scholarly journals A Study of Cloud Mixing and Evolution Using PDF Methods. Part I: Cloud Front Propagation and Evaporation

2006 ◽  
Vol 63 (11) ◽  
pp. 2848-2864 ◽  
Author(s):  
Christopher A. Jeffery ◽  
Jon M. Reisner
Keyword(s):  
Time Scale ◽  
Mixing Time ◽  
A Priori ◽  
Front Propagation ◽  
Eddy Diffusivity ◽  
Droplet Radius ◽  
Size Spectrum ◽  
Evaporation Time ◽  
Probability Density Function Pdf ◽  
Simple Dependence

Abstract The evolution of mean relative humidity (RH) is studied in an isobaric system of clear and cloudy air mixed by an incompressible velocity field. A constant droplet radius assumption is employed that implies a simple dependence of the mixing time scale, τeddy, and the reaction (evaporation) time scale, τreact, on the specifics of the droplet size spectrum. A dilemma is found in the RH e-folding time, τefold, predicted by two common microphysical schemes: models that resolve supersaturation and ignore subgrid correlations, which gives τefold ∼ τreact, and PDF schemes that assume instantaneous evaporation and predict τefold ∼ τeddy. The resolution of this dilemma, Magnussen and Hjertager’s eddy dissipation concept (EDC) model τefold ∼ max(τeddy, τreact), is revealed in the results of 1D eddy diffusivity simulations and a new probability density function (PDF) approach to cloud mixing and evolution in which evaporation is explicitly resolved and the shape of the PDF is not specified a priori. The EDC model is shown to exactly solve the nonturbulent problem of spurious production of cloud-edge supersaturations described by Stevens et al. and produce good results in the more general turbulent case.

MMC-LES of a syngas mixing layer using an anisotropic mixing time scale model

2018 ◽  
Vol 189 ◽  
pp. 311-314 ◽  
Author(s):  
Son Vo ◽  
Andreas Kronenburg ◽  
Oliver T. Stein ◽  
Matthew J. Cleary
Keyword(s):  
Time Scale ◽  
Mixing Time ◽  
Mixing Layer ◽  
Scale Model ◽  
Mixing Time Scale

scholarly journals Broadening of Cloud Droplet Size Spectra by Stochastic Condensation: Effects of Mean Updraft Velocity and CCN Activation

2018 ◽  
Vol 75 (2) ◽  
pp. 451-467 ◽  
Author(s):  
Gaetano Sardina ◽  
Stéphane Poulain ◽  
Luca Brandt ◽  
Rodrigo Caballero
Keyword(s):  
Chemical Composition ◽  
Droplet Size ◽  
Isotropic Turbulence ◽  
Cloud Condensation Nuclei ◽  
Collision Probability ◽  
Droplet Radius ◽  
Droplet Growth ◽  
Size Spectrum ◽  
Similar Reduction ◽  
Size Spectra

Abstract The authors study the condensational growth of cloud droplets in homogeneous isotropic turbulence by means of a large-eddy simulation (LES) approach. The authors investigate the role of a mean updraft velocity and of the chemical composition of the cloud condensation nuclei (CCN) on droplet growth. The results show that a mean constant updraft velocity superimposed onto a turbulent field reduces the broadening of the droplet size spectra induced by the turbulent fluctuations alone. Extending the authors’ previous results regarding stochastic condensation, the authors introduce a new theoretical estimation of the droplet size spectrum broadening that accounts for this updraft velocity effect. A similar reduction of the spectra broadening is observed when the droplets reach their critical size, which depends on the chemical composition of CCN. The analysis of the square of the droplet radius distribution, proportional to the droplet surface, shows that for large particles the distribution is purely Gaussian, while it becomes strongly non-Gaussian for smaller particles, with the left tail characterized by a peak around the haze activation radius. This kind of distribution can significantly affect the later stages of the droplet growth involving turbulent collisions, since the collision probability kernel depends on the droplet size, implying the need for new specific closure models to capture this effect.

Particle Collision Effect on Turbulent Prandtl Number in Gas-Solid Flows

2006 ◽  
Author(s):  
Zohreh Mansoori ◽  
Majid Saffar-Avval ◽  
Hasan Basirat-Tabrizi ◽  
Goodarz Ahmadi ◽  
Payam Ramezani
Keyword(s):  
Prandtl Number ◽  
Turbulent Flow ◽  
Time Scale ◽  
Turbulence Models ◽  
Flow Region ◽  
Eddy Diffusivity ◽  
Particle Collision ◽  
Turbulent Prandtl Number ◽  
Particle Collisions ◽  
Particle Properties

Traditional gas-solid turbulence models using constant or the single-phase gas turbulent Prandtl number cause error in the thermal eddy diffusivity and thermal turbulent intensity fields calculation. The thermo-mechanical turbulence model is based on solving the hydrodynamic transport equations of the turbulent kinetic energy and turbulent time scale, beside the thermal turbulent equations of temperature variance and thermal turbulence time scale. This model has the ability to calculate the turbulent Prandtl number directly by computing the eddy viscosity and the thermal eddy diffusivity through the values of turbulence fluctuation velocity and thermal variances and time scales. A four way Eulerian/Lagrangian formulation was used to study the effect of particle properties on the turbulent flow and thermal fields, as well as on turbulent Prandtl number in a gas-solid developing pipe flow. Inter-particle collisions were included and the Lagrangian trajectory analysis was used. The earlier results showed that turbulent Prandtl number is influenced by the variations of gas and particle properties and also inter-particle collisions in a fully-developed riser. In the current study, the developing gas-solid flow region in a pipe was considered and the variation of turbulent flow field due to inter-particle collision was evaluated.

Estimation and Use of the Radiation Distribution Factor Median for Predicting Uncertainty in the Monte Carlo Ray-Trace Method

2019 ◽  
Vol 141 (6) ◽  
Author(s):  
Mehran Yarahmadi ◽  
J. Robert Mahan ◽  
Kory J. Priestley
Keyword(s):  
Heat Transfer ◽  
Monte Carlo ◽  
A Priori ◽  
Heat Transfer Analysis ◽  
Distribution Factor ◽  
Recent Contribution ◽  
Radiation Distribution ◽  
Radiation Behavior ◽  
Ray Trace ◽  
Probability Density Function Pdf

In a recent contribution, the authors show that the uncertainty in heat transfer results obtained using the Monte Carlo ray-trace (MCRT) method is related to the median of the radiation distribution factor probability density function (PDF). The value of this discovery would be significantly enhanced if the median could be known a priori without first computing the distribution factors. This would allow the user to determine the number of rays required to achieve the desired accuracy of a subsequent heat transfer analysis. The current contribution presents a correlation for the median of the distribution factor PDF as a function of emissivity and the number of surface elements defining an enclosure. The correlation involves a single parameter whose value is unique for a given enclosure geometry. We find that the radiation behavior of a given enclosure can be classified on a scale ranging from reflection-dominated to geometry-dominated. The correlation is shown to work well for reflection-dominated enclosures but less well for geometry-dominated enclosures.

Subgrid Scale Modeling of Turbulence for the Dynamic Procedure Using Finite Difference Method and Its Assessment on the Thermally Stratified Turbulent Channel Flow

2005 ◽  
Vol 73 (3) ◽  
pp. 382-390 ◽  
Author(s):  
Makoto Tsubokura
Keyword(s):  
Heat Flux ◽  
Finite Difference ◽  
Channel Flow ◽  
A Priori ◽  
Turbulent Channel Flow ◽  
Eddy Diffusivity ◽  
Subgrid Scale ◽  
Eddy Simulation ◽  
Scale Modeling ◽  
Subgrid Scale Modeling

Previously proposed methods for subgrid-scale (SGS) stress modeling were re-investigated and extended to SGS heat-flux modeling, and various anisotropic and isotropic eddy viscosity/diffusivity models were obtained. On the assumption that they are used in a finite-difference (FD) simulation, the models were constructed in such a way that they are insensitive to numerical parameters on which calculated flows are strongly dependent in the conventional Smagorinsky model. The models obtained, as well as those previously proposed, were evaluated a priori in a stably stratified open channel flow, which is considered to be a challenging application of large eddy simulation and suitable for testing both SGS stress and heat-flux models. The most important feature of the models proposed is that they are insensitive to the discretized test filtering parameter required in the dynamic procedure of Germano et al. (1991, Phys. Fluids, 3, pp. 1760–1765) in FD simulation. We also found in SGS heat-flux modeling that the effect of the grid (resolved)-scale (GS) velocity gradient plays an important role in the estimation of the streamwise heat flux, and an isotropic eddy diffusivity model with the effect of the GS velocity is proposed.

Diffusive–Nondiffusive Flux Decompositions in Atmospheric Boundary Layers

2020 ◽  
Vol 77 (10) ◽  
pp. 3479-3494
Author(s):  
Tomas Chor ◽  
James C. McWilliams ◽  
Marcelo Chamecki
Keyword(s):  
Boundary Layers ◽  
A Priori ◽  
Eddy Diffusivity ◽  
Turbulent Fluxes ◽  
Atmospheric Boundary Layers ◽  
Convective Boundary ◽  
Atmospheric Sciences ◽  
Diffusive Term ◽  
Novel Method ◽  
Neutral Conditions

AbstractEddy diffusivity models are a common method to parameterize turbulent fluxes in the atmospheric sciences community. However, their inability to handle convective boundary layers leads to the addition of a nondiffusive flux component (usually called nonlocal) alongside the original diffusive term (usually called local). Both components are often modeled for convective conditions based on the shape of the eddy diffusivity profile for neutral conditions. This assumption of shape is traditionally employed due to the difficulty of estimating both components based on numerically simulated turbulent fluxes without any a priori assumptions. In this manuscript we propose a novel method to avoid this issue and estimate both components from numerical simulations without having to assume any a priori shape or scaling for either. Our approach is based on optimizing results from a modeling perspective and taking as much advantage as possible from the diffusive term, thus maximizing the eddy diffusivity. We use our method to diagnostically investigate four different large-eddy simulations spanning different stability regimes, which reveal that nondiffusive fluxes are important even when trying to minimize them. Furthermore, the calculated profiles for both diffusive and nondiffusive fluxes suggest that their shapes change with stability, which is an effect that is not included in most models currently in use. Finally, we use our results to discuss modeling approaches and identify opportunities for improving current models.

Mixing Time Scale Determination in Microchannels Using Reaction Calorimetry

2019 ◽  
Vol 91 (5) ◽  
pp. 622-631 ◽  
Author(s):  
Felix Reichmann ◽  
Kai Vennemann ◽  
Timothy Aljoscha Frede ◽  
Norbert Kockmann
Keyword(s):  
Time Scale ◽  
Mixing Time ◽  
Reaction Calorimetry ◽  
Mixing Time Scale

Mixing Time Scale Measurement With Fast Exothermic Reactions Using Microchannel Reaction Calorimetry

2018 ◽  
Author(s):  
Felix Reichmann ◽  
Yannick Jirmann ◽  
Norbert Kockmann
Keyword(s):  
Heat Flux ◽  
Time Scale ◽  
Mixing Time ◽  
Main Reaction ◽  
Reaction Calorimetry ◽  
Flow Rates ◽  
Exothermic Reactions ◽  
Reaction Progress ◽  
Reaction Calorimeter ◽  
Mixing Time Scale

Continuous reaction calorimetry in microreactors is a powerful technology for the investigation of fast and exothermic reactions regarding thermokinetic data. A Seebeck element based reaction calorimeter has been designed, manufactured, and its performance has been shown in previous works using neutralization reaction in a microreactor made from PVDF-foils [1]. The Seebeck elements allow for spatial and temporal resolution of heat flux profiles across the reactor. Therefore, hot spots and regions of main reaction progress are detected. Finally, heat of reaction has been determined in good agreement with literature data [1]. However, more information can be retrieved related to chemical transformations using the continuously operated reaction calorimeter. In this work, mixing time scale is determined for instantaneous and exothermic reactions. Volumetric flow rate is varied and the region of main reaction progress is shifted within the microreactor. The reaction occurs near the reactor outlet for low flow rates. Here, mixing is dominated by diffusion. However, the reaction and hot spot are shifted towards the reactor inlet for high flow rates as convective mixing regime is reached and secondary flow profile with Dean vortices develop due to curvature of the reaction channel. Finally, mixing time scales can be derived from the location of heat flux peaks. Results display a decrease in mixing time at increased flow rates. Additionally, passive micromixers can be evaluated regarding their efficiency and comparison can be drawn. Moreover, pumps can be characterized and evaluated regarding low-pulsation dosing using the Seebeck element based reaction calorimeter.

A model for the mixing time scale of a turbulent reacting scalar

2003 ◽  
Vol 15 (6) ◽  
pp. 1375 ◽  
Author(s):  
Chong M. Cha ◽  
Philippe Trouillet
Keyword(s):  
Time Scale ◽  
Mixing Time ◽  
Mixing Time Scale
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