Mixed convection in turbulent channels with unstable stratification

Abstract

We study turbulent flows in planar channels with unstable thermal stratification, using direct numerical simulations in a wide range of Reynolds and Rayleigh numbers and reaching flow conditions which are representative of asymptotic developed turbulence. The combined effect of forced and free convection produces a peculiar pattern of quasi--streamwise rollers occupying the full channel thickness with aspect--ratio considerably higher than unity; it has been observed that they have an important redistributing effect on temperature and momentum. The mean values and the variances of the flow variables do not appear to follow Prandtl's scaling in the flow regime near free convection, except for the temperature and vertical velocity fluctuations, which are more affected by turbulent plumes. Nevertheless, we find that the Monin--Obukhov theory still yields a useful representation of the main flow features. In particular, the widely used Businger--Dyer relationships provide a convenient way of accounting for the bulk effects of shear and buoyancy, although individual profiles may vary widely from the alleged trends. Significant deviations are found in DNS with respect to the commonly used parametrization of the mean velocity in the light-wind regime, which may have important practical impact in models of atmospheric dynamics. Finally, for modelling purposes, we devise a set of empirical predictive formulas for the heat flux and friction coefficients which can be used with about $10\%$ maximum error in a wide range of flow parameters.

References in corpus (2)

• Direct numerical simulation of turbulent channel flow up to $R{e}_{\tau }\approx 5200$
• Law of the wall in an unstably stratified turbulent channel flow

Cited by in corpus (16)

• Flow organization and heat transfer in turbulent wall sheared thermal convection
• Topography generation by melting and freezing in a turbulent shear flow
• The effect of Prandtl number on turbulent sheared thermal convection
• Enhancing Heat Transport in Multiphase Rayleigh-Bénard Turbulence by Changing the Plate-Liquid Contact Angles
• On the logarithmic profile of temperature in the atmospheric convective boundary layers
• Streamwise-constant large-scale structures in Couette and Poiseuille flows
• Data-driven algebraic models of the turbulent Prandtl number for buoyancy-affected flow near a vertical surface
• Particle transport and deposition in wall-sheared thermal turbulence
• Temporal modulation on mixed convection in turbulent channels
• Turbulent mixed convection in vertical and horizontal channels
• Much faster heat/mass than momentum transport in rotating Couette flows
• The effect of spanwise heterogeneous surfaces on mixed convection in turbulent channels
• The stability of stratified horizontal flows of carbon dioxide at supercritical pressures
• Plane-layer Rayleigh-Bénard convection up to $Ra=10^{11}$: Near-wall fluctuations and role of initial conditions
• Buoyancy driven dissolution of inclined blocks: Erosion rate and pattern formation
• Multi-scale interactions in turbulent mixed convection drive efficient transport of Lagrangian particles