Analytical ray-tracing in planetary atmospheres

Ground-based astro-geodetic observations and atmospheric occultations, are two examples of observational techniques requiring a scrutiny analysis of atmospheric refraction. In both cases, the measured changes in observables are geometrically related to changes in the photon path and the light time of the received electromagnetic signal. In the context of geometrical optics, the change in the physical properties of the signal are related to the refractive profile of the crossed medium. Therefore, having a clear knowledge of how the refractivity governs the photon path and the light time evolution is of prime importance to clearly understand observational features. Analytical studies usually focused on spherically symmetric atmospheres and only few aimed at exploring the effect of the non-spherical symmetry on the observables. In this paper, we analytically perform the integration of the photon path and the light time of rays traveling across a planetary atmosphere. We do not restrict our attention to spherically symmetric atmospheres and introduce a comprehensive mathematical framework which allows to handle any kind of analytical studies in the context of geometrical optics. To highlight the capabilities of this new formalism, we carry out five realistic applications for which we derive analytical solutions. The accuracy of the method of integration is assessed by comparing our results to a numerical integration of the equations of geometrical optics in the presence of a quadrupolar moment $J_2$. This shows that the analytical solution leads to the determination of the light time and the refractive bending with relative errors at the level of one part in $10^8$ and one part in $10^5$, for typical values of the refractivity and the $J_2$ parameter at levels of $10^{-4}$ and $10^{-2}$, respectively

Exoteric effects at nanoscopic interfaces - Uncommon negative compressibility of nanoporous materials and unexpected cavitation at liquid/liquid interfaces

This PhD thesis is devoted to the investigation of some peculiar effects happening at nanoscopic interfaces between immiscible liquids or liquids and solids via molecular dynamics simulations. The study of the properties of interfaces at a nanoscopic scale is driven by the promise of many interesting technological applications, including: a novel technology for developing both eco-friendly energy storage devices in the form of mechanical batteries, as well as energy dissipation systems and, in particular, shock absorbers for the automotive market; biomedical applications related to cavitation, such as High-Intensity Focused Ultrasound (HIFU) ablation of cancer tissues and localised drug delivery, and many more. The kinetics of phenomena taking places at these scales is typically determined by large free-energy barriers separating the initial and final states, and even intermediate metastable states, when they are present. Because of such barriers, the phenomena we are interested in are "rare events", i.e. the system attempts the crossing of the barrier(s) many times before finally succeeding when an energy fluctuation makes it possible. At the same time, the magnitude of the barrier is determined by the energetics and dynamics of atoms, which forces us to model the system by taking into account both the femtosecond atomistic timescale and the timescale of the relevant phenomena, typically exceeding the former by several orders of magnitude. These longer timescales are inaccessible to standard molecular dynamics, so, in order to tackle this issue, advanced MD techniques need to be employed. The thesis is divided into two parts, corresponding to the main lines of research investigated, which are (I) the interfaces between water and complex nanoporous solids, and (II) planar solid-liquid and liquid-liquid interfaces. Anticipating some results, atomistic simulations helped uncovering the microscopic mechanism behind the (incredibly rare!) giant negative compressibility exhibited by the ZIF-8 metal organic framework (MOF) upon water intrusion. Molecular dynamics simulations also supported experimental results showing how it is possible to change the intermediate intrusion-extrusion performance of ZIF-8 by changing its grain morphology and arrangement, from a fine powder to compact monolith. Free-energy MD calculations allowed to explain the exceptional stability of surface nanobubbles in water, at undersaturated conditions, on a surprisingly wide variety of substrates, characterized by disparate hydrophobicities and gas affinities; and yet, how they catastrophically destabilize in organic solvents. Finally, through simulations, some light was shed upon the working mechanism behind the novelly discovered phenomenon of how the interface between two immiscible liquids can act as a nucleation site for cavitation

Innovative Circular Business Models: A Case from the Italian Fashion Industry

Transition to a sustainable economy signed by a circular vision and culture asks firms for huge investments to innovate their own management, strategies, business models, products, and marketing approaches. The Agenda 2030 and the 17 Sustainable Development Goals (SDG) are an important framework for businesses to change their approach and contribute positively to the global movement to fight climate change. The question is what and how micro, small, and medium enterprises (MSMES) can contribute to reduce their impacts while creating more value for them and their stakeholders. This paper aims to answer to this question presenting a case study from Italy where an artisan small firm is innovating to create more positive impacts in circular terms. The focus will be on circular economy and the firms’ material and energy strategies. In doing so, the paper will try to answer the following questions: how easy is for micro and small firms to apply circular economy strategies to contribute to reduce their environmental impacts? Does their strategy coherently compose energy and material flows? The case study will refer to the fashion system in Italy

Analytical study of the radio signals propagation in planetary atmospheres

The ESA JUICE (JUpiter ICy moons Explorer) mission is planned for launch in 2022 and arrival at Jupiter around 2030. The mission is dedicated to the study of the giant gaseous and its largest moons. While the spacecraft will probe the Jovian system it will be occulted by the atmosphere of Jupiter or its satellites as seen from antennas on Earth. Such a configuration offers a great opportunity to study remotely the physical properties of the occulting atmosphere using radio links as the probe is being occulted. Indeed, non-unity index of refraction causes the electromagnetic waves to depart from the straight line and also impacts the propagation speed of the waves. Both changes modify the wave frequency and conversely, from the time variation of the Doppler measurements the index of refraction profile can be retrieved. In the literature, there are different approaches devoted to the retrieval of the refractive profile from these observables. Let mention, i) the analytic formulation of the Abel inversion which is employed for spherically symmetric atmospheres, and ii) the ray tracing method which is a numerical integration of the fundamental equations of optics and which is well suited for atmospheres with more complicated shapes. Both possess their own advantages and inconveniences. For instance, to invert a complete set of data, the ray tracing method requires more computational time than the Abel transformation. In return, the Abel inversion is based on the spherical symmetry assumption while the ray tracing technique can handle non-radial gradient in the refractive profile. In the context of the future occultations of JUICE by Jupiter, we discuss the benefit of a new formalism based on a full reformulation of the fundamental equations of optics. This new approach let to provide a very comprehensive description of the light trajectory inside a planetary atmosphere with no assumption on the refractive profile. In the special case where the departure from the spherical symmetry is small, we present an analytic solution which is well suited for the data processing of radio occultation experiments. Indeed, this solution can handle the effect of a non-spherically symmetric atmosphere with a low computational cost. We use this solution to process the Cassini Doppler data acquired during an occultation by the oblate atmosphere of Saturn. The validity of the proposed approach is assessed comparing the results with other studies available in the literature

Detection of rapid orbital expansion of Saturn’s moon Titan

The Saturn satellite system is a complex dynamical system with several gravitational interactions happening between the satellites, the rings and the central body, such as resonances, librations and tides. These intricate dynamics carry information on the formation and evolution of the Saturn and Solar systems

Diffusion Kurtosis Imaging of neonatal Spinal Cord in clinical routine

Diffusion kurtosis imaging (DKI) has undisputed advantages over the more classical diffusion magnetic resonance imaging (dMRI) as witnessed by the fast-increasing number of clinical applications and software packages widely adopted in brain imaging. However, in the neonatal setting, DKI is still largely underutilized, in particular in spinal cord (SC) imaging, because of its inherently demanding technological requirements. Due to its extreme sensitivity to non-Gaussian diffusion, DKI proves particularly suitable for detecting complex, subtle, fast microstructural changes occurring in this area at this early and critical stage of development, which are not identifiable with only DTI. Given the multiplicity of congenital anomalies of the spinal canal, their crucial effect on later developmental outcome, and the close interconnection between the SC region and the brain above, managing to apply such a method to the neonatal cohort becomes of utmost importance. This study will (i) mention current methodological challenges associated with the application of advanced dMRI methods, like DKI, in early infancy, (ii) illustrate the first semi-automated pipeline built on Spinal Cord Toolbox for handling the DKI data of neonatal SC, from acquisition setting to estimation of diffusion measures, through accurate adjustment of processing algorithms customized for adult SC, and (iii) present results of its application in a pilot clinical case study. With the proposed pipeline, we preliminarily show that DKI is more sensitive than DTI-related measures to alterations caused by brain white matter injuries in the underlying cervical SC

Heterogeneous cavitation from atomically smooth liquid-liquid interfaces

Pressure reduction in liquids may result in vaporization and bubble formation. This thermodynamic process is termed cavitation. It is commonly observed in hydraulic machinery, ship propellers, and even in medical therapy within the human body. While cavitation may be beneficial for the removal of malign tissue, yet in many cases it is unwanted due to its ability to erode nearly any material in close contact. Current understanding is that the origin of heterogeneous cavitation are nucleation sites where stable gas cavities reside, e.g., on contaminant particles, submerged surfaces or shell stabilized microscopic bubbles. Here, we present the finding of a so far unreported nucleation site, namely the atomically smooth interface between two immiscible liquids. The non-polar liquid of the two has a higher gas solubility and acts upon pressure reduction as a gas reservoir that accumulates at the interface. We describe experiments that clearly reveal the formation of cavitation on non-polar droplets in contact with water and elucidate the working mechanism that leads to the nucleation of gas pockets through simulations.Comment: This preprint has not undergone peer review or any post-submission improvements or corrections. The Version of Record of this article is published in Nature Physics, and is available online at https://doi.org/10.1038/s41567-022-01764-

Experimental Characterization of Two-Phase Flow Instability Thresholds in Helically Coiled Parallel Channels

Among the various types of instabilities affecting vapor generation in boiling systems, Density Wave Oscillation (DWO) occurrence within parallel channels is depicted. Parallel channel instability may represent a critical concern for the operation and safety of the once-through steam generators adopted in GenIII+ and GenIV nuclear reactor concepts. Extensive attention is required to determine the safe operating regime of a two-phase heat exchanger, by evaluating the instability threshold values of system parameters such as thermal power, flow rate, pressure, inlet temperature and exit quality. While the amount of published experimental work in the field of DWOs investigation in parallel straight tubes is overwhelming since the ’60, scarce attempt has been dedicated to the helical-coiled tube geometry. Conversely, coiled pipes are foreseen for applications to steam generators of the next generation NPPs, due to compactness and higher efficiency in heat transfer. The paper deals with the results of an experimental campaign on flow instability occurrence in two electrically heated helically coiled parallel tubes. In the framework of the IRIS project, a full-scale open-loop experimental facility simulating the thermal-hydraulic behavior of a helically coiled steam generator has been built and operated at SIET labs in Piacenza (Italy). The facility comprises two helical tubes (1 m coil diameter, 32 m length, 8 m height), connected via lower and upper headers. In order to excite flow unstable conditions starting from stable operating conditions, supplied electrical power was gradually increased up to the appearance of permanent and regular flow oscillations. Several flow instability threshold conditions were identified, in a test matrix of pressures (80 bar, 40 bar, 20 bar), mass fluxes (600 kg/m2s, 400 kg/m2s, 200 kg/m2s), and inlet subcooling (from -30% up to ~0). The long test section feature and the helical-coiled tube geometry render the present facility a quite unique test case in the outline of two-phase flow instability experimental studies. Parametric effects of the operating pressure, flow rate and inlet subcooling on the threshold power are discussed. The period of oscillations is also discussed