Extreme events in computational turbulence

We have performed direct numerical simulations of homogeneous and isotropic turbulence in a periodic box with 8,192³ grid points. These are the largest simulations performed, to date, aimed at improving our understanding of turbulence small-scale structure. We present some basic statistical results and focus on “extreme” events (whose magnitudes are several tens of thousands the mean value). The structure of these extreme events is quite different from that of moderately large events (of the order of 10 times the mean value). In particular, intense vorticity occurs primarily in the form of tubes for moderately large events whereas it is much more “chunky” for extreme events (though probably overlaid on the traditional vortex tubes). We track the temporal evolution of extreme events and find that they are generally short-lived. Extreme magnitudes of energy dissipation rate and enstrophy occur simultaneously in space and remain nearly colocated during their evolution.Fluid motions encountered in most circumstances are typically turbulent; therefore a good understanding of the subject is essential both for intrinsic scientific reasons and for advancing important technologies, e.g., improving jet engine performance. The difficulty of the subject (1, 2) has unfortunately consigned our present understanding to be partial at best. A milestone of turbulence theory consists of the similarity hypotheses of Kolmogorov (3, 4) and their various descendant scaling theories. In refs. 5 and 6, one can find a fair summary of the theoretical ideas as well as the considerable experimental work devoted to assessing their veracity. Rapid advances in computing power in recent decades have made computations increasingly important in advancing our understanding of the subject. Key quantities that cannot yet be measured in experiments can instead be computed by the so-called direct numerical simulation (DNS; e.g., see ref. 7), in which the exact equations of motion based on mass and momentum conservation are integrated numerically in time and space. The DNS data are capable of providing a wealth of quantitative detail (see, e.g., ref. 8) and improved qualitative understanding. In this paper, we present results from the largest DNS, to date, of isotropic turbulence aimed at the small-scale structure, rendered statistically stationary by large-scale forcing. We focus on the extreme events (to be made more precise momentarily).Turbulent flows consist of disorderly fluctuations in all measurable properties over a range of scales in both space and time. These fluctuations produce a combination of changes in shape and orientation of an infinitesimal fluid element and can affect quantities of practical interest, such as the tendency of tiny water vapor droplets to collide and grow to millimeter-size rain drops in atmospheric clouds (9). One of the key fluctuating quantities is the energy dissipation rate, ? = 2νs_ijs_ij, where ν is the kinematic viscosity, s_ij is the rate of strain given by ½(??u_i/??x_j + ??u_j/??x_i), u_i being the velocity in the direction i, and we have used Einstein’s summation convention. The second key quantity is the turbulent vorticity, ω→=∇×u→. From ω→, one can define the instantaneous enstrophy Ω = ω_iω_i, whose mean value multiplied by the kinematic viscosity is equal to the mean dissipation rate, although their interrelationship is more complex locally. Here, we focus on ε and Ω. The mean energy dissipation rate is equal to the rate of energy transfer from large scales of turbulence to small scales, and is thus a key quantity that characterizes the nonlinear interaction among the various scales excited in a turbulent flow. An extremely high dissipation rate (which implies high strain rate) can, for example, tear apart a flame surface and lead to local extinction in reacting systems (10). Fluctuations of enstrophy are likewise crucial in attempts to describe the structure of turbulence as (primarily) a collection of vortices of various intensities, sizes, and spacing (11).Our focus on small-scale properties ϵ and Ω suggests that it is adequate to consider homogeneous and isotropic turbulence, which is what we do here. The simulations are performed using well-established Fourier pseudospectral methods on 3D periodic domains (8). Advances in computing have allowed simulations to increase in size from 512³ grid points in the 1990s to 4,096³ in 2003 (12). The present simulations on an 8,192³ box (see Simulation and Data Processing Methodology) push toward attaining higher Reynolds numbers. See the legend for 13, 14). As the flow Reynolds number increases (which is the main reason for increasing the size of the simulations), ϵ and Ω fluctuate more and more wildly in space and time. This property of intermittency is thought to be a central trait of turbulence, and indeed of all strongly nonlinear systems. For the Reynolds numbers attained in the present simulations, ϵ and Ω display fluctuations of the order up to 10⁵ times their mean values. These extreme events are very fast-changing and require frequent sampling. We capture them faithfully and study them here: We not only study several standard statistical characteristics as well as their spatial and temporal nature but also discuss questions of their spatial simultaneity and geometric shape. We show that they are quite different in character from those that have been routinely designated as extreme events until now. We proceed below by elaborating on the concept of extreme events and the challenges they pose. Questions of accuracy, including effects of numerical resolution, statistical convergence, and machine precision, are addressed briefly in Supporting Information.

Table 1.

Parameters of the 8,192³ simulation performed in this paper

Physico-chemical alterations of urine in experimental hyperoxaluria: a biochemical approach with fucoidan

Urinary supersaturation-induced crystal formation has been attributed as one of the key factor for the pathogenesis/progression of lithogenesis. This study was aimed at investigating whether fucoidan, a naturally occurring sulfated glycosaminoglycan, could ameliorate the biochemical changes in urine induced by stone formation. Two groups of male albino Wistar rats (120+/-20 g) received 0.75% ethylene glycol (EG) for 28 days to induce hyperoxaluria, and one of them received sulfated polysaccharides (fucoidan from Fucus vesiculosus, 5 mg kg(-1), s.c.), commencing from the 8(th) day of the experimental period. One group was maintained as normal control group and another group served as drug control, which received sulfated polysaccharides. The urine collected from all the groups was analysed for changes in pH, volume, oxalate, calcium, phosphorus, uric acid, magnesium, citric acid and glycosaminoglycans. Urinary crystals were analysed with a light microscope. Renal tissues were studied under polarized light for deposition of crystals and also analysed for their oxalate and calcium content. The changes in extracellular matrix on crystal deposition were also evaluated. The urinary pH and volume were altered in rats treated with EG along with an increase in weight of the kidney. Further, administration of EG to rats increased the supersaturation of urine by escalating the levels of the stone-forming constituents, such as oxalate, calcium, phosphorus and uric acid, which was completely restored by fucoidan treatment. The decrease in the inhibitors, like citrate, magnesium and glycosaminoglycans, in urine was prevented by the co-treatment with fucoidan. In hyperoxaluric rats, there was an increased excretion of calcium oxalate monohydrate crystals in urine along with crystal deposition in renal tissues; this was prevented by fucoidan treatment. Fucoidan administration reversed even the tissue levels of calcium and oxalate. The increased accumulation of collagen and expression of transforming growth factor-beta(1) in hyperoxaluria was normalized on fucoidan administration. These results suggest that the physico-chemical alterations in urine produced during hyperoxaluria can be reversed by fucoidan administration.     

Antimicrobial effects of a new therapeutic liquid dentifrice formulation on oral bacteria including odorigenic species.

The control of oral malodor is well-recognized in efforts to improve oral health. Antimicrobial formulations can mitigate oral malodor, however, procedures to assess effects on oral bacteria including those implicated in halitosis are unavailable. This investigation examined the antimicrobial effects of a new liquid triclosan/copolymer dentifrice (test) formulation that demonstrated significant inhibition of oral malodor in previous organoleptic clinical studies. Procedures compared antimicrobial effects of the test and control formulations on a range of oral micro-organisms including members implicated in halitosis, substantive antimicrobial effects of formulations with hydroxyapatite as a surrogate for human teeth and ex vivo effects on oral bacteria from human volunteers. With Actinomyces viscosus, as a model system, the test formulation demonstrated a dose-dependent effect. At these concentrations the test formulation provided significant antimicrobial effects on 13 strains of oral bacteria including those implicated in bad breath at selected posttreatment time points. Treatment of hydroxyapatite by the test dentifrice resulted in a significant and substantive antimicrobial effect vs. controls. Oral bacteria from subjects treated ex vivo with the test dentifrice resulted in significant reductions in cultivable oral bacteria and odorigenic bacteria producing hydrogen sulfide. In summary, microbiological methods adapted to study odorigenic bacteria demonstrate the significant antimicrobial effects of the test (triclosan/copolymer) dentifrice with laboratory and clinical strains of oral bacteria implicated in bad breath.     

Oleandrin induces apoptosis in human, but not in murine cells: dephosphorylation of Akt, expression of FasL, and alteration of membrane fluidity

Common practice to evaluate the efficacy of any compound as drug is done in cell-based in vitro system followed by in vivo murine model prior to clinical trial in human. Cardiac glycosides are very effective to kill human cells, but not murine cells. In this report, we describe the comparative molecular mechanism of oleandrin, a cardiac glycoside action in human and murine cells. Treatment with oleandrin facilitated nuclear translocation of FKHR in human, but not murine cells by dephosphorylating Akt. It activated MAPK and JNK in human, but not in murine cells and also induced expression of FasL leads to apoptosis in human cells as detected by assaying caspases activation, PARP cleavage, nuclear fragmentation, and annexin staining. Oleandrin interacted with human plasma membrane as evaluated by HPLC, altered its fluidity as detected by DPH binding, inhibited Na+/K+-ATPase activity, and increased intracellular free Ca2+ level followed by calcineurin activity only in human, but not in murine cells. Results suggest that human plasma membrane might be different than murine, which interact with oleandrin that disturb Na+/K+-ATPase pump resulting in the calcification followed by induction of Ca2+-dependent cellular responses such as apoptosis.     

Cytotoxicity and genotoxicity assessment of Euphorbia hirta in MCF-7 cell line model using comet assay

Objective

To evaluate the cytotoxicity and genotoxicity activity of Euphorbia hirta (E. hirta) in MCF-7 cell line model using comet assay.

Methods

The cytotoxicity of E. hirta extract was investigated by employing brine shrimp lethality assay and the genotoxicity of E. hirta was assessed by using Comet assay.

Results

Both toxicity tests exhibited significant toxicity result. In the comet assay, the E. hirta extract exhibited genotoxicity effects against MCF-7 DNA in a time-dependent manner by increasing mean percentage of DNA damage. The extract of E. hirta showed significant toxicity against brine shrimp with an LC₅₀ value of 620.382 µg/mL (24 h). Comparison with positive control potassium dichromate signifies that cytotoxicity exhibited by the methanol extract might have moderate activity.

Conclusion

The present work confirmed the cytotoxicity and genotoxicity of E. hirta. However, the observed toxicity of E. hirta extracts needs to be confirmed in additional studies.     

A sero-epidemiologic study of a water-borne epidemic of viral hepatitis in Kolhapur City, India.

Between February and March 1981 an epidemic of hepatitis occurred in Kolhapur City, Maharashtra State, India. Approximately 1200 cases of jaundice were reported; more than 300 patients were hospitalized and three died. The epidemiological investigations showed a distinct concentration of cases in municipal ward E of the city. Investigations of the ward E water supply system disclosed gross contamination of raw water with sewage at source. The serological studies revealed that the aetiological agent responsible for this epidemic was neither hepatitis A virus nor hepatitis B virus but was likely to have been due to a non-A, non-B hepatitis virus.