Turbulent boundary layer in high Rayleigh number convection in air.
Flow visualizations and particle image velocimetry measurements in the boundary layer of a Rayleigh-Bénard experiment are presented for the Rayleigh number Ra=1.4×1010. Our visualizations indicate that the appearance of the flow structures is similar to ordinary (isothermal) turbulent boundary layers. Our particle image velocimetry measurements show that vorticity with both positive and negative sign is generated and that the smallest flow structures are 1 order of magnitude smaller than the boundary layer thickness. Additional local measurements using laser Doppler velocimetry yield turbulence intensities up to I=0.4 as in turbulent atmospheric boundary layers. From our observations, we conclude that the convective boundary layer becomes turbulent locally and temporarily although its Reynolds number Re≈200 is considerably smaller than the value 420 underlying existing phenomenological theories. We think that, in turbulent Rayleigh-Bénard convection, the transition of the boundary layer towards turbulence depends on subtle details of the flow field and is therefore not universal.
Technische Universitaet Ilmenau, Institute of Thermodynamics and Fluid Mechanics, P.O. Box 100565, 98684 Ilmenau, Germany.
Ronald
R
du Puits
Technische Universitaet Ilmenau, Institute of Thermodynamics and Fluid Mechanics, P.O. Box 100565, 98684 Ilmenau, Germany.
Ling
L
Li
Technische Universitaet Ilmenau, Institute of Thermodynamics and Fluid Mechanics, P.O. Box 100565, 98684 Ilmenau, Germany.
Christian
C
Resagk
Technische Universitaet Ilmenau, Institute of Thermodynamics and Fluid Mechanics, P.O. Box 100565, 98684 Ilmenau, Germany.
André
A
Thess
German Aerospace Center, Institute of Propulsion Technology, 51170 Koeln, Germany.
Christian
C
Willert