Transitional boundary layers in low-Prandtl-number convection

Abstract

The boundary layer structure of the velocity and temperature fields in turbulent Rayleigh-Benard flows in closed cylindrical cells of unit aspect ratio is revisited from a transitional and turbulent viscous boundary layer perspective. When the Rayleigh number is large enough, the dynamics at the bottom and top plates can be separated into an impact region of downwelling plumes, an ejection region of upwelling plumes and an interior region away from the side walls. The latter is dominated by the shear of the large-scale circulation (LSC) roll which fills the whole cell and continuously varies its orientation. The working fluid is liquid mercury or gallium at a Prandtl number Pr=0.021 for Rayleigh numbers between Ra=3e+5 and 4e+8. The generated turbulent momentum transfer corresponds to macroscopic flow Reynolds numbers with values between 1800 and 46000. It is shown that the viscous boundary layers for the largest Rayleigh numbers are highly transitional and obey properties that are directly comparable to transitional channel flows at friction Reynolds numbers Re_tau slightly below 100. The transitional character of the viscous boundary layer is also underlined by the strong enhancement of the fluctuations of the wall stress components with increasing Rayleigh number. An extrapolation of our analysis data suggests that the friction Reynolds number Re_tau in the velocity boundary layer can reach values of 200 for Ra beyond 1e+11. Thus the viscous boundary layer in a liquid metal flow would become turbulent at a much lower Rayleigh number than for turbulent convection in gases and gas mixtures.

16 pages, 15 figures in reduced quality, accepted for Phys. Rev. Fluids

References in corpus (5)

• Rotations and cessations of the large-scale circulation in turbulent Rayleigh-Benard convection
• Boundary layer structure in turbulent Rayleigh-Benard convection
• Global and local statistics in turbulent convection at low Prandtl numbers
• Local boundary layer scales in turbulent Rayleigh-Benard convection
• Logarithmic temperature profiles of turbulent Rayleigh-Bénard convection in the classical and ultimate state for a Prandtl number of 0.8

Cited by in corpus (7)

• Thermal boundary layer structure in low-Prandtl-number turbulent convection
• Mean Temperature Profiles in Turbulent Thermal Convection
• Non-Boussinesq low-Prandtl number convection with a temperature-dependent thermal diffusivity
• Geophysical flows over topography, a playground for laboratory experiments
• Flow organization in laterally unconfined Rayleigh-Bénard turbulence
• Predicting transition ranges to turbulent viscous boundary layers in low Prandtl number convection flows
• The transition to the ultimate regime of thermal convection from a stochastic one-dimensional turbulence perspective