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The behavior of moist Rayleigh-Bénard convection is investigated using a Boussinesq model with a simplified thermodynamics for phase transitions. This idealized configuration makes the problem accessible to high-resolution three-dimensional direct numerical simulations without small-scale parameterizations of the turbulence for extended layers with aspect ratios up to 64. Our study is focused on the frequently observed conditionally unstable environment that is stably stratified for unsaturated air, but is unstable for cloudy air. We find that no sharp threshold for the transition to convective turbulence exists, a situation similar to wall-bounded shear flows. Rather, the transition depends on the amplitude of the initial perturbation of the quiescent equilibrium and on the aspect ratio of the convective domain. In contrast to the classical dry Rayleigh-Bénard case, convection is highly asymmetric with respect to the vertical direction. Moist upwelling air inside turbulent cloud aggregates is surrounded by ambient regions of slowly descending unsaturated air. It is also found that conditionally unstable moist convection is inefficient at transporting energy. Our study suggests that there is an upper bound on the Nusselt number in moist convection that is lower than that of the classical dry case. [ABSTRACT FROM AUTHOR]

Molecular machines belonging to the AAA+ superfamily of ATPases use NTP hydrolysis to remodel their versatile substrates. The presence of an insertion sequence defines the major phylogenetic pre-sensor I insertion (pre-SIi) AAA+ superclade. In the bacterial σ54-dependent enhancer binding protein phage shock protein F (PspF) the pre-SIi loop adopts different conformations depending on the nucleotide-bound state. Single amino acid substitutions within the dynamic pre-SIi loop of PspF drastically change the ATP hydrolysis parameters, indicating a structural link to the distant hydrolysis site. We used a site-specific protein–DNA proximity assay to measure the contribution of the pre-SIi loop in σ54-dependent transcription and demonstrate that the pre-SIi loop is a major structural feature mediating nucleotide state-dependent differential engagement with Eσ54. We suggest that much, if not all, of the action of the pre-SIi loop is mediated through the L1 loop and relies on a conserved molecular switch, identified in a crystal structure of one pre-SIi variant and in accordance with the high covariance between some pre-SIi residues and distinct residues outside the pre-SIi sequence. [ABSTRACT FROM AUTHOR]

Abstract: AAA+ proteins are ubiquitous mechanochemical ATPases that use energy from ATP hydrolysis to remodel their versatile substrates. The AAA+ characteristic hexameric ring assemblies raise important questions about if and how six often identical subunits coordinate hydrolysis and associated motions. The PspF AAA+ domain, PspF1–275, remodels the bacterial σ54–RNA polymerase to activate transcription. Analysis of ATP substrate inhibition kinetics on ATP hydrolysis in hexameric PspF1–275 indicates negative homotropic effects between subunits. Functional determinants required for allosteric control identify: (i) an important link between the ATP bound ribose moiety and the SensorII motif that would allow nucleotide-dependent α-helical α/β subdomain dynamics; and (ii) establishes a novel regulatory role for the SensorII helix in PspF, which may apply to other AAA+ proteins. Consistent with functional data, homotropic control appears to depend on nucleotide state-dependent subdomain angles imposing dynamic symmetry constraints in the AAA+ ring. Homotropic coordination is functionally important to remodel the σ54 promoter. We propose a structural symmetry-based model for homotropic control in the AAA+ characteristic ring architecture. [Copyright &y& Elsevier]

Bacterial sigma (σ) factors confer gene specificity upon the RNA polymerase, the central enzyme that catalyses gene transcription. The binding of the alternative σ factor σ54 confers upon the RNA polymerase special functional and regulatory properties, making it suited for control of several major adaptive responses. Here, we summarize our current understanding of the interactions the σ54 factor makes with the bacterial transcription machinery. [ABSTRACT FROM AUTHOR]

Transcriptional initiation invariably involves the transition from a closed RNA polymerase (RNAP) promoter complex to a transcriptional competent open complex. Activators of the bacterial σ54-RNAP are AAA+ proteins that couple ATP hydrolysis to restructure the σ54-RNAP promoter complex. Structures of the σ54 activator PspF AAA+ domain (PspF1-275) bound to σ54 show two loop structures proximal to σ54 as follows: the σ54 contacting the GAFTGA loop 1 structure and loop 2 that classifies σ54 activators as pre-sensor 1 β-hairpin AAA+ proteins. We report activities for PspF1-275 mutated in the AAA+ conserved sensor I threonine/asparagine motif (PspF1-275T148A, PSpF1-275N149A, and PspF1-275N149S) within the second region of homology. We show that sensor I asparagine plays a direct role in ATP hydrolysis. However, low hydrolysis rates are sufficient for functional output in vitro. In contrast, PspF1-275T148A has severe defects at the distinct step of σ54 promoter restructuring. This defect is not because of the failure of PspF1-275T148A to stably engage with the closed σ54 promoter, indicating (i) an important role in ATP hydrolysis-associated motions during energy coupling for remodeling and (ii) distinguishing PspF1-275T148A from PSpF1-275 variants involved in signaling to the GAFTGA loop 1, which fail to stably engage with the promoter. Activities of loop 2 PspF1-275 variants are similar to those of PspF1-275T148A suggesting a functional signaling link between Thr148 and loop 2. In PspF1-275 this link relies on the conserved nucleotide state-dependent interaction between the Walker B residue Glu108 and Thr148. We propose that hydrolysis is relayed via Thr148 to loop 2 creating motions that provide mechanical force to the GAFTGA loop 1 that contacts σ54. [ABSTRACT FROM AUTHOR]

The article explores how the business model for low-cost airports has become a viable tool for customer acquisition, marketing and public relations in the commercial aeronautics market. The model employed by Frankfurt-Hahn airport, a low-cost carrier based from Germany is given as a paradigm to illustrate how low-cost carriers can afford to offer cheap tickets and make a profit at the same time only if they keep their costs down. Briefly discussed are the factors behind the airport's success such as its flexibility in terms of its operations, its highly-motivated staff and the nature of its business infrastructure.

The catalytic AAA+ domain (PspF1–275) of an enhancer-binding protein is necessary and sufficient to contact σ54-RNA polymerase holoenzyme (Eσ54), remodel it, and in so doing catalyze open promoter complex formation. Whether ATP binding and hydrolysis is coordinated between subunits of PspF and the precise nature of the nucleotide(s) bound to the oligomeric forms responsible for substrate remodeling are unknown. We demonstrate that ADP stimulates the intrinsic ATPase activity of PspF1–275 and propose that this heterogeneous nucleotide occupancy in a PspF1–275 hexamer is functionally important for specific activity. Binding of ADP and ATP triggers the formation of functional PspF1–275 hexamers as shown by a gain of specific activity. Furthermore, ATP concentrations congruent with stoi- chiometric ATP binding to PspF1–275 inhibit ATP hydrolysis and Eσ54-promoter open complex formation. Demonstration of a heterogeneous nucleotide-bound state of a functional PspF1–275·Eσ54 complex provides clear biochemical evidence for heterogeneous nucleotide occupancy in this AAA+ protein. Based on our data, we propose a stochastic nucleotide binding and a coordinated hydrolysis mechanism in PspF1� hexamers. [ABSTRACT FROM AUTHOR]

Abstract: Initiation of transcription is a major point of transcriptional regulation and invariably involves the transition from a closed to an open RNA polymerase (RNAP) promoter complex. In the case of the σ54-RNAP, this multi step process requires energy, provided by ATP hydrolysis occurring within the AAA+ domain of enhancer binding proteins (EBPs). Typically, EBPs have an N-terminal regulatory domain, a central AAA+ domain that directly contacts σ54 and a C-terminal DNA binding domain. The following AAA+ EBP crystal structures have recently become available: heptameric AAA+ domains of NtrC1 and dimeric NtrC1 with its regulatory domain, hexameric AAA+ domains of ZraR with DNA binding domains, apo and nucleotide bound forms of the AAA+ domain of PspF as well as a cryo-EM structure of the AAA+ domain of PspF complexed with σ54. These AAA+ domains reveal the structural conservation between EBPs and other AAA+ domains. EBP specific structural features involved in substrate remodelling are located proximal to the pore of the hexameric ring. Parallels with the substrate binding elements near the central pore of other AAA+ members are drawn. We propose a structural model of EBPs in complex with a σ54-RNAP-promoter complex. [Copyright &y& Elsevier]

Bacterial enhancer-binding proteins (EBP) activate transcription by hydrolyzing ATP to restructure the σ54-RNA polymerase–promoter complex. We compare six high resolution structures (<2.1Å) of the AAA+ domain of EBP phage shock protein F (PspF) including apo, AMPPNP, Mg2+-ATP, and ADP forms. These structures permit a description of the atomic details underpinning the origins of the conformational changes occurring during ATP hydrolysis. Conserved regions of PspF''s AAA+ domain respond distinctively to nucleotide binding and hydrolysis, suggesting functional roles during the hydrolysis cycle, which completely agree with those derived from activities of PspF mutated at these positions. We propose a putative atomic switch that is responsible for coupling structural changes in the nucleotide-binding site to the repositioning of the σ54-interacting loops. Striking similarities in nucleotide-specific conformational changes and atomic switch exist between PspF and the large T antigen helicase, suggesting conservation in the origin of those events amongst AAA+ proteins. [Copyright &y& Elsevier]

Abstract: Our current knowledge regarding age and sex differences in sensation seeking is based on studies, which all use the SSS-V. The aim of the present study therefore was to validate the results reported by Zuckerman, Eysenck, and Eysenck (1978) while applying an alternative instrument. We also examined the relevance of sociodemographic factors in sensation seeking. German data were collected from 1949 subjects (47% female; aged 16–79years) constituting a representative population sample of Germany. Sensation Seeking was measured using the Arnett Inventory of Sensation Seeking (AISS), which consists of two subscales: Novelty and Intensity constituting a Total Score. The results clearly confirm the postulated age and sex differences. Significant age declines occurred on all three scales for both sexes. Males generally scored higher than females in all age groups. The age and sex differences are independent of sociodemographic factors. On the whole, sociodemographic factors explain only a small part of the variance in sensation seeking. [Copyright &y& Elsevier]

Transcription activation by bacterial σ54-dependent enhancer-binding proteins (EBPs) requires their tri-nucleotide hydrolysis to restructure the σ54 RNA polymerase (RNAP). EBPs share sequence similarity with guanine nucleotide binding-proteins and ATPases associated with various cellular activities (AAA) proteins, especially in the mononucleotide binding P-loop fold. Using the phage shock protein F (PspF) EBP, we identify P-loop residues responsible for nucleotide binding and hydrolysis, consistent with their roles in other P-loop NTPases. We show the refined low-resolution structure of an EBP, PspF, revealing a hexameric ring organisation characteristic of AAA proteins. Functioning of EBPs involves ATP binding, higher oligomer formation and ATP hydrolysis coupled to the restructuring of the RNAP. This is thought to be a highly coordinated multi-step process, but the nucleotide-driven mechanism of oligomerisation and ATP hydrolysis is little understood. Our kinetic and structural data strongly suggest that three PspF dimers assemble to form a hexamer upon nucleotide binding. During the ATP hydrolysis cycle, both ATP and ADP are bound to oligomeric PspF, in line with a sequential hydrolysis cycle. We identify a putative R-finger, and show its involvement in ATP hydrolysis. Substitution of this arginine residue results in nucleotide-independent formation of hexameric rings, structurally linking the putative R-finger and, by inference, a specific nucleotide interaction to the control of PspF oligomerisation. [Copyright &y& Elsevier]

We study the instantaneous and local energy injection in a turbulent shear flow driven by volume forces. The energy injection can be both positive and negative. Extremal events are related to coherent streaks. The probability distribution is asymmetric, deviates slightly from a Gaussian shape and depends on the position in shear direction. The probabilities for positive and negative injection are exponentially related, but the prefactor in the exponent varies across the shear layer. [Copyright &y& Elsevier]

As part of a larger survey involving a representative sample of 2.050 Germans aged 14 to 92 years, the prevalence of somatoform complaints among young people (N=189,14 to 25 years old) was analyzed separately.The instrument used was the SOMS 2 (Screening for somatoform disorders), a German questionnaire developed by Rief et al. (1997). It could be shown that somatoform complaints are less prevalent among young people, but still occur frequently.Most commonly mentioned were headaches, facial pain,... [ABSTRACT FROM AUTHOR]

Copyright of Forstwissenschaftliches Centralblatt is the property of Springer Nature and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Summary In the present study the interpersonal problems of a community sample of n=766 elderly at the age of 61 to 92 years were investigated by means of the German version of the Inventory of Interpersonal Problems (IIP-C; Horowitz et al. 1994). The item analysis revealed that the IIP-C is suitable for the elderly. Beyond the age of 60, we found no considerable age-specific or sex-specific effects. The elderly from East-Germany, however, described themselves with more social openess, feeling more close to other people, and with more social competence. With the results of this study, that are largely representative for the general German population over 60 years, we present references for studies on interpersonal problems in clinical subgroups of the elderly. [ABSTRACT FROM AUTHOR]

Copyright of Social & Preventive Medicine is the property of Springer Nature and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

A correction to the article "Lagrangian mixing dynamics at the cloudy-clear air interface," by B. Kumar and colleagues published in an issue of the "Journal of Atmospheric Science."

In the field of magnetohydrodynamics, turbulent duct flows in a transverse magnetic field are of considerable interest. Their velocity fields exhibit electromagnetic boundary layers and anisotropy due to damping of turbulent fluctuations by the Lorentz force. The weak induced magnetic field is slaved to the velocity field (quasistatic limit) in laboratory experiments since the magnetic Reynolds number is small. Here we consider flow conditions with finite magnetic Reynolds number in direct numerical simulations, whereby the full induction equation needs to be solved. The modification of the imposed transverse field by the flow and other differences to the quasistatic results are briefly discussed. [ABSTRACT FROM AUTHOR]

We investigate the large-scale circulation (LSC) in a turbulent Rayleigh-Bénard convection flow in a cubic closed convection cell by means of direct numerical simulations at a Rayleigh number Ra = 106. The numerical studies are conducted for single flow trajectories up to 105 convective free-fall times to obtain a sufficient sampling of the four discrete LSC states, which can be summarized to one macrostate, and the two crossover configurations which are taken by the flow in between for short periods. We find that large-scale dynamics depends strongly on the Prandtl number Pr of the fluid which has values of 0.1, 0.7, and 10. Alternatively, we run an ensemble of 3600 short-term direct numerical simulations to study the transition probabilities between the discrete LSC states. This second approach is also used to probe the Markov property of the dynamics. Our ensemble analysis gave strong indication of Markovianity of the transition process from one LSC state to another, even though the data are still accompanied by considerable noise. It is based on the eigenvalue spectrum of the transition probability matrix, further on the distribution of persistence times and the joint distribution of two successive microstate persistence times. This article is part of the theme issue 'Mathematical problems in physical fluid dynamics (part 1)'. [ABSTRACT FROM AUTHOR]

We explore the transport mechanisms of heat in two- and three-dimensional turbulent convection flows by means of the long-term evolution of Lagrangian coherent sets. They are obtained from the spectral clustering of trajectories of massless fluid tracers that are advected in the flow. Coherent sets result from trajectories that stay closely together under the dynamics of the turbulent flow. For longer times, they are always destroyed by the intrinsic turbulent dispersion of material transport. Here, this constraint is overcome by the application of evolutionary clustering algorithms that add a time memory to the coherent set detection and allows individual trajectories to leak in or out of evolving clusters. Evolutionary clustering thus also opens the possibility to monitor the splits and mergers of coherent sets. These rare dynamic events leave clear footprints in the evolving eigenvalue spectrum of the Laplacian matrix of the trajectory network in both convection flows. The Lagrangian trajectories reveal the individual pathways of convective heat transfer across the fluid layer. We identify the long-term coherent sets as those fluid flow regions that contribute least to heat transfer. Thus, our evolutionary framework defines a complementary perspective on the slow dynamics of turbulent superstructure patterns in convection flows that were recently discussed in the Eulerian frame of reference. The presented framework might be well suited for studies in natural flows, which are typically based on sparse information from drifters and probes. [ABSTRACT FROM AUTHOR]

We analyze large‐scale patterns in three‐dimensional turbulent convection in a horizontally extended square convection cell by means of Lagrangian particle trajectories calculated in direct numerical simulations. Different Lagrangian computational methods, i.e. finite‐time Lyapunov exponents, spectral and density‐based clustering and transfer operator approaches, are used to detect these large‐scale structures, which are denoted as turbulent superstructures of convection. [ABSTRACT FROM AUTHOR]

The application of a uniform external magnetic field on the turbulent duct flow of an electrically conducting fluid leads to several interesting changes in the structure and the mean charateristics of the flow. This is fairly well understood from the existing studies of duct flows in the low magnetic Reynolds number ( R m) limit. In this paper, we present the results for magnetohydrodynamic duct flow at moderate R m obtained from direct numerical simulations (DNS). Several differences are observed to occur in this case as compared to low R m flows, such as increased Hartmann layer thickness and enhanced large scale turbulence in the core region of the duct cross-section due to partial expulsion of magnetic flux. (© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

The intermittency of a passive scalar advected by three-dimensional Navier-Stokes turbulence at a Taylor-scale Reynolds number of 650 is studied using direct numerical simulations on a 4096³ grid; the Schmidt number is unity. By measuring scalar increment moments of high orders, while ensuring statistical convergence, we provide unambiguous evidence that the scaling exponents saturate to 1.2 for moment orders beyond about 12, indicating that scalar intermittency is dominated by the most singular shocklike cliffs in the scalar field. We show that the fractal dimension of the spatial support of steep cliffs is about 1.8, whose sum with the saturation exponent value of 1.2 adds up to the space dimension of 3, thus demonstrating a deep connection between the geometry and statistics in turbulent scalar mixing. The anomaly for the fourth and sixth order moments is comparable to that in the Kraichnan model for the roughness exponent of 4/3. [ABSTRACT FROM AUTHOR]

Abstract: Transparent heating plates, consisting of glass coated with a transparent conductive metal oxide, are applied in large aspect ratio turbulent Rayleigh-Bénard convection (RBC) to investigate the large-scale patterns of velocity fields with optical flow measurement techniques across the whole horizontal cross section. The square convection cell with an aspect ratio Γ = L/h = 10 was tested inside the scaled convective airflow laboratory experiment (SCALEX) facility which enables experiments with gases as working fluids for pressures of up to 10 bar to achieve very high Rayleigh numbers Ra. For the current study, Ra = 2 × 104 was applied. The velocity fields are measured with 2D3C particle image velocimetry (PIV). The possibility of reliable PIV measurements with reproducible homogenous temperature boundary conditions was demonstrated in the SCALEX facility. The seeding of the tracer particles, their illumination and data evaluation are addressed in detail. The final comparison of experimental data and numerical simulations shows a good agreement for the probability density functions of the horizontal velocity components. Deviations for the vertical out-of-plane velocity component and their dependence on the thickness of the laser sheet are discussed in detail and quantified by measurements with light sheets of different thickness.Graphical abstract: [ABSTRACT FROM AUTHOR]

If a fluid flow is driven by a weak Gaussian random force, the nonlinearity in the Navier-Stokes equations is negligibly small and the resulting velocity field obeys Gaussian statistics. Nonlinear effects become important as the driving becomes stronger and a transition occurs to turbulence with anomalous scaling of velocity increments and derivatives. This process has been described by Yakhot and Donzis [Phys. Rev. Lett. 119, 044501 (2017)] for homogeneous and isotropic turbulence. In more realistic flows driven by complex physical phenomena, such as instabilities and nonlocal forces, the initial state itself, and the transition to turbulence from that initial state, is much more complex. In this paper, we discuss the Reynolds-number dependence of moments of the kinetic energy dissipation rate of orders 2 and 3 obtained in the bulk of thermal convection in the Rayleigh-Bénard system. The data are obtained from three-dimensional spectral element direct numerical simulations in a cell with square cross section and aspect ratio 25 by Pandey et al. [Nat. Commun. 9, 2118 (2018)]. Different Reynolds numbers 1≲Reℓ≲1000 which are based on the thickness of the bulk region ℓ and the corresponding root-mean-square velocity are obtained by varying the Prandtl number Pr from 0.005 to 100 at a fixed Rayleigh number Ra=105. A few specific features of the data agree with the theory. The normalized moments of the kinetic energy dissipation rate En show a nonmonotonic dependence for small Reynolds numbers before obeying the algebraic scaling prediction for the turbulent state. Implications and reasons for this behavior are discussed. [ABSTRACT FROM AUTHOR]

In Rayleigh‐Bénard convection, turbulent superstructures are large‐scale patterns of circulation rolls created by hot ascending and cold descending thermal plumes. The evolution of these large‐scale patterns happens on very large time scales τ [1]. Spectral clustering applied to Lagrangian particle trajectories on time intervals smaller than τ can be used to create clusters displaying a structure similar to the patterns detected in the Eulerian frame of reference [2]. However, this technique is unfeasible for the analysis of the evolution of turbulent superstructures due to turbulent dispersion. Therefore, we test the application of concepts of evolutionary spectral clustering [3] on Lagrangian particle trajectories to analyze the long‐term dynamics of turbulent superstructures in the Lagrangian frame of reference. [ABSTRACT FROM AUTHOR]

The global transport of heat and momentum in turbulent convection is constrained by thin thermal and viscous boundary layers at the heated and cooled boundaries of the system. This bottle neck is thought to be lifted once the boundary layers them selves become fully turbulent at very high values of the Rayleigh number Ra—the dimensionless parameter that describes the vigor of convective turbulence. Laboratory experiments in cylindrical cells for Ra≳1012have reported different outcomes on the putative heat transport law. Here we show, by direct numerical simulations of three-dimensional turbulent Rayleigh–Bénard convection flows in a slender cylindrical cell of aspect ratio 1/10, that the Nusselt number—the dimensionless measure of heat transport—follows the classical power law of Nu=(0.0525±0.006)×Ra0.331±0.002up to Ra=1015. Intermittent fluctuations in the wall stress, a blue print of turbulence in the vicinity of the boundaries, manifest at all Ra considered here, increasing with increasing Ra, and suggest that an abrupt transition of the boundary layer to turbulence does not take place. [ABSTRACT FROM AUTHOR]

We explore heat transport properties of turbulent Rayleigh- Be'nard convection in horizontally extended systems by using deep-learning algorithms that greatly reduce the number of degrees of freedom. Particular attention is paid to the slowly evolving turbulent superstructures--so called because they are larger in extent than the height of the convection layer--which appear as temporal patterns of ridges of hot upwelling and cold downwelling fluid, including defects where the ridges merge or end. The machine-learning algorithm trains a deep convolutional neural network (CNN) with U-shaped architecture, consisting of a contraction and a subsequent expansion branch, to reduce the complex 3D turbulent superstructure to a temporal planar network in the midplane of the layer. This results in a data compression by more than five orders of magnitude at the highest Rayleigh number, and its application yields a discrete transport network with dynamically varying defect points, including points of locally enhanced heat flux or "hot spots." One conclusion is that the fraction of heat transport by the superstructure decreases as the Rayleigh number increases (although they might remain individually strong), correspondingly implying the increased importance of small-scale background turbulence. [ABSTRACT FROM AUTHOR]

An extreme dissipation event in the bulk of a closed three-dimensional turbulent convection cell is found to be correlated with a strong reduction of the large-scale circulation flow in the system that happens at the same time as a plume emission event from the bottom plate. The reduction in the large-scale circulation opens the possibility for a nearly frontal collision of down- and upwelling plumes and the generation of a high-amplitude thermal dissipation layer in the bulk. This collision is locally connected to a subsequent high-amplitude energy dissipation event in the form of a strong shear layer. Our analysis illustrates the impact of transitions in the large-scale structures on extreme events at the smallest scales of the turbulence, a direct link that is observed in a flow with boundary layers. We also show that detection of extreme dissipation events which determine the far-tail statistics of the dissipation fields in the bulk requires long-time integrations of the equations of motion over at least a hundred convective time units. [ABSTRACT FROM AUTHOR]

Abstract We study hydrodynamic and magnetohydrodynamic channel flows by means of large eddy simulations (LES) on the basis of a second-order finite-volume scheme in collocated variable arrangement with a focus on the impact of numerical diffusion on the subgrid-scale (SGS) modeling. It is found that a mixed SGS model, which is based on the velocity increment tensor and an eddy viscosity model, performs best and is able to capture near-wall regions of energy backscatter from small to larger scales. Thereby, it improves the accuracy of the LES computations significantly. Our studies suggest that the mixed SGS model is thus applicable for a wide class for shear flow problems in liquid metal flows where finite-volume methods with collocated grids are applied. [ABSTRACT FROM AUTHOR]

Rayleigh–Bénard convection and Taylor–Couette flow are two canonical flows that have many properties in common. We here compare the two flows in detail for parameter values where the Nusselt numbers, i.e. the thermal transport and the angular momentum transport normalized by the corresponding laminar values, coincide. We study turbulent Rayleigh–Bénard convection in air at Rayleigh number Ra=107 and Taylor–Couette flow at shear Reynolds number ReS=2×104 for two different mean rotation rates but the same Nusselt numbers. For individual pairwise related fields and convective currents, we compare the probability density functions normalized by the corresponding root mean square values and taken at different distances from the wall. We find one rotation number for which there is very good agreement between the mean profiles of the two corresponding quantities temperature and angular momentum. Similarly, there is good agreement between the fluctuations in temperature and velocity components. For the heat and angular momentum currents, there are differences in the fluctuations outside the boundary layers that increase with overall rotation and can be related to differences in the flow structures in the boundary layer and in the bulk. The study extends the similarities between the two flows from global quantities to local quantities and reveals the effects of rotation on the transport. This article is part of the themed issue ‘Toward the development of high-fidelity models of wall turbulence at large Reynolds number’. [ABSTRACT FROM AUTHOR]

To understand turbulent convection at very high Rayleigh numbers typical of natural phenomena, computational studies in slender cells are an option if the needed resources have to be optimized within available limits. However, the accompanying horizontal confinement affects some properties of the flow. Here, we explore the characteristics of turbulent fluctuations in the velocity and temperature fields in a cylindrical convection cell of aspect ratio 0.1 by varying the Prandtl number P r between 0.1 and 200 at a fixed Rayleigh number R a = 3 × 1 0 10 , and find that the fluctuations weaken with increasing P r , quantitatively as in aspect ratio 25. The probability density function (PDF) of temperature fluctuations in the bulk region of the slender cell remains mostly Gaussian, but increasing departures occur as P r increases beyond unity. We assess the intermittency of the velocity field by computing the PDFs of velocity derivatives and of the kinetic energy dissipation rate, and find increasing intermittency as P r decreases. In the bulk region of convection, a common result applicable to the slender cell, large aspect ratio cells, as well as in 2D convection, is that the turbulent Prandtl number decreases as P r − 1 / 3 . • Slender convection cells in numerical experiments allow to achieve high Rayleigh numbers. • Dependence of various flow properties on the Prandtl number is similar to those in extended convection domains. • Temperature and velocity fluctuations exhibit similar distribution in cells of aspect ratio 0.1 and 25. • Turbulent Prandtl number increases with decreasing molecular Prandtl number with a powerlaw exponent of −1/3. [ABSTRACT FROM AUTHOR]

A combined convolutional autoencoder–recurrent neural network machine learning model is presented to directly analyze and forecast the dynamics and low-order statistics of the local convective heat flux field in a two-dimensional turbulent Rayleigh–Bénard convection flow at Prandtl number Pr = 7 and Rayleigh number Ra = 10 7 . Two recurrent neural networks are applied for the temporal advancement of turbulent heat transfer data in the reduced latent data space, an echo state network, and a recurrent gated unit. Thereby, our work exploits the modular combination of three different machine learning algorithms to build a fully data-driven and reduced model for the dynamics of the turbulent heat transfer in a complex thermally driven flow. The convolutional autoencoder with 12 hidden layers is able to reduce the dimensionality of the turbulence data to about 0.2% of their original size. Our results indicate a fairly good accuracy in the first- and second-order statistics of the convective heat flux. The algorithm is also able to reproduce the intermittent plume-mixing dynamics at the upper edges of the thermal boundary layers with some deviations. The same holds for the probability density function of the local convective heat flux with differences in the far tails. Furthermore, we demonstrate the noise resilience of the framework. This suggests that the present model might be applicable as a reduced dynamical model that delivers transport fluxes and their variations to coarse grids of larger-scale computational models, such as global circulation models for atmosphere and ocean. [ABSTRACT FROM AUTHOR]

Abstract: The statistical properties of velocity gradients in a wall-bounded turbulent channel flow are discussed on the basis of three-dimensional direct numerical simulations. Our analysis is concentrated on the trend of the statistical properties of the local enstrophy and the energy dissipation rate with increasing distance from the wall. We detect a sensitive dependence of the largest amplitudes of both fields (which correspond with the tail of the distribution) on the spectral resolution. The probability density functions of each single field as well as their joint distribution vary significantly with increasing distance from the wall. The largest fluctuations of the velocity gradients are found in the logarithmic layer. This is in agreement with recent experiments which observe a bursting of hairpin vortex packets into the logarithmic region. [Copyright &y& Elsevier]

Abstract: Polymer dynamics in a turbulent flow is a problem spanning several orders of magnitude in length and time scales. A microscopic simulation covering all those scales from the polymer segment to the inertial scale of turbulence remains improbable within the foreseeable future. We propose a multiscale simulation strategy to enhance the spatio-temporal resolution of the local Lagrangian turbulent flow by matching two different simulation techniques, i.e. direct numerical simulation for the flow as a whole, and the lattice Boltzmann method coupled to polymer dynamics at the Kolmogorov dissipation scale. Local turbulent flows sampled by Lagrangian tracer particles in the direct numerical simulation are reproduced in the lattice Boltzmann model with a finer resolution, by supplying the latter with both the correct initial condition as well as the correct time-dependent boundary condition, sampled from the former. When combined with a Molecular Dynamics simulation of a polymer chain in the lattice Boltzmann model, it provides a strategy to simulate the passive dynamics of a polymer chain in a turbulent flow covering all these scales. Although this approach allows for a fairly realistic model of the macromolecule, the back-coupling to the flow on the large scales is missing. [Copyright &y& Elsevier]