Small-scale universality in fluid turbulence

Turbulent flows in nature and technology possess a range of scales. The largest scales carry the memory of the physical system in which a flow is embedded. One challenge is to unravel the universal statistical properties that all turbulent flows share despite their different large-scale driving mechanisms or their particular flow geometries. In the present work, we study three turbulent flows of systematically increasing complexity. These are homogeneous and isotropic turbulence in a periodic box, turbulent shear flow between two parallel walls, and thermal convection in a closed cylindrical container. They are computed by highly resolved direct numerical simulations of the governing dynamical equations. We use these simulation data to establish two fundamental results: (i) at Reynolds numbers Re ∼ 10² the fluctuations of the velocity derivatives pass through a transition from nearly Gaussian (or slightly sub-Gaussian) to intermittent behavior that is characteristic of fully developed high Reynolds number turbulence, and (ii) beyond the transition point, the statistics of the rate of energy dissipation in all three flows obey the same Reynolds number power laws derived for homogeneous turbulence. These results allow us to claim universality of small scales even at low Reynolds numbers. Our results shed new light on the notion of when the turbulence is fully developed at the small scales without relying on the existence of an extended inertial range.An enduring notion in the phenomenology of turbulence is the universality of small scales. It has been taken for granted in theoretical approaches (e.g., refs. 1–8) and analyzed in numerical simulations (9–11) as well as various laboratory experiments (e.g., refs. 5 and 12). The standard paradigm is that whereas the large scales are nonuniversal, reflecting the circumstances of their generation, an increasingly weaker degree of nonuniversality is imparted to small scales with increasing separation between the large and small scales. This scale separation is thought to increase with the flow Reynolds number, so a proper test of universality has been thought to require very high Reynolds numbers. Consequently, many substantial efforts have been made to produce such high-Reynolds-number flows (e.g., ref. 12).Here, we show evidence for an alternative point of view: If one resolves small scales accurately, one observes, even at low Reynolds numbers, universal scaling of velocity gradients that manifest primarily at small scales. We stress that small-scale dynamics are strongly nonlinear even in low-Reynolds-number flows driven by large-scale forcing. There is thus considerable merit in measuring or simulating low-Reynolds-number flows much more accurately than has been the practice and exploring the evidence for universality (or lack thereof), instead of advancing as inevitable the notion that useful lessons about universality are possible only at very high Reynolds numbers. Indeed, another result of this paper is that there exists a threshold Reynolds number above which Gaussian-like fluctuations tend to assume intermittent characteristics of fully developed flows and that these features can be extracted by accessing increasingly smaller scales even if the Reynolds numbers are quite moderate. The latter result is especially important for purposes of identifying a fixed point in certain renormalization group expansion procedures (8).     

A Re‐examination of the MDM2/p53 Interaction Leads to Revised Design Criteria for Novel Inhibitors

The general model of epitope‐type MDM2 inhibitor was developed based on the structural information on the complexes between MDM2 and various low molecular weight ligands found in the PDB database. Application of this model to our in‐house library has led us to a new scaffold capable of interrupting protein–protein interactions. A synthetic library based on this and related scaffolds resulted in new classes of compounds that possess biochemical and cellular activity and good pharmacokinetic properties. We assume that such general approach to PPI inhibitors design may be useful for the development of inhibitors of various PPI types, including Bcl/XL.     

The influence of the sacrificial agent nature on transformations of the Zn(OH)2/Cd0.3Zn0.7S photocatalyst during hydrogen production under visible light

Photocatalysts based on zinc hydroxide and a solid solution of CdS and ZnS were prepared via the precipitation method and used for photocatalytic hydrogen production from aqueous solutions of inorganic (Na₂S/Na₂SO₃) and organic (ethanol) sacrificial agents. The photocatalysts were tested in cyclic experiments for hydrogen evolution and studied using X-ray diffraction (XRD), UV-Vis diffuse reflectance spectroscopy, high-resolution transmission electron microscopy (HRTEM), energy-dispersive X-ray spectroscopy (EDX), and X-ray photoelectron spectroscopy (XPS) techniques. Different transformations of the β-Zn(OH)₂ co-catalyst were observed in the presence of inorganic and organic sacrificial agents; namely, ZnS was formed in Na₂S/Na₂SO₃ solution, whereas the formation of ε-Zn(OH)₂ was detected in solution with ethanol. The composite Zn(OH)₂/Cd_1−xZn_xS photocatalysts have great potential in various photocatalysis processes (e.g., hydrogen production, CO₂ reduction, and the oxidation of organic contaminants) under visible light.

The nature of the sacrificial agent affects the transformations of a Zn(OH)₂ co-catalyst during photocatalytic hydrogen production.     

Turning weak emitters into outstanding luminescent materials using rigid host media

The incorporation into rigid silica host structures leads successfully to a significant luminescence enhancement of two zinc(ii) dipyrrins, known to be weak emitters in solution. One of these complexes shows a fluorescence efficiency of 55% and prolonged photo-stability once entrapped in silica, demonstrating high potential for applications in energy conversion.

Through incorporation into host–guest silica structures, typically poorly emissive zinc(ii) dipyrrins exhibit up to 100-fold fluorescence enhancement and prolonged photostability.

The search for ever more efficient and sustainable energetic solutions requires the development of new materials that are capable of high conversion rates with the least possible energy losses. Such a target is desirable, and currently tackled, for different types of conversion processes, e.g. light-to-electricity and electricity-to-light, and the primary step typically undertaken is the design of molecular dyes with controlled light absorption and emission bandwidths, alongside high transition efficiencies.¹ Among the various classes of dyes studied so far, dipyrromethenes, often called dipyrrins, present particularly attractive advantages over others, such as simple, high-yielding and cheap synthetic routes, wide coordinative versatility towards semi-metals like boron and several metal ions, and finely tuneable optical activity through structural modification on the pyrrole rings by introduction of specific substituents.^2–4 Alongside boron dipyrrins, the most extensively studied dipyrrin-based dyes so far, metal dipyrrins have recently gained attention and have been the subject of a growing number of studies on the application of their optical properties for energy conversion.^5–8 One of the main drives for studies on metal dipyrrins is the possibility of (i) exploiting the presence of two or three dipyrrin-based chromophores in a complex, as opposed to bodipy dyes bearing only one, thus enhancing the light absorbing capability, and (ii) making use of specific central metal ions to further manipulate the optical properties and the coordination geometries.^1,4 Zinc(ii) dipyrrins, among other metal complexes, are particularly attractive because of the presence of an earth-abundant central ion and the ability to form supramolecular structures and coordination polymers,^9–12 commonly exhibiting a linear geometry, due to the easy functionalization on the meso-position of the dipyrrin. Despite their strong absorption in the blue-green region, though, with molar extinction coefficients up to above 300 000 M⁻¹ cm⁻¹, they are typically weak emitters because of the strongly competitive non-radiative deactivation pathways, introduced by the distorted tetrahedral coordination and a certain structural flexibility.⁵One of the possible solutions that has been recently adopted to improve the emission quantum yields of zinc complexes is the use of bulky coordinating ligands that are able to impose a high structural rigidity to the whole complexes. This strategy involves the design of suitable multi-steps synthetic routes, with the aim of expanding the dipyrrin core from the α- or the β-positions, so to introduce sterically hindering groups.^7,13 An alternative solution, which we are presenting in this communication, for increasing the luminescence efficiency without having to introduce lengthy synthetic modifications is to impart the desired rigidity by encapsulation of the complexes into a spatially constraining host structure. This may, in principle, also help circumventing another strict limitation to fluorescence efficiencies that homoleptic bis(dipyrrinato) zinc complexes suffer from: the presence of symmetry-breaking charge transfer excited states, which become the lowest-lying states in polar solvents and, being prone to non-radiative decay, lead to fluorescence quenching.⁶Therefore, we chose two among the weakest bis(dipyrrinato) Zn(ii) emitters to build host–guest materials, where the role of the host is played by mesoporous silica of COK-12 type.¹⁴ The structures and the optical absorption of the two homoleptic complexes, 1 and 2, are shown in Fig. 1. They were first reported, respectively, by Nishihara''s and Thompson''s groups,^6,7 and showed fluorescence quantum yields below 1% in mildly polar solvents, such as chloroform and dichloromethane, and of 2% (complex 1)¹³ and 16% (complex 2) in cyclohexane, the least polar solvents used in the above studies (see also Open in a separate windowFig. 1Chemical structures of the complexes 1 and 2, and absorption spectra recorded from 10⁻⁶ M dichloromethane solutions.Photophysical properties of the zinc complexes

TREC/KREC Levels and T and B Lymphocyte Subpopulations in COVID-19 Patients at Different Stages of the Disease

Background: T and B cell-mediated immunity can be assessed using T cell receptor excision circle (TREC) and Kappa-deleting recombination excision circle (KREC) analysis, respectively, and successful implementation of this method requires evaluation of the correlation between the TREC frequencies and T cell subsets as well as KREC levels and B lymphocyte subsets. The aim of the present study was to evaluate the correlation between the TREC/KREC concentrations and T/B lymphocyte subsets at different stages of COVID-19. Methods: We examined 33 patients in the acute stage of COVID-19 (including 8 patients with poor outcomes) and 33 COVID-19 survivors. TREC/KREC concentrations were measured using quantitative real-time PCR. T/B lymphocyte subsets were determined using flow cytometry. Results: Blood TREC and KREC levels were found to be significantly lower in the acute stage of COVID-19 compared to control values. Moreover, a zero blood TREC level was a predictor of a poor disease outcome. Reductions in CD3⁺CD4⁺CD45RO⁻CD62L⁻ and CD3⁺CD8⁺CD45RO⁻CD62L⁻ T cell counts (as well as in the main fractions of B1 and B2 B cells) indicated a favorable outcome in COVID-19 patients in the acute stage of the disease. Decreased CD3⁺CD4⁺CD45RO⁻CD62L⁺ and CD3⁺CD8⁺CD45RO⁻CD62L⁺ T cell frequencies and increased CD3⁺CD8⁺CD45RO⁻CD62L⁻ cell counts were found to indicate a poor outcome in patients with acute COVID-19. These patients were also found to have increased B1 cell counts while demonstrating no changes in B2 cell counts. The levels of effector T cell subsets an naïve B cells were normal in COVID-19 survivors. The most pronounced correlations between TREC/KREC levels and T/B cell subsets counts were observed in COVID-19 survivors: there were positive correlations with naïve T and B lymphocytes and negative correlations with central and effector memory T cell subsets. Conclusions: The assessment of correlations between TREC and T cell subsets as well as KREC levels and B cell subset counts in patients with acute COVID-19 and COVID-19 survivors has shown that blood concentrations of TREC and KREC are sensitive indicators of the stage of antigen-independent differentiation of adaptive immunity cells. The results of the TREC and KREC analysis correlated with the stages of COVID-19 and differed depending on the outcome of COVID-19.     

Transfer hydrogenation of nitroarenes using cellulose filter paper-supported Pd/C by filtration as well as sealed methods

A reductive filter paper for selective nitro reduction has been prepared by modification of a pristine cellulose filter paper by Pd/C nanoparticles, as a portable catalyst. The reaction was performed in two different set-ups including (i) filtration and (ii) sealed systems, in the presence of ammonium formate and ex situ generated hydrogen gas reducing agents, respectively. In the sealed system in the presence of H₂ gas, the halogenated nitroarenes were completely reduced, while in the filtration system, different derivatives of the nitroarenes were selectively reduced to aryl amines. In both systems, the reduction of nitroarenes to aryl amines was performed with high efficiency and selectivity, comparable to a heterogeneous system. Reaction parameters were comprehensively designed using Design Expert software and then studied. The properties of the catalytic filter paper were studied in detail from the points of view of swellability, shrinkage, reusability, and stability against acidic, alkaline, and oxidative reagents.

A novel and efficient catalytic filtration has been developed for the selective reduction of nitro compounds on a Pd/C-doped cellulose filter paper.