Aspects of Two-Level Systems under External Time Dependent Fields

The dynamics of two-level systems in time-dependent backgrounds is under consideration. We present some new exact solutions in special backgrounds decaying in time. On the other hand, following ideas of Feynman, Vernon and Hellwarth, we discuss in detail the possibility to reduce the quantum dynamics to a classical Hamiltonian system. This, in particular, opens the possibility to directly apply powerful methods of classical mechanics (e.g. KAM methods) to study the quantum system. Following such an approach, we draw conclusions of relevance for ``quantum chaos'' when the external background is periodic or quasi-periodic in time.Comment: To appear in J. Phys. A. Mathematical and Genera

Multiple classical limits in relativistic and nonrelativistic quantum mechanics

The existence of a classical limit describing interacting particles in a second-quantized theory of identical particles with bosonic symmetry is proved. This limit exists in addition to a previously established classical limit with a classical field behavior, showing that the limit $\hbar \to 0$ of the theory is not unique. An analogous result is valid for a free massive scalar field: two distinct classical limits are proved to exist, describing a system of particles or a classical field. The introduction of local operators in order to represent kinematical properties of interest is shown to break the permutation symmetry under some localizability conditions, allowing the study of individual particle properties.Comment: 13 page

T cells in sputum of asthmatic patients are activated independently of disease severity or control

Background T cells play an important role in bronchial asthma. Although airway CD4+ T cells have been extensively studied previously, there are hardly any studies relating CD8+ T cell activation and disease symptoms. Objectives The aim of this study was to analyse the association between T cell activation in induced sputum T cells and asthma severity and control; and to evaluate T cell subpopulations in the same subgroups. Methods Fifty allergic asthmatic patients were recruited and lung function testing was performed. Airway cells were obtained by sputum induction via inhalation of hypertonic saline solution. CD3, CD4, CD8, CD28, CD25 and CD69 were studied by flow cytometry in whole induced sputum and peripheral blood cells. Results Total induced sputum T cells and CD8+ T cells had a higher relative percentage of the activation markers CD25 and CD69 in comparison with peripheral blood. In sputum, the relative percentage of CD25 was higher in CD4+ T cells when compared to CD8+ T cells and the reverse was true regarding CD69. However, neither disease severity nor control were associated with the relative percentage of CD25 or CD69 expression on T cells in sputum. Conclusions Both CD4+ and CD8+ T cells are activated in the lungs and peripheral blood of asthmatic patients. However, with the possible exception of CD69+ CD8+ T lymphocytes in the sputum, there is no association between T cell activation phenotype in the target organ and disease severity or control.info:eu-repo/semantics/publishedVersio

Temperature Field Validation on Supercritical Coaxial Injection

In the present manuscript, we report on the numerical simulation of coaxial nitrogen injection under subcritical and supercritical conditions, where the inner stream is recessed concerning the coaxial one. An incompressible but variable density description of supercritical phenomena is pursued as an alternative to more established compressible formulations, in which we extend past analysis focusing on the evaluation of density to the temperature field validation, according to the available experimental data. The results indicate that recirculation regions are formed as expected at either the inner jet axis or at the post tip between both streams. Furthermore, it is found that the transition threshold between the blockage effect depends on the momentum ratio and differs between subcritical and supercritical conditions.Fundação para a Ciência e a Tecnologiainfo:eu-repo/semantics/publishedVersio

Influence of Dimensionless Temperature on Droplet Impact onto Heated Liquid Films for Subcooled Boiling Regimes

Heat and mass transfer mechanisms related to multiphase flows occur in several applications such as spray cooling, quenching, internal combustion engines and plasma spraying. These mechanisms have become increasingly important due to the need of achieving higher heat rate coefficients associated with phase-change processes, such as evaporation and condensation. Specifically, the phenomenon of droplet impact onto non-heated liquid films has been extensively researched, both experimentally and numerically. However, the influence of temperature on droplet impact and liquid film stability has been overlooked in the literature, which is a focal point in understanding interfacial phenomena. The main objective of this work is to experimentally study droplet impact onto heated liquid films. Therefore, an experimental facility was designed for this purpose. A borosilicate glass surface is used to contain the liquid film. This surface is placed above an aluminium block with four embedded cartridge heaters of 250W each, heating the liquid film by conduction. Immersion type-k thermocouples are employed for liquid film temperature measurements. Liquid film evaporation rates are calculated in order to ensure the liquid film thickness prior to the droplet impact. Water and n-decane are the fluids adopted due to their differences in thermophysical properties and saturation temperature. The impact conditions are 100 < W e < 300, 0.5 < h* < 1.5, and a dimensionless temperature of θ < 0.6. Qualitative analysis is performed regarding crater and central jet evolution, and quantitative data regarding evaporation rate and central jet height are measured. The dimensionless temperature affects the droplet impact phenomena, creating recirculation zones near the crater and the impact surface, and affecting the crater formation. The central jet height increases with increasing values of θ for h* = 1.0 and h* = 1.5, whereas for h* = 0.5, the measurements do not follow a similar tendency. The emerging time of the central jet is delayed for higher values of θ, meaning that future studies regarding crater evolution should be considered. The dimensionless temperature also promotes central jet breakup, as well increasing the number of secondary droplets originated from the breakup.Fundação para a Ciência e a Tecnologiainfo:eu-repo/semantics/publishedVersio

Influence of Vapour Bubbles Size and Spacing on Droplet Impact Outcomes under Subcooled Boiling Regimes

Multiphase flows are characterised as heterogeneous mixtures of two or more phases, such as gas-liquid or solid-liquid. These are extremely complex due to the underlying dynamics that may occur, which include interfacial phenomena, such as boiling and evaporation, and interactions between phases. Specifically, droplet impact onto heated wetted surfaces has been overlooked in the literature, which involves heat and mass transfer mechanisms related to sensible heating, condensation and evaporation. The droplet impact phenomenon occurs in several industrial applications, such as internal combustion engines, electronics cooling devices, heat exchangers, among others. In numerical terms, improving efficiency and reliability of simulations is of interest to understand the behaviour of mathematical models for complex physical systems. The main objective of this work is to numerically simulate droplet impact onto a heated liquid film in the presence of vapour bubbles in a 2D-axisymmetric assumption. The numerical model solves the incompressible mass, momentum and energy equations coupled with the VOF method and height functions for accurately capturing the interface. Phase-change processes such as evaporation and condensation are neglected for the current simulations. Water and n-decane are the fluids adopted for the simulation. The impact conditions are D0 = 3.0 mm, h* = 0.5, and U0 = 3.0 m/s. Different vapour bubble sizes, (Dv), and spacings, (xv), are studied to evaluate the vapour bubble phenomena and crown geometrical parameters. An initial vapour bubble is positioned on the axis of symmetry, xs = 0, and, therefore, in the droplet trajectory. Results indicate that vapour bubbles have a lower influence on crown diameter, and a higher influence on crown height. Increasing the size of vapour bubbles leads to a decrease in both crown diameter and height, whereas an increase in the vapour bubbles spacing causes an increase in the crown height and diameter. The initial vapour bubble positioning in relation to the axis of symmetry is a factor that should also be considered in future studies.Fundação para a Ciência e a Tecnologiainfo:eu-repo/semantics/publishedVersio

Locally variable turbulent Prandtl number considerations on the modeling of Liquid Rocket Engines operating above the critical point

The general idea behind the present work is to study the injection of a cryogenic liquid numerically into rocket engines, where propellant conditions are above the thermodynamic critical point, for a non-reactive case. The singular behavior of thermodynamic and transport properties at and around the critical point makes this a most challenging task. While mass diffusivity, surface tension, and latent heat are zero at the critical point, isentropic compressibility, specific heat, and thermal conductivity tend to infinity. As a result, the distinction between liquid and solid phases disappears. Ultimately, the fluid has liquid-like density and gas-like properties, mass diffusion replaces vaporization as a governing parameter, and it dominates over jet atomization. Henceforth, any model used incorporates as close as possible to reality, the variation of thermodynamic and transport properties. An incompressible variable-density flow is simulated using Favre averages (FANS) with a locally variable turbulent Prandtl number, taking into account the potential core, transition, and the self-similar region of the jet. The use of a turbulence model with a variable turbulent Prandtl number arises from the ineffectiveness in predicting observed anisotropies in the thermal eddy diffusivity fields when this value is taken as a constant. Favre averaged conservation equations for mass, momentum, and energy are coupled with the k- two-equation turbulence model and discretized following the third order upwind QUICK scheme. Stability and accuracy of the results are maintained through a careful selection of the parameters involved in the models. The use of the conservation equation for energy is justified as an indirect means to evaluate the thermal field. Results are compared with experimental cases for validation purposes as well as LES computations for performance comparison and evaluation of the degree of model complexity needed to achieve satisfactory results.info:eu-repo/semantics/publishedVersio

Effect of Vapour Bubble Initial Displacement on Droplet Impact onto Liquid Films

Heat and mass transfer mechanisms regarding droplet impact have been extensively studied for dry surfaces, and in terms of droplet evaporation/combustion during free-fall. On the contrary, the droplet impact phenomena onto heated liquid films is a topic overlooked in the literature and requires further understanding in terms of hydrodynamics and phase change. Therefore, this work focuses on numerically simulating droplet impact onto liquid films in the presence of vapour bubbles. The crown height, diameter and overall development are analysed as a function of the position of the bubble related to the axis of symmetry. Results show that the crown overall growth is affected if the vapour bubbles detach and contact the crown wall. The size of the vapour bubbles also influences the detachment from the surface.Fundação para a Ciência e a Tecnologiainfo:eu-repo/semantics/publishedVersio

Injector Wall Heat Transfer Quantification in Supercritical Nitrogen Injection

Pressure and temperature increase in combustion chambers of Liquid Rocket Engines (LRE’s), while enhancing injection and combustion efficiencies, leads to both fuels and oxidizers to exceed their critical point conditions, entering the domain of supercritical fluid flows. Over the past 20 years, many physical models are developed for the simulation of supercritical nitrogen injection, which is validated with the experimental data from [1] and [2]. However, regardless of the sophistication employed in RANS, LES [3] or DNS-based [4] approaches, unrealistic top hat density profiles appear in the computations, which have in common the consideration of adiabatic injector walls. The present work has the objective of quantifying the influence of injector wall heat transfer for the considered experimental conditions, contributing for a more accurate representation of the physical phenomena in LRE's combustion chambers. For this purpose, a RANS-based approach is followed combining the accuracy of a multiparameter equation of state for nitrogen with an incompressible, but variable density approach description of the mixing conditions.Figure 1 depicts a comparison of the results obtained for the centerline density decay for case 4 from [1]. The injector diameter normalizes the axial distance from the injector. In this figure, the origin corresponds to the entrance of the combustion chamber. For the case of the adiabatic injector walls, it can be observed a potential core until x/D = 7.5, as opposed to what is depicted in the same figure for the experimental data. Experimentally no core is predicted, and the axial density starts to decrease as soon as the beginning of the combustion chamber. If, on the other hand, the isothermal injector is considered, it can be seen that no top hat profile appears, and the numerical results closely replicate the experimental behavior of the jet.It is shown that injector heat transfer phenomena actively changes the topology of the jet mixing, contributing to improved performance of numerical solvers.info:eu-repo/semantics/publishedVersio