A novel ensemble method for retrieving properties of warm cloud in 3-D using ground-based scanning radar and zenith radiances

We present a novel method for retrieving high-resolution, three-dimensional (3-D) nonprecipitating cloud fields in both overcast and broken-cloud situations. The method uses scanning cloud radar and multiwavelength zenith radiances to obtain gridded 3-D liquid water content (LWC) and effective radius (re) and 2-D column mean droplet number concentration (Nd). By using an adaption of the ensemble Kalman filter, radiances are used to constrain the optical properties of the clouds using a forward model that employs full 3-D radiative transfer while also providing full error statistics given the uncertainty in the observations. To evaluate the new method, we first perform retrievals using synthetic measurements from a challenging cumulus cloud field produced by a large-eddy simulation snapshot. Uncertainty due to measurement error in overhead clouds is estimated at 20% in LWC and 6% in re, but the true error can be greater due to uncertainties in the assumed droplet size distribution and radiative transfer. Over the entire domain, LWC and re are retrieved with average error 0.05–0.08 g m-3 and ~2 μm, respectively, depending on the number of radiance channels used. The method is then evaluated using real data from the Atmospheric Radiation Measurement program Mobile Facility at the Azores. Two case studies are considered, one stratocumulus and one cumulus. Where available, the liquid water path retrieved directly above the observation site was found to be in good agreement with independent values obtained from microwave radiometer measurements, with an error of 20 g m-2.

Structure of the planetary boundary layer over southeast England: modeling and measurements

The Weather Research and Forecasting model was applied to analyze variations in the planetary boundary layer (PBL) structure over Southeast England including central and suburban London. The parameterizations and predictive skills of two nonlocal mixing PBL schemes, YSU and ACM2, and two local mixing PBL schemes, MYJ and MYNN2, were evaluated over a variety of stability conditions, with model predictions at a 3 km grid spacing. The PBL height predictions, which are critical for scaling turbulence and diffusion in meteorological and air quality models, show significant intra-scheme variance (> 20%), and the reasons are presented. ACM2 diagnoses the PBL height thermodynamically using the bulk Richardson number method, which leads to a good agreement with the lidar data for both unstable and stable conditions. The modeled vertical profiles in the PBL, such as wind speed, turbulent kinetic energy (TKE), and heat flux, exhibit large spreads across the PBL schemes. The TKE predicted by MYJ were found to be too small and show much less diurnal variation as compared with observations over London. MYNN2 produces better TKE predictions at low levels than MYJ, but its turbulent length scale increases with height in the upper part of the strongly convective PBL, where it should decrease. The local PBL schemes considerably underestimate the entrainment heat fluxes for convective cases. The nonlocal PBL schemes exhibit stronger mixing in the mean wind fields under convective conditions than the local PBL schemes and agree better with large-eddy simulation (LES) studies.

Joint retrievals of cloud and drizzle in marine boundary layer clouds using ground-based radar, lidar and zenith radiances

Active remote sensing of marine boundary-layer clouds is challenging as drizzle drops often dominate the observed radar reflectivity. We present a new method to simultaneously retrieve cloud and drizzle vertical profiles in drizzling boundary-layer clouds using surface-based observations of radar reflectivity, lidar attenuated backscatter, and zenith radiances under conditions when precipitation does not reach the surface. Specifically, the vertical structure of droplet size and water content of both cloud and drizzle is characterised throughout the cloud. An ensemble optimal estimation approach provides full error statistics given the uncertainty in the observations. To evaluate the new method, we first perform retrievals using synthetic measurements from large-eddy simulation snapshots of cumulus under stratocumulus, where cloud water path is retrieved with an error of 31 g m−2 . The method also performs well in non-drizzling clouds where no assumption of the cloud profile is required. We then apply the method to observations of marine stratocumulus obtained during the Atmospheric Radiation Measurement MAGIC deployment in the Northeast Pacific. Here, retrieved cloud water path agrees well with independent three-channel microwave radiometer retrievals, with a root mean square difference of 10–20 g m−2.

Langmuir turbulence and surface heating in the ocean surface boundary layer

This study uses large-eddy simulation to investigate the structure of the ocean surface boundary layer (OSBL) in the presence of Langmuir turbulence and stabilizing surface heat fluxes. The OSBL consists of a weakly stratified layer, despite a surface heat flux, above a stratified thermocline. The weakly stratified (mixed) layer is maintained by a combination of a turbulent heat flux produced by the wave-driven Stokes drift and downgradient turbulent diffusion. The scaling of turbulence statistics, such as dissipation and vertical velocity variance, is only affected by the surface heat flux through changes in the mixed layer depth. Diagnostic models are proposed for the equilibrium boundary layer and mixed layer depths in the presence of surface heating. The models are a function of the initial mixed layer depth before heating is imposed and the Langmuir stability length. In the presence of radiative heating, the models are extended to account for the depth profile of the heating.

Solução da equação de difusão-advecção para uma CLP não-homogênea e não-estacionária pelo método GILTT

Neste trabalho é apresentada a solução da equação de difusão-advecção transiente para simular a dispersão de poluentes na Camada Limite Planetária. A solução é obtida através do método analítico GILTT (Generalized Integral Laplace Transform Technique) e da técnica de inversão numérica da quadratura de Gauss. A validação da solução é comprovada utilizando as concentraçãos obtidas a partir do modelo com as obtidas experimentalmente pelo Experimento de Copenhagen. Nesta comparação foram utilizados os perfis de vento potencial e logaritmo e os parâmetros de turbulência propostos por Degrazia et al (1997) [19] e (2002) [17]. Os melhores resultados foram obtidos utilizando o perfil de vento potencial e o coeficiente de difusão propostos por Degrazia et al (1997). A influência da velocidade vertical é mostrada através do comportamento das concentrações de poluentes na pluma. Além disso, as velocidades verticais e longitudinais geradas pelo Large Eddy Simulation (LES) foram colocadas no modelo para poder simular uma camada limite turbulenta mais realística, a qual apresentou resultados satisfatórios quando comparados com os disponíveis na literatura.

Simulations of incompressible fluid flows by a least squares finite element method

In this work simulations of incompressible fluid flows have been done by a Least Squares Finite Element Method (LSFEM) using velocity-pressure-vorticity and velocity-pressure-stress formulations, named u-p-ω) and u-p-τ formulations respectively. These formulations are preferred because the resulting equations are partial differential equations of first order, which is convenient for implementation by LSFEM. The main purposes of this work are the numerical computation of laminar, transitional and turbulent fluid flows through the application of large eddy simulation (LES) methodology using the LSFEM. The Navier-Stokes equations in u-p-ω and u-p-τ formulations are filtered and the eddy viscosity model of Smagorinsky is used for modeling the sub-grid-scale stresses. Some benchmark problems are solved for validate the numerical code and the preliminary results are presented and compared with available results from the literature. Copyright © 2005 by ABCM.

Simulação numérica do escoamento de jatos impingentes axissimétricos confinados

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Simulação numérica de escoamentos de fluidos com efeitos de rotação

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Simulação de grandes escalas de escoamentos incompressíveis com transferência de calor e massa por um método de elementos finitos de subdomínio

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Simulação numérica de escoamentos de fluidos pelo método de elementos finitos baseado em volumes de controle em malhas não estruturadas

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Simulação numérica de escoamentos de fluidos pelo método de elementos finitos de mínimos quadrados

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Simulação numérica de escoamentos de fluídos pelo método de elementos finitos baseado em volumes de controle com a técnica de passo fracionado

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Engine LES with fuel-spray modeling

Pós-graduação em Engenharia Mecânica - FEG

Analisi numerica della combustione di gas a basso potere calorifico per applicazioni a microturbine a gas

Il presente lavoro si occupa dell’analisi numerica di combustione di gas a basso potere calorifico (gas di sintesi derivanti da pirolisi di biomasse). L’analisi è stata condotta su due principali geometrie di camera di combustione. La prima è un bruciatore sperimentale da laboratorio adatto allo studio delle proprietà di combustione del singas. Esso è introdotto in camera separatamente rispetto ad una corrente d’aria comburente al fine di realizzare una combustione non-premiscelata diffusiva in presenza di swirl. La seconda geometria presa in considerazione è la camera di combustione anulare installata sulla microturbina a gas Elliott TA 80 per la quale si dispone di un modello installato al banco al fine dell’esecuzione di prove sperimentali. I principali obbiettivi conseguiti nello studio sono stati la determinazione numerica del campo di moto a freddo su entrambe le geometrie per poi realizzare simulazioni in combustione mediante l’utilizzo di diversi modelli di combustione. In particolare è stato approfondito lo studio dei modelli steady laminar flamelet ed unsteady flamelet con cui sono state esaminate le distribuzioni di temperatura e delle grandezze tipiche di combustione in camera, confrontando i risultati numerici ottenuti con altri modelli di combustione (Eddy Dissipation ed ED-FR) e con i dati sperimentali a disposizione. Di importanza fondamentale è stata l’analisi delle emissioni inquinanti, realizzata per entrambe le geometrie, che mostra l’entità di tali emissioni e la loro tipologia. Relativamente a questo punto, il maggior interesse si sposta sui risultati ottenuti numericamente nel caso della microturbina, per la quale sono a disposizione misure di emissione ottenute sperimentalmente. Sempre per questa geometria è stato inoltre eseguito il confronto fra microturbina alimentata con singas a confronto con le prestazioni emissive ottenute con il gas naturale. Nel corso dei tre anni, l’esecuzione delle simulazioni e l’analisi critica dei risultati ha suggerito alcuni limiti e semplificazioni eseguite sulle griglie di calcolo realizzate per lo studio numerico. Al fine di eliminare o limitare le semplificazioni o le inesattezze, le geometrie dei combustori e le griglie di calcolo sono state migliorate ed ottimizzate. In merito alle simulazioni realizzate sulla geometria del combustore della microturbina Elliott TA 80 è stata condotta dapprima l’analisi numerica di combustione a pieno carico per poi analizzare le prestazioni ai carichi parziali. Il tutto appoggiandosi a tecniche di simulazione RANS ed ipotizzando alimentazioni a gas naturale e singas derivato da biomasse. Nell’ultimo anno di dottorato è stato dedicato tempo all’approfondimento e allo studio della tecnica Large Eddy Simulation per testarne una applicazione alla geometria del bruciatore sperimentale di laboratorio. In tale simulazione è stato implementato l’SGS model di Smagorinsky-Lilly completo di combustione con modelli flamelet. Dai risultati sono stati estrapolati i profili di temperatura a confronto con i risultati sperimentali e con i risultati RANS. Il tutto in diverse simulazioni a diverso valore del time-step imposto. L’analisi LES, per quanto migliorabile, ha fornito risultati sufficientemente precisi lasciando per il futuro la possibilità di approfondire nuovi modelli adatti all’applicazione diretta sulla MTG.

A LES study of a modified Ahmed body geometry

A numerical study using Large Eddy Simulation Coherent Structure Model (LES-CSM), of the flow around a simplified Ahmed body, has been done in this work of thesis. The models used are two salient geometries from the experimental investigation performed in [1], and consist, in particular, in two notch-back body geometries. Six simulation are carried out in total, changing Reynolds number and back-light angle of the model’s rear part. The Reynolds numbers used, based on the height of the models and the free stream velocity, are Re = 10000, Re = 30000 and Re = 50000. The back-light angles of the slanted surface with respect to the horizontal roof surface, that characterizes the vehicle, are taken as B = 31.8◦ and B = 42◦ respectively. The experimental results in [1] have shown that, depending on the parameter B, asymmetric and symmetric averaged flow over the back-light and in the wake for a symmetric geometry can be observed. The aims of the present work of master thesis are principally two. The first aim is to investigate and confirm the influence of the parameter B on the presence of the asymmetry of the averaged flow, and confirm the features described in the experimental results. The second important aspect is to investigate and observe the influence of the second variable, the Reynolds number, in the developing of the asymmetric flow itself. The results have shown the presence of the mentioned asymmetry as well as an influence of the Reynolds number on it.