLarge Sample
X.-L. Qiu and P. Tong,Phys. Rev. E 66, 026308 (2002).
unpublished
Re = (L / t1)(L / )
A.) The important components have been identified:1.) top and bottom boundary layers2.) “plumes”3.) large-scale circulation
B.) The nature of the interactions between boundary layers, plumes, and large scale circulation is not so clear.
C.) Models yielding relationships between, the Nusselt number, Rayleigh number, Prandtl number, and Reynolds number (of the LSC) are at best good approximations, but for large R miss important physics.
LSC Reversals
E. Brown, A. Nikolaenko, and G. A., Phys. Rev. Lett., submitted.
Ti = <T> + cos( i/4 + )
Rotation
Cessation
probability distribution of for reorientations with /<> < 0.25
A.) LSC “reversal” can occur via 1.) rotation of the vorticity vector (“rotation) 2.) shrinking of the vorticity vector, followed by re-development with a new orientation (“cessation”)
B.) Cessation is followed by re-development of the LSC in a circulation plane with an arbitrary new orientation, i.e. P() ~ constant.
C.) Rotation through an angle has a powerlaw probability distribution P() ~ with ~ 4.
D.) Reversals are Poisson distributed.
More LSC Dynamics
D. Funfschilling and G. A., Phys. Rev. Lett. 92, 194502 (2004).
Hot plumes/rollsnear the bottom plateappear dark
Cold plumes/rolls Near the top plateAppear bright
Shadowgraph
mirror
pin holeand LED ligh source
beam-splitter
RayleighBénard cell
2o tilt
lens
lens
34
37
t = 0.0s
Correlation Functions:
0.3s
0.6s 0.9s
1.2s 1.5s
t = 0.0s
t = 0.9s
t
YXVelocity
22
+
=
⎟⎠
⎞⎜⎝
⎛
=ΘY
Xarctan
The maximum of the correlation Function is located at X, Y Relative to its origin (center).
Speed
Direction of 2 deg. tilt
Highest point
Lowest pointdirection of plumemovement andpresumed direction ofcirculation of LSF
Θ
Viewed from Above:
The angle of the plane of the large-scale-flow circulation, and its time correlation function
R = 7.0 x 108
Conclusion
Near the center of the top and bottom plate the large-scale flow direction oscillates about the vertical axis of the cell.
This oscillation has the same frequency as the periodic signals seen by others in measurements at individual points.
The frequency yields a Reynolds number consistent with measurements by other methods and the GL model.
Assumption:Near the center of the top and bottom plate plumes/rolls follow the large-scale flow