scholarly journals Velocity and acceleration statistics in rapidly rotating Rayleigh–Bénard convection

2018 ◽  
Vol 857 ◽  
pp. 374-397 ◽  
Author(s):  
Hadi Rajaei ◽  
Kim M. J. Alards ◽  
Rudie P. J. Kunnen ◽  
Herman J. H. Clercx
Keyword(s):  
Heat Transfer ◽  
Boundary Layers ◽  
Flow Structures ◽  
Regime Transition ◽  
Bénard Convection ◽  
Benard Convection ◽  
Lagrangian Velocity ◽  
Neutrally Buoyant ◽  
Rayleigh Benard Convection ◽  
Rayleigh Benard

Background rotation causes different flow structures and heat transfer efficiencies in Rayleigh–Bénard convection. Three main regimes are known: rotation unaffected, rotation affected and rotation dominated. It has been shown that the transition between rotation-unaffected and rotation-affected regimes is driven by the boundary layers. However, the physics behind the transition between rotation-affected and rotation-dominated regimes are still unresolved. In this study, we employ the experimentally obtained Lagrangian velocity and acceleration statistics of neutrally buoyant immersed particles to study the rotation-affected and rotation-dominated regimes and the transition between them. We have found that the transition to the rotation-dominated regime coincides with three phenomena; suppressed vertical motions, strong penetration of vortical plumes deep into the bulk and reduced interaction of vortical plumes with their surroundings. The first two phenomena are used as confirmations for the available hypotheses on the transition to the rotation-dominated regime while the last phenomenon is a new argument to describe the regime transition. These findings allow us to better understand the rotation-dominated regime and the transition to this regime.

scholarly journals Axially homogeneous Rayleigh–Bénard convection in a cylindrical cell

2011 ◽  
Vol 691 ◽  
pp. 52-68 ◽  
Author(s):  
Laura E. Schmidt ◽  
Enrico Calzavarini ◽  
Detlef Lohse ◽  
Federico Toschi ◽  
Roberto Verzicco
Keyword(s):  
Heat Transfer ◽  
Aspect Ratio ◽  
Boundary Layers ◽  
Temperature Fields ◽  
Cylindrical Cell ◽  
Bénard Convection ◽  
Benard Convection ◽  
Velocity And Temperature Fields ◽  
Rayleigh Benard Convection ◽  
Rayleigh Benard

AbstractPrevious numerical studies have shown that the ‘ultimate regime of thermal convection’ can be attained in a Rayleigh–Bénard cell when the kinetic and thermal boundary layers are eliminated by replacing both lateral and horizontal walls with periodic boundary conditions (homogeneous Rayleigh–Bénard convection). Then, the heat transfer scales like $\mathit{Nu}\ensuremath{\sim} {\mathit{Ra}}^{1/ 2} $ and turbulence intensity as $\mathit{Re}\ensuremath{\sim} {\mathit{Ra}}^{1/ 2} $, where the Rayleigh number $\mathit{Ra}$ indicates the strength of the driving force (for fixed values of $\mathit{Pr}$, which is the ratio between kinematic viscosity and thermal diffusivity). However, experiments never operate in unbounded domains and it is important to understand how confinement might alter the approach to this ultimate regime. Here we consider homogeneous Rayleigh–Bénard convection in a laterally confined geometry – a small-aspect-ratio vertical cylindrical cell – and show evidence of the ultimate regime as $\mathit{Ra}$ is increased: in spite of the lateral confinement and the resulting kinetic boundary layers, we still find $\mathit{Nu}\ensuremath{\sim} \mathit{Re}\ensuremath{\sim} {\mathit{Ra}}^{1/ 2} $ at $\mathit{Pr}= 1$. Further, it is shown that the system supports solutions composed of modes of exponentially growing vertical velocity and temperature fields, with $\mathit{Ra}$ as the critical parameter determining the properties of these modes. Counter-intuitively, in the low-$\mathit{Ra}$ regime, or for very narrow cylinders, the numerical simulations are susceptible to these solutions, which can dominate the dynamics and lead to very high and unsteady heat transfer. As $\mathit{Ra}$ is increased, interaction between modes stabilizes the system, evidenced by the increasing homogeneity and reduced fluctuations in the root-mean-square velocity and temperature fields. We also test that physical results become independent of the periodicity length of the cylinder, a purely numerical parameter, as the aspect ratio is increased.

scholarly journals Heat transfer in rotating Rayleigh–Bénard convection with rough plates

2017 ◽  
Vol 830 ◽  
Author(s):  
Pranav Joshi ◽  
Hadi Rajaei ◽  
Rudie P. J. Kunnen ◽  
Herman J. H. Clercx
Keyword(s):  
Heat Transfer ◽  
Boundary Layer ◽  
Ekman Layer ◽  
Ekman Pumping ◽  
Ekman Boundary Layer ◽  
Bénard Convection ◽  
Roughness Elements ◽  
Benard Convection ◽  
Rayleigh Benard Convection ◽  
Rayleigh Benard

This experimental study focuses on the effect of horizontal boundaries with pyramid-shaped roughness elements on the heat transfer in rotating Rayleigh–Bénard convection. It is shown that the Ekman pumping mechanism, which is responsible for the heat transfer enhancement under rotation in the case of smooth top and bottom surfaces, is unaffected by the roughness as long as the Ekman layer thickness $\unicode[STIX]{x1D6FF}_{E}$ is significantly larger than the roughness height $k$. As the rotation rate increases, and thus $\unicode[STIX]{x1D6FF}_{E}$ decreases, the roughness elements penetrate the radially inward flow in the interior of the Ekman boundary layer that feeds the columnar Ekman vortices. This perturbation generates additional thermal disturbances which are found to increase the heat transfer efficiency even further. However, when $\unicode[STIX]{x1D6FF}_{E}\approx k$, the Ekman boundary layer is strongly perturbed by the roughness elements and the Ekman pumping mechanism is suppressed. The results suggest that the Ekman pumping is re-established for $\unicode[STIX]{x1D6FF}_{E}\ll k$ as the faces of the pyramidal roughness elements then act locally as a sloping boundary on which an Ekman layer can be formed.

scholarly journals Heat transfer mechanisms in bubbly Rayleigh-Bénard convection

2009 ◽  
Vol 80 (2) ◽  
Author(s):  
Paolo Oresta ◽  
Roberto Verzicco ◽  
Detlef Lohse ◽  
Andrea Prosperetti
Keyword(s):  
Heat Transfer ◽  
Bénard Convection ◽  
Benard Convection ◽  
Rayleigh Benard Convection ◽  
Rayleigh Benard ◽  
Transfer Mechanisms
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