Statistics of coherent structures in two-dimensional turbulent Rayleigh-Bénard convection

Abstract

Characterization of coherent structures in turbulent Rayleigh-Bénard convection using statistical measures is presented in the present work. Numerical simulations are carried out in a two-dimensional (2D) rectangular cell with aspect ratio 2 using air as the working fluid across four decades of Rayleigh number. The absence of one lateral dimension leads to entrapment of plumes which are consequently emitted in the form of thermal jets. Axial nonuniformity in thermal boundary layers is eliminated at high Rayleigh numbers. The so-called slope and 99% methods produce identical boundary layer thicknesses whose power law variation confirms theoretical inverse-Nu scaling. Turbulent kinetic energy budget unveils a transport-dissipation balance near the walls with buoyancy production nearly sustaining turbulent fluctuations in the bulk region. A higher threshold for the correlation between the vertical velocity and temperature results in faster convergence of plume and background share of dissipation, while decay in the volume fraction of the plume region continues. Exponential distribution of temperature fluctuations suggests the presence of hard turbulence at very large Rayleigh numbers with wider tails recording extreme fluctuating events. Changes in plume emission and its subsequent motion not only influence boundary layer instabilities but also cause departure from the −5/3 law in the frequency spectra.

Investigation of flow dynamics and heat transfer mechanism in turbulent Rayleigh–Bénard convection over multi-scale rough surfaces

“…The volume-time-averaged contributions of A K , T K and P S being negligible result in an exact balance between global buoyancy production (P B ) and turbulent dissipation ( ). In table 2, a direct comparison is made between the present rough case and the smooth surface case (Chand et al 2019) for volume-time-averaged P B , and the Nusselt number. Note that in the present roughness configuration, there is a slight digression from the expected balance between the global P B and .…”

“…It can be observed that with increase in δ, the scaling exponents for both V pl and pl increase in absolute sense, while for V bg and bg , they appear to be more stable. Further, to explain how roughness alters plume regions, variation of V pl and pl with Ra is compared with the smooth surface case (Chand et al 2019) for δ = 1 % and 5 % in figure 7. It is clear that irrespective of δ, both V pl and pl are higher for the roughened surface with slower decay over increasing Ra.…”

“…A comparison is made for the variation of (a) V pl and (b) pl with Ra between rough and smooth surfaces for δ = 1 and 5 %. Symbols in red and blue represent data for smooth (Chand et al 2019) plume generation spots is its biggest asset in influencing flow structures and dynamics. In smooth surfaces, plume eruption is driven purely by buoyancy, whereas for rough surfaces, sharp edges promote flow separation and act as the active source of plume emission (Stringano et al 2006).…”

“…In figure 16, horizontally averaged r.m.s. of temperature (σ θ ) and vertical fluctuations (σ v ) at mid-height of the cell (y = 0.5) are shown along with the smooth case counterparts (Chand et al 2019) as a function of Ra. It is clearly evident that σ θ is magnified in the presence of the rough surfaces, while σ v is comparable to that observed in the smooth case.…”

Turbulence Structure and Momentum Exchange in Compound Channel Flows With Shore Ice Covered on the Floodplains

“…The coherent vortices carry the high-momentum fluid from the main channel into the shallow floodplains, thereby enhancing the transverse scalar and momentum exchange. Previous studies show that the horizontal coherent vortices can be also detected at the confluence of two rivers (Uijttewaal & Booij, 2000;Yuan et al, 2016), in the turbulent Rayleigh-Benard convection (Chand et al, 2019), around the groynes of a trapezoidal open channel (Zhang et al, 2019), as well as in the wakes of an island (Alaee et al, 2007). Recently, Juez et al (2019) conducted a comprehensive study in an open asymmetric compound channel to investigate the large vortical structures and mass exchange near the mixing interface under different channel characteristics.…”