scholarly journals Concentration Fluctuations from Localized Atmospheric Releases

2020 ◽  
Vol 177 (2-3) ◽  
pp. 461-510
Author(s):  
Massimo Cassiani ◽  
Matteo B. Bertagni ◽  
Massimo Marro ◽  
Pietro Salizzoni
Keyword(s):  
Transport Equation ◽  
Point Sources ◽  
Concentration Fluctuations ◽  
Analytical Models ◽  
Particle Methods ◽  
Navier Stokes ◽  
Numerical Resolution ◽  
Concentration Time Series ◽  
Atmospheric Releases ◽  
Analytical Relations

Abstract We review the efforts made by the scientific community in more than seventy years to elucidate the behaviour of concentration fluctuations arising from localized atmospheric releases of dynamically passive and non-reactive scalars. Concentration fluctuations are relevant in many fields including the evaluation of toxicity, flammability, and odour nuisance. Characterizing concentration fluctuations requires not just the mean concentration but also at least the variance of the concentration in the location of interest. However, for most purposes the characterization of the concentration fluctuations requires knowledge of the concentration probability density function (PDF) in the point of interest and even the time evolution of the concentration. We firstly review the experimental works made both in the field and in the laboratory, and cover both point sources and line sources. Regarding modelling approaches, we cover analytical, semi-analytical, and numerical methods. For clarity of presentation we subdivide the models in two groups, models linked to a transport equation, which usually require a numerical resolution, and models mainly based on phenomenological aspects of dispersion, often providing analytical or semi-analytical relations. The former group includes: large-eddy simulations, Reynolds-averaged Navier–Stokes methods, two-particle Lagrangian stochastic models, PDF transport equation methods, and heuristic Lagrangian single-particle methods. The latter group includes: fluctuating plume models, semi-empirical models for the concentration moments, analytical models for the concentration PDF, and concentration time-series models. We close the review with a brief discussion highlighting possible useful additions to experiments and improvements to models.

Wave Impact Loads on the Bottom of Flat Decks

2021 ◽  
Author(s):  
Daniel de Oliveira Costa ◽  
Julia Araújo Perim ◽  
Bruno Guedes Camargo ◽  
Joel Sena Sales Junior ◽  
Antonio Carlos Fernandes ◽  
...  
Keyword(s):  
Scale Effects ◽  
Offshore Structures ◽  
Model Tests ◽  
Analytical Models ◽  
Navier Stokes ◽  
Impact Loads ◽  
Wave Impact ◽  
Wave Channel ◽  
Waves And Currents ◽  
The Impact

Abstract Slamming events due to wave impact on the underside of decks might lead to severe and potentially harmful local and/or global loads in offshore structures. The strong nonlinearities during the impact require a robust method for accessing the loads and hinder the use of analytical models. The use of computation fluid dynamics (CFD) is an interesting alternative to estimate the impact loads, but validation through experimental data is still essential. The present work focuses on a flat-bottomed model fixed over the mean free surface level submitted to regular incoming waves. The proposal is to reproduce previous studies through CFD and model tests in a different reduced scale to provide extra validation and to identify possible non-potential scale effects such as air compressibility. Numerical simulations are performed in both experiments’ scales. The numerical analysis is performed with a marine dedicated flow solver, FINE™/Marine from NUMECA, which features an unsteady Reynolds-averaged Navier-Stokes (URANS) solver and a finite volume method to build spatial discretization. The multiphase flow is represented through the Volume of Fluid (VOF) method for incompressible and nonmiscible fluids. The new model tests were performed at the wave channel of the Laboratory of Waves and Currents (LOC/COPPE – UFRJ), at the Federal University of Rio de Janeiro.

scholarly journals Stellar mass spectrum within massive collapsing clumps

2018 ◽  
Vol 611 ◽  
pp. A89 ◽  
Author(s):  
Yueh-Ning Lee ◽  
Patrick Hennebelle
Keyword(s):  
Large Scale ◽  
Equations Of State ◽  
Initial Conditions ◽  
Peak Position ◽  
Density Fluctuations ◽  
Initial Mass Function ◽  
Analytical Models ◽  
Numerical Resolution ◽  
Tidal Forces ◽  
The Imf

Context. Understanding the origin of the initial mass function (IMF) of stars is a major problem for the star formation process and beyond. Aim. We investigate the dependence of the peak of the IMF on the physics of the so-called first Larson core, which corresponds to the point where the dust becomes opaque to its own radiation. Methods. We performed numerical simulations of collapsing clouds of 1000 M⊙ for various gas equations of state (eos), paying great attention to the numerical resolution and convergence. The initial conditions of these numerical experiments are varied in the companion paper. We also develop analytical models that we compare to our numerical results. Results. When an isothermal eos is used, we show that the peak of the IMF shifts to lower masses with improved numerical resolution. When an adiabatic eos is employed, numerical convergence is obtained. The peak position varies with the eos, and using an analytical model to infer the mass of the first Larson core, we find that the peak position is about ten times its value. By analyzing the stability of nonlinear density fluctuations in the vicinity of a point mass and then summing over a reasonable density distribution, we find that tidal forces exert a strong stabilizing effect and likely lead to a preferential mass several times higher than that of the first Larson core. Conclusions. We propose that in a sufficiently massive and cold cloud, the peak of the IMF is determined by the thermodynamics of the high-density adiabatic gas as well as the stabilizing influence of tidal forces. The resulting characteristic mass is about ten times the mass of the first Larson core, which altogether leads to a few tenths of solar masses. Since these processes are not related to the large-scale physical conditions and to the environment, our results suggest a possible explanation for the apparent universality of the peak of the IMF.

From Navier–Stokes to Stokes by means of particle methods

2006 ◽  
Vol 214 (1) ◽  
pp. 264-283 ◽  
Author(s):  
Anthony Beaudoin ◽  
Serge Huberson ◽  
Elie Rivoalen
Keyword(s):  
Particle Methods ◽  
Navier Stokes

Models of non-Boussinesq lock-exchange flow

2011 ◽  
Vol 675 ◽  
pp. 1-26 ◽  
Author(s):  
R. ROTUNNO ◽  
J. B. KLEMP ◽  
G. H. BRYAN ◽  
D. J. MURAKI
Keyword(s):  
Free Boundary ◽  
Shallow Water ◽  
Shallow Water Equations ◽  
Numerical Solutions ◽  
Stokes Equations ◽  
Water System ◽  
Analytic Solutions ◽  
Analytical Models ◽  
Navier Stokes ◽  
Exchange Flow

Nearly all analytical models of lock-exchange flow are based on the shallow-water approximation. Since the latter approximation fails at the leading edges of the mutually intruding fluids of lock-exchange flow, solutions to the shallow-water equations can be obtained only through the specification of front conditions. In the present paper, analytic solutions to the shallow-water equations for non-Boussinesq lock-exchange flow are given for front conditions deriving from free-boundary arguments. Analytic solutions are also derived for other proposed front conditions – conditions which appear to the shallow-water system as forced boundary conditions. Both solutions to the shallow-water equations are compared with the numerical solutions of the Navier–Stokes equations and a mixture of successes and failures is recorded. The apparent success of some aspects of the forced solutions of the shallow-water equations, together with the fact that in a real fluid the density interface is a free boundary, shows the need for an improved theory of lock-exchange flow taking into account non-hydrostatic effects for density interfaces intersecting rigid boundaries.

scholarly journals On singular limits arising in the scale analysis of stratified fluid flows

2016 ◽  
Vol 26 (03) ◽  
pp. 419-443 ◽  
Author(s):  
Eduard Feireisl ◽  
Rupert Klein ◽  
Antonín Novotný ◽  
Ewelina Zatorska
Keyword(s):  
Transport Equation ◽  
Stokes System ◽  
Stokes Equations ◽  
Potential Temperature ◽  
Fluid Flows ◽  
Navier Stokes ◽  
Flat Torus ◽  
Navier Stokes Equations ◽  
Singular Limits ◽  
Order Variation

We study the low Mach low Freude numbers limit in the compressible Navier–Stokes equations and the transport equation for evolution of an entropy variable — the potential temperature [Formula: see text]. We consider the case of well-prepared initial data on “flat” torus and Reynolds number tending to infinity, and the case of ill-prepared data on an infinite slab. In both cases, we show that the weak solutions to the primitive system converge to the solution to the anelastic Navier–Stokes system and the transport equation for the second-order variation of [Formula: see text].

Calculation of Transition in Adverse Pressure Gradient Flow by Conditioned Equations

1996 ◽  
Author(s):  
J. Steelant ◽  
E. Dick
Keyword(s):  
Pressure Gradient ◽  
Transport Equation ◽  
Stokes Equations ◽  
Growth Parameter ◽  
Gradient Flow ◽  
Leading Edge ◽  
Adverse Pressure Gradient ◽  
Transitional Flow ◽  
Navier Stokes ◽  
Navier Stokes Equations

Conditionally averaged Navier-Stokes equations are used to describe transitional flow in adverse pressure gradient combined with a transport equation for the intermittency factor γ. A transport equation developped in earlier work has been modified to eliminate the use of a distance along a streamline. An extension of the correlations is proposed to determine the spot growth parameter in adverse pressure gradient. This approach is verified against flows over a flat plate with an elliptical leading edge.

Concentration fluctuations and fluxes in plumes from point sources in a turbulent boundary layer

1982 ◽  
Vol 117 ◽  
pp. 1-26 ◽  
Author(s):  
J. E. Fackrell ◽  
A. G. Robins
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Near Field ◽  
Ground Level ◽  
Turbulent Fluxes ◽  
Point Sources ◽  
Concentration Fluctuations ◽  
Vertical Diffusion ◽  
Eddy Diffusivities ◽  
Concentration Probability

Measurements have been made of concentration fluctuations and turbulent fluxes for two passive plumes from an elevated and a ground-level source in a turbulent boundary layer. For the concentration fluctuations, results are presented for the variance, the intermittency, peak values of concentration, probability-density functions and spectra. The balance of terms in the variance transport equation is examined, as is the overall level of fluctuations along the plume. It is shown that most of the production of fluctuations occurs very near the source. Then, the level of fluctuation decays, roughly in accordance with a balance between advection and dissipation. For the turbulent fluxes of concentration, results are presented for the vertical and lateral fluxes, with the associated behaviour of the vertical and lateral eddy diffusivities. The balance of terms in the transport equations for the fluxes is examined. The essential differences between vertical diffusion from ground-level and elevated sources and between near-field and far-field behaviour are shown to be due to the relative importance of the advection and diffusion terms in these equations.

Sign in / Sign up

Export Citation Format

Share Document