3,110 research outputs found
Correction to: A T-junction device allowing for two simultaneous orthogonal views: application to bubble formation and break-up (Microfluidics and Nanofluidics, (2018), 22, 8, (85), 10.1007/s10404-018-2101-1)
No full textThe article ‘A T-junction device allowing for two simultaneous orthogonal views: application to bubble formation and break-up’, written by Davide Caprini, Giorgia Sinibaldi, Luca Marino, Carlo Massimo Casciola was originally published electronically on the publisher’s internet portal (currently SpringerLink) on 30 July 2018 without open access
Costruzione di stati di ground su spaziotempi statici dotati di bordo di tipo tempo: libertà nascoste e divergenze infrarosse.
No full textIn this work we review the construction of ground states focusing on a real scalar field whose dynamics is ruled by the Klein-Gordon equation on a large class of static spacetimes with a time-like boundary. As in the analysis of the classical equations of motion, when enough isometries are present, via a mode expansion the construction of two-point correlation functions boils down to solving a second order, ordinary differential equation on an interval of the real line. Using the language of Sturm-Liouville theory, most compelling is the scenario when one endpoint of such interval is classified as a limit circle, as it often happens when one is working on globally hyperbolic spacetimes with a timelike boundary. In this case, beyond initial data, one needs to specify a boundary condition both to have a well-defined classical dynamics and to select a corresponding ground state. Here, we take into account boundary conditions of Robin type by using well-known results from Sturm-Liouville theory, but we go beyond the existing literature by exploring an unnoticed freedom that emerges from the intrinsic arbitrariness of secondary solutions at a limit circle endpoint. Accordingly, we show that infinitely many one-parameter families of sensible dynamics are admissible. In other words, we emphasize that physical constraints guaranteeing the construction of full-fledged ground states do not, in general, fix one such state unambiguously. In addition, we provide, in full detail, an example on (1+1)-half Minkowski spacetime to spell out the rationale in a specific scenario where analytic formulae can be obtained.\\
Then, on , we construct the two-point correlation functions for the ground and thermal states admitting generalized -boundary conditions. We follow the prescription we have developed for two different choices of secondary solutions. For each of them, we obtain a family of admissible boundary conditions parametrized by . We study how they affect the response of a static Unruh-DeWitt detector. The latter not only perceives variations of , but also distinguishes between the two families of secondary solutions in a qualitatively different, and rather bizarre, fashion. Our results highlight once more the existence of a freedom in choosing boundary conditions at a timelike boundary which is greater than expected and with a notable associated physical significance.\\
To conclude, we observe that, depending on the assigned boundary condition of Robin type, this procedure does not always lead to the existence of a suitable bi-distribution due to the presence of infrared divergences. As a concrete example we consider a Bertotti-Robinson spacetime in two different coordinate patches. In one case we show that infrared divergences do not occur only for Dirichlet boundary conditions as one might expect a priori, while, in the other case, we prove that they occur only when Neumann boundary conditions are imposed at the time-like boundary
Experimental analysis on the noise of propellers for small uav
No full textMain goal of this paper is an experimental analysis on the aeroacoustic features of propellers aimed at the propulsion of small unmanned aerial vehicles. The investigation concerns the comparison between the acoustic signature of optimized and conventional propellers driven by brushless electric motors. This topic is particularly relevant due to the dramatic increase of the UAV market for civil and military purposes with multiple operational issues. The quietness and the efficiency of the propulsive system are key aspects in the design of advanced aerial vehicles and very often can lead to the success or failure of the mission. In addition to the experimental research, a numerical analysis based on the compact formulation of the Ffowcs-Williams/Hawking equation and a simple model for the broadband noise, is presented and discussed
On the interplay between boundary conditions and the Lorentzian Wetterich equation
No full textIn the framework of the functional renormalization group and of the perturbative, algebraic approach to quantum field theory (pAQFT), in D'Angelo et al. [Ann. Henri Poinc. 25 (2024) 2295-2352] it has been derived a Lorentzian version of a flow equation à la Wetterich, which can be used to study nonlinear, quantum scalar field theories on a globally hyperbolic spacetime. In this work, we show that the realm of validity of this result can be extended to study interacting scalar field theories on globally hyperbolic manifolds with a timelike boundary. By considering the specific examples of half-Minkowski spacetime and of the Poincaré patch of Anti-de Sitter, we show that the form of the Lorentzian Wetterich equation is strongly dependent on the boundary conditions assigned to the underlying field theory. In addition, using a numerical approach, we are able to provide strong evidences that there is a qualitative and not only a quantitative difference in the associated flow and we highlight this feature by considering Dirichlet and Neumann boundary conditions on half-Minkowski spacetime
Sound source mechanisms in under‐expanded impinging jets
No full textExperiments on the aeroacoustics of an under-expanded supersonic jet impinging on a flat plate are presented and thoroughly discussed. A wide range of nozzle pressure ratios and of nozzle-to-plate distances has been analyzed with particular attention to the behavior of the discrete component of the noise. The investigation has been carried out by means of acoustic, particle image velocimetry and wall pressure measurements. The analysis of the relationship between the acoustic data and the fluid dynamic fields allows to examine the different source mechanisms of the discrete component of the noise and to evaluate the link between the jet flow structure and the acoustic tone features. Specifically, two ranges of nozzle pressure ratio have been observed showing different acoustic behaviors, characterized by distinct mechanisms of discrete noise generation. These regions are separated by a range of nozzle pressure ratios where impinging tones are not observed. The present experimental data extend previously published results, improving the analysis of the connection between fluid dynamic and acoustic fields and leading to a better comprehension of the impinging tone source mechanisms
An assessment of the impact of possible CAP reform scenarios on Romanian agriculture
No full textUsing a simplified model, with key-variable the prices of two different possible scenarios of CAP reform after 2013 (moderate and radical), this paper present a comparison between the price effects of implementation of each reform scenario at 2015 horizon on Romanian agriculture. This short analysis shows that, under the presented hypotheses, the net welfare effect, due to the price changes, for the selected products, is positive in both reform scenarios, yet greater in the case of the radical reform. Integrated in the large context of Romanian development, it seems that the influence of CAP reform upon agriculture and rural areas will be most likely a gradual one: an interpenetration between the two scenarios is foreseeable, starting with the moderate reform that will dominate the period around 2013, the reform measures acquiring a more radical character afterwards.CAP reform, Romania, welfare effects, Agricultural and Food Policy,
Hidden freedom in the mode expansion on static spacetimes
No full textWe review the construction of ground states focusing on a real scalar field
whose dynamics is ruled by the Klein-Gordon equation on a large class of static
spacetimes. As in the analysis of the classical equations of motion, when
enough isometries are present, via a mode expansion the construction of
two-point correlation functions boils down to solving a second order, ordinary
differential equation on an interval of the real line. Using the language of
Sturm-Liouville theory, most compelling is the scenario when one endpoint of
such interval is classified as a limit circle, as it often happens when one is
working on globally hyperbolic spacetimes with a timelike boundary. In this
case, beyond initial data, one needs to specify a boundary condition both to
have a well-defined classical dynamics and to select a corresponding ground
state. Here, we take into account boundary conditions of Robin type by using
well-known results from Sturm-Liouville theory, but we go beyond the existing
literature by exploring an unnoticed freedom that emerges from the intrinsic
arbitrariness of secondary solutions at a limit circle endpoint. Accordingly,
we show that infinitely many one-parameter families of sensible dynamics are
admissible. In other words, we emphasize that physical constraints guaranteeing
the construction of full-fledged ground states do not, in general, fix one such
state unambiguously. In addition, we provide, in full detail, an example on -half Minkowski spacetime to spell out the rationale in a specific
scenario where analytic formulae can be obtained.Comment: 24 pages, 3 fig
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