Identification of large coherent structures in supersonic axisymmetric wakes

Direct numerical simulation data of supersonic axisymmetric wakes are analysed for the existence of large coherent structures. Wakes at Ma ¼ 2:46 are considered with results being presented for cases at Reynolds numbers ReD ¼ 30; 000 and 100,000. Criteria for identification of coherent structures in freeshear flows found in the literature are compiled and discussed, and the role of compressibility is addressed. In particular, the ability and reliability of visualisation techniques intended for incompressible shear-flows to educe meaningful structures in supersonic wakes is scrutinised. It is shown that some of these methods retain their usefulness for identification of vortical structures as long as the swirling rate is larger than the local compression and expansion rates in the flow field. As a measure for the validity of this condition in a given flow the ‘vortex compressibility parameter’ is proposed which is derived here. Best ‘visibility’ of coherent structures is achieved by employing visualisation techniques and proper orthogonal decomposition in combination with the introduction of artificial perturbations (forcing of the wake). The existence of both helical and longitudinal structures in the shear layer and of hairpin-like structures in the developing wake is demonstrated. In addition, elongated tubes of streamwise vorticity are observed to emanate from the region of recirculating flo

investigating ice microphysical processes by combining multi-frequency and polarimetric Doppler radar observations with Lagrangian Monte-Carlo particle modelling

Clouds and precipitation strongly impact society and the earth system by influencing the water cycle, determining fresh water availability or causing natural disasters such as floods or droughts. However, many aspects of precipitation formation are still poorly understood, causing large uncertainties in the prediction of precipitation. Especially the microphysical processes, which describe the nucleation of cloud particle and their growth into precipitation lack understanding. As globally 63% of precipitation originates from the ice phase, increasing the understanding of ice microphysical processes is crucial to improve precipitation forecast. The dendritic growth layer (DGL), located at temperatures between −20 and −10 ° C, plays an important role in the formation of precipitation. Previous studies have found an in particle size and number concentration through depositional growth, aggregation and secondary ice processes. This dissertation investigates ice microphysical processes in the DGL by combining polarimetric and multi-frequency Doppler cloud radar observations with Monte-Carlo Lagrangian particle modelling. Study I presents a statistical analysis of a three-month polarimetric and multi-frequency Doppler radar dataset. This combination of radar measurements allows to observe the full evolution of ice particle growth, as the polarimetric measurements are indicators of depositional growth and possible secondary ice processes, while the multi-frequency approach gives an indication of the increase particle in size through aggregation and riming. The statistical analysis revealed an increase of aggregate size at −15 ° C. The mean size of aggregates is found to be correlated to an updraft with a maximum of approximately 0.1 m s −1 at −14 ° C. The radar observations further indicate the growth of plate-like ice crystals at −15 ° C. Unexpectedly, aggregation is found to increase in the DGL alongside an increase in ice particle number concentration. This simultaneous increase necessitates a source of new ice particles, as aggregation is expected to decrease the total number of ice particles. Secondary ice processes, such as collisional fragmentation provide one explanation for this increase in ice particle size. Another possible explanation might be that small ice particles sediment from colder temperatures into the DGL and enhance the number concentration locally. The third explanation is linked to the observed updraft, as this updraft might increase the super-saturation with respect to ice at −15 ° C, leading to the activation of ice nucleating particles and a subsequent increase in ice particle number and growth of plate-like particles. Unfortunately, radar observations do not observe the formation of particles directly, it is difficult to predict the origin of the particles responsible for the increase in particle concentration and observed polarimetric signatures further. With the observational dataset as a constrain, Study II uses the Monte-Carlo Lagrangian particle model McSnow to investigate the origin of the increase in ice particle number concentration in the DGL further. The comparison of the observations and McSnow simulations indicate that the particles responsible for the polarimetric signatures and increase in number concentration need to be nucleated at temperatures close to −15 ° C. This might indicate that in the observed clouds, sedimenting ice particles into the DGL play a lesser role. The McSnow simulations further indicate that neither collisional fragmentation nor new ice particles due to activation of ice nucleating particles can explain the observed multi-frequency and polarimetric observations. A combination of both processes might explain the observed signatures. This dissertation shows the potential of a combination of radar observations and modelling for increasing the understanding of microphysical processes in clouds. However, further laboratory studies are needed in order to further constrain the processes in the DGL and validate the findings of this dissertation

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Tato disertační práce se zabývá tématem jak finanční trhy vnímají budoucnost rozvoje eurozóny. Názor trhu v oblasti rozšíření eurozóny je hodnocen s využitím dvou různých metod. První přístup, metoda Statických pravděpodobnostních kalkulátorů (SPC), je založena na existující metodologii, která byla dříve použita pro staré členské státy eurozóny. Tato statická metoda má však pár nedostatků, kterým se druhá, zcela nově vytvořená, metodologie snaží předejít. Druhá metoda, tzv. Dynamický pravděpodobnostní kalkulátor (DPC), je založena na indikátoru odvozeném z krátkodobé dynamiky budoucí úrokových diferenciálů. Jak SPC tak DPC jsou aplikovány na finanční deriváty (budoucí úrokové swapy) obchodované ve čtyřech středoevropských zemích: České republice, Maďarsku, Polsku a Slovensku. Deriváty obchodovaná v některých dalších Evropských zemí byly použity pro ověření robustnosti zkoumaných dvou metod. Nová DPC metodologie je konceptuálně založena na předpokladu reprezentativního jedince averzního vůči nejistotě. Tato metoda je založena na modelu všeobecné rovnováhy za předpokladu nedokonalých preferencí. Předpoklad nedokonalých preferencí dovoluje výskyt mnohočetných rovnovážných stavů na základě pouze jedné množiny ekonomických fundamentů. Nemožnost určení jednoznačného rovnovážného stavu vysvětluje...This thesis deals with assessing how financial markets perceive prospects of future euro area enlargement. Market views on such an enlargement are measured using two different approaches. The first approach, the static probability calculators method (SPC), is based on an existing method that was previously used for the old EU Member States. In order to overcome inherent shortcomings of this method, a second, completely new methodology with an indicator that is based on short-term dynamics of forward spreads was developed, further referred to as dynamic probability calculators (DPC). Both the SPC and DPC are applied to data from four Central Eastern European countries: the Czech Republic, Hungary, Poland, and Slovakia. In addition, data of other European countries were used to assess the robustness of the two approaches. The new methodology is conceptually based on the notion of ambiguity-averse agents. Specifically, it attempts to apply the framework of incomplete preferences, developing a general equilibrium framework, which allows for multiple equilibria supported by one set of fundamentals. This equilibrium indeterminacy offers a way to reconcile shortterm fluctuations of market prices with a relatively stable underlying economic environment and expectations. The thesis concludes with a...Institut ekonomických studiíInstitute of Economic StudiesFaculty of Social SciencesFakulta sociálních vě

Failure of Sterne- and Pasteur-Like Strains of Bacillus anthracis to Replicate and Survive in the Urban Bluebottle Blow Fly Calliphora vicina under Laboratory Conditions

Britta von Terzi, Peter C. B. Turnbull, Wolfgang Beyer, University of Hohenheim, Institute of Environmental and Animal Hygiene, Stuttgart, GermanySteve E. Bellan, Center for Computational Biology and Bioinformatics, University of Texas at Austin, Austin, Texas, United States of AmericaThis study aimed to elucidate the bacteriological events occurring within the gut of Calliphora vicina, selected as the European representative of blow flies held responsible for the spread of anthrax during epidemics in certain parts of the world. Green-fluorescent-protein-carrying derivatives of Bacillus anthracis were used. These lacked either one of the virulence plasmids pXO1 and pXO2 and were infected, or not infected, with a worm intestine phage (Wip4) known to influence the phenotype and survival of the pathogen. Blood meals were prepared for the flies by inoculation of sheep blood with germinated and, in case of pXO2+ strains, encapsulated cells of the four B. anthracis strains. After being fed for 4 h an initial 10 flies were externally disinfected with peracetic acid to ensure subsequent quantitation representing ingested B. anthracis only. Following neutralization, they were crushed in sterile saline. Over each of the ensuing 7 to 10 days, 10 flies were removed and processed the same way. In the absence of Wip4, strains showed steady declines to undetectable in the total B. anthracis counts, within 7–9 days. With the phage infected strains, the falls in viable counts were significantly more rapid than in their uninfected counterparts. Spores were detectable in flies for longer periods than vegetative bacteria. In line with the findings in both biting and non-biting flies of early workers our results indicate that B. anthracis does not multiply in the guts of blow flies and survival is limited to a matter of days.This work was funded by grant BE 2157/3-1 of the German Research Foundation (DFG). SEB was funded by a United States National Institute of General Medical Sciences MIDAS grant U01GM087719 to Lauren A. Meyers and Alison P. Galvani. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.Center for Computational Biology and BioinformaticsEmail: [email protected]

Drivers for optimum sizing of wind turbines for offshore wind farms

Large-scale exploitation of offshore wind energy is deemed essential to provide its expected share to electricity needs of the future. To achieve the same, turbine and farm-level optimizations play a significant role. Over the past few years, the growth in the size of turbines has massively contributed to the reduction in costs. However, growing turbine sizes come with challenges in rotor design, turbine installation, supply chain, etc. It is, therefore, important to understand how to size wind turbines when minimizing the levelized cost of electricity (LCoE) of an offshore wind farm. Hence, this study looks at how the rated power and rotor diameter of a turbine affect various turbine and farm-level metrics and uses this information in order to identify the key design drivers and how their impact changes with setup. A multi-disciplinary design optimization and analysis (MDAO) framework is used to perform the analysis. The framework uses low-fidelity models that capture the core dependencies of the outputs on the design variables while also including the trade-offs between various disciplines of the offshore wind farm. The framework is used, not to estimate the LCoE or the optimum turbine size accurately, but to provide insights into various design drivers and trends. A baseline case, for a typical setup in the North Sea, is defined where LCoE is minimized for a given farm power and area constraint with the International Energy Agency 15 MW reference turbine as a starting point. It is found that the global optimum design, for this baseline case, is a turbine with a rated power of 16 MW and a rotor diameter of 236 m. This is already close to the state-of-the-art designs observed in the industry and close enough to the starting design to justify the applied scaling. A sensitivity study is also performed that identifies the design drivers and quantifies the impact of model uncertainties, technology/cost developments, varying farm design conditions, and different farm constraints on the optimum turbine design. To give an example, certain scenarios, like a change in the wind regime or the removal of farm power constraint, result in a significant shift in the scale of the optimum design and/or the specific power of the optimum design. Redesigning the turbine for these scenarios is found to result in an LCoE benefit of the order of 1 %–2 % over the already optimized baseline. The work presented shows how a simplified approach can be applied to a complex turbine sizing problem, which can also be extended to metrics beyond LCoE. It also gives insights into designers, project developers, and policy makers as to how their decision may impact the optimum turbine scale.</p

The importance of particle size distribution and internal structure for triple-frequency radar retrievals of the morphology of snow

The accurate representation of ice particles is essential for both remotely sensed estimates of clouds and precipitation and numerical models of the atmosphere. As it is typical in radar retrievals to assume that all snow is composed of aggregate snowflakes, both denser rimed snow and the mixed-phase cloud in which riming occurs may be under-diagnosed in retrievals and therefore difficult to evaluate in weather and climate models. Recent experimental and numerical studies have yielded methods for using triple-frequency radar measurements to interrogate the internal structure of aggregate snowflakes and to distinguish more dense and homogeneous rimed particles from aggregates. In this study we investigate which parameters of the morphology and size distribution of ice particles most affect the triple-frequency radar signature and must therefore be accounted for in order to carry out triple-frequency radar retrievals of snow. A range of ice particle morphologies are represented, using a fractal representation for the internal structure of aggregate snowflakes and homogeneous spheroids to represent graupel-like particles; the mass-size and area-size relations are modulated by a density factor. We find that the particle size distribution (PSD) shape parameter and the parameters controlling the internal structure of aggregate snowflakes both have significant influences on triple-frequency radar signature and are at least as important as that of the density factor. We explore how these parameters may be allowed to vary in order to prevent triple-frequency radar retrievals of snow from being over-constrained, using two case studies from the Biogenic Aerosols - Effects of Clouds and Climate (BAECC) 2014 field campaign at Hyytiala, Finland. In a case including heavily rimed snow followed by large aggregate snowflakes, we show that triple-frequency radar measurements provide a strong constraint on the PSD shape parameter, which can be estimated from an ensemble of retrievals; however, resolving variations in the PSD shape parameter has a limited impact on estimates of snowfall rate from radar. Particle density is more effectively constrained by the Doppler velocity than triple-frequency radar measurements, due to the strong dependence of particle fall speed on density. Due to the characteristic signatures of aggregate snowflakes, a third radar frequency is essential for effectively constraining the size of large aggregates. In a case featuring rime splintering, differences in the internal structures of aggregate snowflakes are revealed in the triple-frequency radar measurements. We compare retrievals assuming different aggregate snowflake models against in situ measurements at the surface and show significant uncertainties in radar retrievals of snow rate due to changes in the internal structure of aggregates. The importance of the PSD shape parameter and snowflake internal structure to triple-frequency radar retrievals of snow highlights that the processes by which ice particles interact may need to be better understood and parameterized before triple-frequency radar measurements can be used to constrain retrievals of ice particle morphology.Peer reviewe

DES study of blade trailing edge cutback cooling performance with various lip thicknesses

Three-dimensional detached-eddy simulation (DES) study has been carried out to evaluate the cooling performance of a trailing-edge cutback turbine blade with various lip thickness to slot height ratios (t/H). By adopting the shear-stress transport (SST) k-ω turbulence model, the numerical investigations were performed at two successive steps: first, to validate simulation results from an existing cutback turbine blade model with staggered circular pin-fins arrays inside the cooling passage against experimental measurements and other available numerical predictions; second, to understand the effects of the lip thickness to the slot height ratio on the blade trailing-edge cooling performance. It was found from the model validations that at two moderate blowing ratios of 0.5 and 1.1, DES predicted film cooling effectiveness are in very good agreement with experimental data. Further comparisons of four various t/H ratios (t/H = 0.25, 0.5, 1.0, 1.5) have revealed that the thermal mixing process between the ‘cold’ coolant gas and the ‘hot’ mainstream flow in the near wake region of the exit slot has been greatly intensified with the increase of the t/H ratio. As a result, it causes a rapid decay of the adiabatic film cooling effectiveness downstream of the blade trailing-edge. The observed vortex shedding and its characteristics in the near wake region are found to play an important role in determining the dynamic process of the ‘cold’ and the ‘warm’ airflow mixing, which in turn have significant influences on the prediction accuracy of the near-wall heat transfer performance. As the four t/H ratio increases from 0.25 to 1.5, DES predicts the decrease of main shedding frequencies as fs = 3.69, 3.2, 2.21, and 1.49 kHz, corresponding to Strouhal numbers St = 0.15, 0.20, 0.23, and 0.22, respectively. These results are in good agreement with available experimental measurements

The determinants of electronic traceability adoption: a firm-level analysis of French agribusiness

International audienceThis paper aims to understand what factors influence firms to adopt electronic traceability systems (ETS) and notably the respective effects of the firm's internal characteristics, its vertical relations and its external environment. Traceability systems based on information and communication technologies (ICT) allow firms to collect, track, stock and transfer information on a range of product attributes. This study contributes to further understand traceability adoption by applying ICT adoption models to the case of ETS, and by using an original dataset, the 2002 ICT Survey, representative of all French agribusiness. The results suggest that a firm's degree of complexity (growing size, belonging to a group) and the development of its information system play a significant role in its adoption behavior. Moreover, they show that ETS adoption is more driven by a firm's narrow relations with specialized suppliers and downstream processors than by retailers

Numerical investigation of transitional and turbulent backward-facing step flows

Transitional and turbulent flows over a backward-facing step are physically highly complex. Apart from vastly different mean flow regimes and the rapid generation of turbulence, additional complexities arise from the presence of large coherent structures. For the present study, the mean flow, turbulence statistics and the origin of large coherent structures were investigated using Direct Numerical Simulations and turbulence modeling approaches. The latter included Large Eddy Simulations (LES) and state-of-the-art Reynolds-Averaged Navier-Stokes (RANS) computations. Wall-distance independent forms of the RANS models were developed, validated and calibrated. The ability of computing the step flows investigated and the associated computational costs were evaluated, for both LES and RANS. By employing harmonic forcing of the shear layer and a Fourier analysis in time and in the lateral direction the generation of coherent structures was linked to specific hydrodynamic instabilities. Comparison with references in the literature, resolution and domain size studies, and variations of inflow conditions established an accurate description of the mean flow and turbulence quantities and the level of sensitivity of the flow field to boundary conditions. From the controlled environment of the simulations, a simplified scenario was proposed for the creation of large coherent structures in transitional and turbulent step flows. The scenario suggests that Kelvin-Helmholtz, elliptical and centrifugal instabilities may be the relevant physical mechanisms for the observed primary, secondary and tertiary instabilities of the shear layer, respectively. The onset of the elliptical instability can also be described as a fundamental resonance of two waves. A cascade of subharmonic resonances is regarded to be responsible for vortex mergings and the generation of low frequency waves in the flow field. Furthermore, the simulations indicate that a three-dimensional global instability of the time and spanwise averaged separation bubble may be present. It was observed that the range of all unstable lateral wavelengths has a short-wave cutoff depending on Reynolds number and an upper bound on the order of the reattachment length