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

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

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

Assessment of Higher-Order RANS Closures in a Decelerated Planar Wall-Bounded Turbulent Flow

A reference DNS database is presented, which includes third- and fourth-order moment budgets for unstrained and strained planar channel flow. Existing RANS closure models for third- and fourth-order terms are surveyed, and new model ideas are introduced. The various models are then compared with the DNS data term by term using a priori testing of the higher-order budgets of turbulence transport, velocity-pressure-gradient, and dissipation for both the unstrained and strained databases. Generally, the models for the velocity-pressure-gradient terms are most in need of improvement

A non-hybrid method for the PDF equations of turbulent flows on unstructured grids

In probability density function (PDF) methods of turbulent flows, the joint PDF of several flow variables is computed by numerically integrating a system of stochastic differential equations for Lagrangian particles. A set of parallel algorithms is proposed to provide an efficient solution of the PDF transport equation, modeling the joint PDF of turbulent velocity, frequency and concentration of a passive scalar in geometrically complex configurations. An unstructured Eulerian grid is employed to extract Eulerian statistics, to solve for quantities represented at fixed locations of the domain (e.g. the mean pressure) and to track particles. All three aspects regarding the grid make use of the finite element method (FEM) employing the simplest linear FEM shape functions. To model the small-scale mixing of the transported scalar, the interaction by exchange with the conditional mean model is adopted. An adaptive algorithm that computes the velocity-conditioned scalar mean is proposed that homogenizes the statistical error over the sample space with no assumption on the shape of the underlying velocity PDF. Compared to other hybrid particle-in-cell approaches for the PDF equations, the current methodology is consistent without the need for consistency conditions. The algorithm is tested by computing the dispersion of passive scalars released from concentrated sources in two different turbulent flows: the fully developed turbulent channel flow and a street canyon (or cavity) flow. Algorithmic details on estimating conditional and unconditional statistics, particle tracking and particle-number control are presented in detail. Relevant aspects of performance and parallelism on cache-based shared memory machines are discussed.Comment: Accepted in Journal of Computational Physics, Feb. 20, 200