Wind and boundary layers in Rayleigh-Benard convection. I: analysis and modeling

The aim of this paper is to contribute to the understanding and to model the processes controlling the amplitude of the wind of Rayleigh-Benard convection. We analyze results from direct simulation of an L/H = 4 aspect-ratio domain with periodic sidewalls at Ra = 1e5; 1e6; 1e7; 1e8 and at Pr = 1 by decomposing independent realizations into wind and fluctuations. It is shown that deep inside the thermal boundary layer, horizontal heat-fuxes exceed the average vertical heat-fux by a factor 3 due to the interaction between the wind and the mean temperature field. These large horizontal heat-fluxes are responsible for spatial temperature differences that drive the wind by creating pressure gradients. The wall fluxes and turbulent mixing in the bulk provide damping. Using the DNS results to parameterise the unclosed terms, a simple model capturing the essential processes governing the wind structure is derived. The model consists of two coupled differential equations for wind velocity and temperature amplitude. The equations indicate that the formation of a wind structure is inevitable due to the positive feedback resulting from the interaction between the wind and temperature field. Furthermore, the wind velocity is largely determined by the turbulence in the bulk rather than by the wall-shear stress. The model reproduces the Ra dependence of wind Reynolds number and temperature amplitude

A simple eddy viscosity formulation for turbulent boundary layers near smooth walls

The aim of this study is to improve the prediction of near-wall mean streamwise velocity profile $U^+$ by using a simple method. The $U^+$ profile is obtained by solving the momentum equation which is written as an ordinary differential equation. An eddy viscosity formulation based on a near-wall turbulent kinetic energy $k^+$ function (R. Absi, Analytical solutions for the modeled $k$-equation, ASME J. Appl. Mech. \textbf{75}, 044501, 2008) and the van Driest mixing length equation (E.R. van Driest, On turbulent flow near a wall, J. Aero. Sci. \textbf{23}, 1007, 1956) is used. The parameters obtained from the $k^+$ profiles are used for the computation of $U^+$ (variables with the superscript of + are those nondimensionalized by the wall friction velocity $u_\tau$ and the kinematic viscosity $\nu$). Comparisons with DNS data of fully-developed turbulent channel flows for $109 < Re_{\tau} < 2003$ show good agreement (where $Re_{\tau}$ denotes the friction Reynolds number defined by $u_\tau$, $\nu$ and the channel half-width $\delta$)

Spectral analysis of boundary layers in Rayleigh-Benard convection

A combined experimental and numerical study of the boundary layer in a 4:1 aspect-ratio Rayleigh-B\'{e}nard cell over a four-decade range of Rayleigh numbers has been undertaken aimed at gaining a better insight into the character of the boundary layers. The experiments involved the simultaneous Laser Doppler Anemometry (LDA) measurements of fluid velocity at two locations, i.e. in the boundary layer and far away from it in the bulk, for Rayleigh numbers varying between $1.6 \times 10^7$ and $2.4 \times 10^9$. In parallel, direct numerical simulations (DNS) have been performed for the same configuration for Rayleigh numbers between $7.0 \times 10^4$ and $7.7 \times 10^7$. The temperature and velocity probability density functions and the power spectra of the horizontal velocity fluctuations measured in the boundary layer and in the bulk flow are found to be practically identical. Except for the smallest Rayleigh numbers, the spectra in the boundary layer and in the bulk central region are continuous and have a wide range of active scales. This indicates that both the bulk and the boundary layers are turbulent in the $\textrm{Ra}$ number range considered. However, molecular effects can still be observed and the boundary layer does not behave like a classical shear-driven turbulent boundary layer.Comment: 10 pages, 6 figures, Accepted for publication in Phys. Rev.

Wind and boundary layers in Rayleigh-Benard convection. Part 2: boundary layer character and scaling

The effect of the wind of Rayleigh-Benard convection on the boundary layers is studied by direct numerical simulation of an L/H=4 aspect-ratio domain with periodic side boundary conditions for Ra={10^5, 10^6, 10^7} and Pr=1. It is shown that the kinetic boundary layers on the top- and bottom plate have some features of both laminar and turbulent boundary layers. A continuous spectrum, as well as significant forcing due to Reynolds stresses indicates undoubtedly a turbulent character, whereas the classical integral boundary layer parameters -- the shape factor and friction factor (the latter is shown to be dominated by the pressure gradient) -- scale with Reynolds number more akin to laminar boundary layers. This apparent dual behavior is caused by the large influence of plumes impinging onto and detaching from the boundary layer. The plume-generated Reynolds stresses have a negligible effect on the friction factor at the Rayleigh numbers we consider, which indicates that they are passive with respect to momentum transfer in the wall-parallel direction. However, the effect of Reynolds stresses cannot be neglected for the thickness of the kinetic boundary layer. Using a conceptual wind model, we find that the friction factor C_f should scale proportional to the thermal boundary layer thickness as C_f ~ lambda_Theta, while the kinetic boundary layer thickness lambda_u scales inversely proportional to the thermal boundary layer thickness and wind Reynolds number lambda_u ~ lambda_Theta^{-1} Re^{-1}. The predicted trends for C_f and \lambda_u are in agreement with DNS results

Urinary activity of N-acetyl-beta-D-glucosaminidase in workers exposed to the effects of mercury

U skupini 100 ispitanika koji rade u pogonu elektrolize određivana je aktivnost urinarne N-acetil-beta-D-glukozaminidaze i koncentracija žive u urinu. Utvrđeno je da je pomenuta enzimska aktivnost oko dva puta veća u usporedbi sa vrijednostima koje su određene u 100 ispitanika iz kontrolne skupine. Ta je razlika bila statistički značajna, ali nije utvrđeno postojanje pozitivne korelacije između aktivnosti urinarne N-acetil-beta-D-glukozarninidaze i urinarne koncentracije žive. Diskutirano je o mogućnosti primjene ovog biokemijsko-toksikološkog testa u profesionalnoj toksikologiji, odnosno medicini rada.Mercury concentration in urine and urinary activity of N-acetyl-beta-D-gfucosaminidase were determined in a group of 100 workers from an electrolysis workshop. The enzyme activities measured were twice as high as those of 100 control subjects. The difference was statistically significant. There was no correlation between urinary activity of N-acetyl-beta-D-glucosaminidase and mercury concentration in urine. The applicability of this biochemical-toxicological test in occupational health practice is discussed

Status of the BOLD reference library of DNA barcodes of caddisflies (Insecta: Trichoptera) from the Western Balkans

Background and purpose: Available data in research literature suggest that the Western Balkan countries hold a rich diversity of caddisflies. Assessment and biomonitoring of such rich diversity could be facilitated through DNA-based high-throughput approaches like DNA metabarcoding that depend on the availability of comprehensive reference libraries. Materials and methods: We assessed the status of the COI barcode sequence data for a total of 112 caddisflies species in the investigated region by determining the gaps in representative sequences in the Barcode of Life Data System (BOLD) and examining the accuracy of available records using the Barcode, Audit and Grade System (BAGS). Results: Results revealed a considerable underrepresentation of surveyed geographic region in BOLD records for the target insect group. Moreover, the large majority of the species records were rated “discordant” (72.80% grade E), and only 15.20% were classified as “consolidated concordance or basal concordance” (3.20% grade A and 12.00% B). Approximately 3.20% of the records pertaining to species occurring in multiple BINs (Barcode Index Number) and 8.80% were poorly represented (i.e., less than three specimens, grade D). A fraction of the species graded discordant were deemed concordant after detailed inspection of individual data, decreasing by 14.07%. Conclusions: The assessment of the current state of BOLD entries indicated that DNA barcoding is still not widely applied in Albania, Bosnia and Herzegovina, Montenegro, North Macedonia, Serbia, and Slovenia, emphasizing that Croatia has the most barcoded caddisflies species. The finding that available BOLD Trichopteran records for investigated countries were mainly graded as “discordant” indicates the need for better quality control of reference libraries

Urinary cotinine as marker of passive tobacco smoking

Radi objektivizacije opasnosti pušenja roditelja po zdravlje njihove djece u 205 učenika oba spola, dobi 10-12 godina određena je, kolorimetrijskom metodom sa barbiturnom kiselinom (OBA), koncentracija kotinina u urinu. Dobiveni rezultati su korelirani sa podacima dobivenim iz ankete. Utvrđena je signifikantna razlika između srednje vrijednosti koncentracije kotinina za djecu roditelja nepušača (3,2 µmol/L) i djece u kojih puši jedan roditelj (5,8 µmol/L). Koncentracija kotinina je još veća ako puše oba roditelja (7,8 µmol/L) i najveća u djece, pasivnih pušača, koji nemaju posebnu sobu za učenje i spavanje, a oba su im roditelja pušači (9,2 µmol/L). Nije dobivena statistički značajna razlika u koncentracijama kotinina u djevojčica i dječaka.To provide an objective measure of the hazard smoking parents represent to their children\u27s health, cotinine concentration in urine was measured by the colorimetric method using barbituric acid (DBA). A total of 205 children, aged 10-12, were examined. The results of laboratory tests were correlated with the data collected by interview. A significant difference in the average value of cotinine concentration was demonstrated between the children whose parents did not smoke (3.2 µmol/L) and chose whose one parent smoked (5.8 µmol/L). An even larger concentration was recorded when both parents smoked (7.8 µmol/L). The largest cotinine concentration was determined in the urine of children - passive smokers whose both parents smoked and who did not have a room of their own (9.2 µmol/L). The difference in cotinine concentration between girls and boys was not statistically significant

Determination of an efficient and reliable method for PCR detection of borrelial DNA from engorged ticks

Since ticks have been recognized as one of the most important vectors of pathogens causing serious diseases in humans, a number of studies have focused on identifying the pathogen composition as well as transmission and infection mechanisms. Although a plethora of detection methods is available today, PCR-based approach is regarded as the most sensitive and rapid. However, common challenges in molecular analyses conducted on ticks are weak amplification results because of present inhibitors, either from a mammalian bloodmeal or a male DNA in female reproductive organs. Present study aimed to evaluate which body part of an engorged tick is the most preferable as a starting material in DNA extraction for molecular detection of Borrelia burgdorferi sensu lato, causative agent of Lyme borreliosis. We analyzed 58 Ixodes ricinus ticks removed from patients in The Center for Emergency Medical Assistance of the Sarajevo Canton, Bosnia and Herzegovina. Our findings suggest using the anterior half of semi-engorged and fully-engorged ticks for DNA extraction with the purpose of Borrelia detection

Scientometrics and academia

The social significance and quality of every human activity are proportional to its usefulness to the social community. Science belongs to the very top of the processes and events in the history of humankind that strongly influenced the development of society, which over time transformed it and contributed to the common good. Science produced new knowledge that made it possible for billions of people to rise out of poverty, develop industrialization and mass communication, eradicate many dangerous diseases for humankind, and enable humans to leave their footprints on the moon. Science is a human activity that produces new knowledge presented through innovations, patents, and publications, aimed at solving the problems facing humanity. Read more in the PDF.