Towards DNS of the Ultimate Regime of Rayleigh--B\'enard Convection

In this contribution we have briefly introduced the problem of turbulent thermal convection with a particular look at its transition to the ultimate regime and the resolution requirements needed for the direct numerical simulation of this flow.Comment: 10 pages, 6 figure

How small-scale flow structures affect the heat transport in sheared thermal convection

We investigate the counter-intuitive initial decrease and subsequent increase in the Nusselt number Nu with increasing wall Reynolds number Re-w in the sheared Rayleigh-Benard (RB) system by studying the energy spectra of convective flux and turbulent kinetic energy for Rayleigh number Ra = 10(7), Prandtl number Pr = 1.0 and inverse Richardson numbers 0 <= 1/Ri <= 10. These energy spectra show two distinct high-energy regions corresponding to the large-scale superstructures in the bulk and small-scale structures in the boundary layer (BL) regions. A greater separation between these scales at the thermal BL height correlates to a higher Nu and indicates that the BLs are more turbulent. The minimum Nu, which occurs at 1/Ri = 1.0, is accompanied by the smallest separation between the large- and small-scale structures at the thermal BL height. At 1/Ri = 1.0, we also observe the lowest value of turbulent kinetic energy normalized with the square of friction velocity within the thermal BL. Additionally, we find that the domain size has a limited effect on the heat and momentum transfer in the sheared RB system as long as the domain can accommodate the small-scale convective structures at the thermal BL height, signifying that capturing the large-scale superstructures is not essential to obtain converged values of Nu and shear Reynolds number Re-tau. When the domain is smaller than these small-scale convective structures, the overall heat and momentum transfer reduces drastically

Breakdown of the large-scale circulation in gamma=1/2 rotating Rayleigh-Bénard flow

Experiments and simulations of rotating Rayleigh-Bénard convection in cylindrical samples have revealed an increase in heat transport with increasing rotation rate. This heat transport enhancement is intimately related to a transition in the turbulent flow structure from a regime dominated by a large-scale circulation (LSC), consisting of a single convection roll, at no or weak rotation to a regime dominated by vertically aligned vortices at strong rotation. For a sample with an aspect ratio G=D/L=1 (D is the sample diameter and L is its height) the transition between the two regimes is indicated by a strong decrease in the LSC strength. In contrast, for G=1/2, Weiss and Ahlers J. Fluid Mech. 688 461 (2011)] revealed the presence of a LSC-like sidewall temperature signature beyond the critical rotation rate. They suggested that this might be due to the formation of a two-vortex state, in which one vortex extends vertically from the bottom into the sample interior and brings up warm fluid while another vortex brings down cold fluid from the top; this flow field would yield a sidewall temperature signature similar to that of the LSC. Here we show by direct numerical simulations for G=1/2 and parameters that allow direct comparison with experiment that the spatial organization of the vertically aligned vortical structures in the convection cell do indeed yield (for the time average) a sinusoidal variation of the temperature near the sidewall, as found in the experiment. This is also the essential and nontrivial difference with the G=1 sample, where the vertically aligned vortices are distributed randomly

Warmtetransport in een roterende turbulente stroming

Hoe hangt bet warmtetransport in een vat met een fluIdum (vloeistof of gas), dat van beneden verwarmd en van boven gekoeld wordt, af van het temperatuurverschil tussen de bodem en bet deksel? Vloeistoffysici houden zich hier al meer dan een eeuw mee bezig. Theoretisch, experimenteel en numeriek inzicht is in de laatste tien jaar in een stroomversnelling gekomen, zie de overzichtsartikelen [1—3]. De vraag in dit artikel is, wat er gebeurt als bet systeem ook nog roteert? Deze vraag is relevant voor het warmtetransport in de atmosfeer en in de oceaan. Wij laten zien dat met toenemende rotatie bet warmtetransport eerst toeneemt en daarna dramatisch inzakt

Recognition in the Investigation of Crime

(česky) Rigorózní práce s názvem "Rekognice ve vyšetřování trestné činnosti" se zabývá interakcí a komunikací účastníků trestního řízení a tím, jaký má vliv kvalita těchto procesů na rekognici. V teoretické části je čtenářům přiblíženo trestní řízení, všechny procesní náležitosti a faktory, které průběh vyšetřování ovlivňují. Jsou zde také definována práva a povinnosti všech důležitých účastníků trestního řízení. Úvodní část je dále věnována komunikaci a jejím vlivům na úspěšné vedení výslechu a rekognice. Následně je zde pojednáno o psychologických aspektech procesních úkonů v přípravném řízení, přičemž v závěru teoretické části jsou podrobně popsány faktory, které mají vliv na vedení rekognice. V empirické části bylo kladeno za cíl zjistit, jaký vliv má způsob a kvalita podané instrukce na úspěšné provedení rekognice pachatele v simultánním a kombinovaném formátu. Empirická část byla uskutečněna v metodologické podobě experimentu. Probandi byli rozděleni do skupin podle daných kritérií a byl jim prezentován videozáznam trestného činu. Dále byla provedena rekognice a cílem bylo zjistit, jaký vliv má instrukce a podnětový materiál na výsledek rekognice.(in English): The thesis " Recognition in the investigation of crime" is studies the interaction and communication of criminal proceeding participants and its influence on the quality of those proceedings on recognition. The theoretical part deals with criminal proceedings such as all the trial requirements and factors which affect police investigation. Rights and responsibilities of all participants of trials are defined. Furthermore, communication, its types and categories and -most of all -its impact on successful police interrogation is examined. In conclusion of the theoretical part we describe the aspects which control the investigation and potential offender recognition. In the empirical part the aim was to find out what impact the quality of instructions has on successful offender recognition in simultaneous and combined format. The empirical part was performed in an experiment. The research participants were devided into categories according to chosen standards. Then they were presented with a video recording of a criminal offence followed by recognition. We targeted at discovering whether the instructions and the way they are given influence the results of recognition.Department of PsychologyKatedra psychologieFilozofická fakultaFaculty of Art

Serial mediational models tested.

<p>Serial mediational models tested.</p

Sharp transitions in rotating turbulent convection:Lagrangian acceleration statistics reveal a second critical Rossby number

In Rayleigh–Bénard convection (RBC) for fluids with Prandtl number Pr≳1, rotation beyond a critical (small) rotation rate is known to cause a sudden enhancement of heat transfer, which can be explained by a change in the character of the boundary layer (BL) dynamics near the top and bottom plates of the convection cell. Namely, with increasing rotation rate, the BL signature suddenly changes from Prandtl–Blasius type to Ekman type. The transition from a constant heat transfer to an almost linearly increasing heat transfer with increasing rotation rate is known to be sharp and the critical Rossby number Roc occurs typically in the range 2.3≲Roc≲2.9 (for Rayleigh number Ra=1.3×109, Pr=6.7, and a convection cell with aspect ratio Γ=DH=1, with D the diameter and Hthe height of the cell). The explanation of the sharp transition in the heat transfer points to the change in the dominant flow structure. At 1/Ro≲1/Roc (slow rotation), the well-known large-scale circulation (LSC) is found: a single domain-filling convection roll made up of many individual thermal plumes. At 1/Ro≳1/Roc (rapid rotation), the LSC vanishes and is replaced with a collection of swirling plumes that align with the rotation axis. In this paper, by numerically studying Lagrangian acceleration statistics, related to the small-scale properties of the flow structures, we reveal that this transition between these different dominant flow structures happens at a second critical Rossby number, Roc2≈2.25(different from Roc1≈2.7 for the sharp transition in the Nusselt number Nu; both values for the parameter settings of our present numerical study). When statistical data of Lagrangian tracers near the top plate are collected, it is found that the root-mean-square values and the kurtosis of the horizontal acceleration of these tracers show a sudden increase at Roc2. To better understand the nature of this transition we compute joint statistics of the Lagrangian velocity and acceleration of fluid particles and vertical vorticity near the top plate. It is found that for Ro≳2.25 there is hardly any correlation between the vertical vorticity and extreme acceleration events of fluid particles. For Ro≲2.25, however, vortical regions are much more prominent and extreme horizontal acceleration events are now correlated to large values of positive (cyclonic) vorticity. This suggests that the observed sudden transition in the acceleration statistics is related to thermal plumes with cyclonic vorticity developing in the Ekman BL and subsequently becoming mature and entering the bulk of the flow for Ro≲2.25

Comparison of computational codes for direct numerical simulations of turbulent Rayleigh–Bénard convection

\u3cp\u3eComputational codes for direct numerical simulations of Rayleigh–Bénard (RB) convection are compared in terms of computational cost and quality of the solution. As a benchmark case, RB convection at Ra=10\u3csup\u3e8\u3c/sup\u3e and Pr=1 in a periodic domain, in cubic and cylindrical containers is considered. A dedicated second-order finite-difference code (AFID/RBFLOW) and a specialized fourth-order finite-volume code (GOLDFISH) are compared with a general purpose finite-volume approach (OPENFOAM) and a general purpose spectral-element code (NEK5000). Reassuringly, all codes provide predictions of the average heat transfer that converge to the same values. The computational costs, however, are found to differ considerably. The specialized codes AFID/RBFLOW and GOLDFISH are found to excel in efficiency, outperforming the general purpose flow solvers NEK5000 and OPENFOAM by an order of magnitude with an error on the Nusselt number Nu below 5%. However, we find that Nu alone is not sufficient to assess the quality of the numerical results: in fact, instantaneous snapshots of the temperature field from a near wall region obtained for deliberately under-resolved simulations using NEK5000 clearly indicate inadequate flow resolution even when Nu is converged. Overall, dedicated special purpose codes for RB convection are found to be more efficient than general purpose codes.\u3c/p\u3