6,132 research outputs found
Feedback local optimality principle applied to rocket vertical landing VTVL
Vertical landing is becoming popular in the last fifteen years, a technology known under the acronym VTVL, Vertical Takeoff and Vertical Landing [1,2]. The interest in such landing technology is dictated by possible cost reductions [3,4], that impose spaceship’s recycling. The rockets are not generally de- signed to perform landing operations, rather their design is aimed at takeoff operations, guaranteeing a very high forward acceleration to gain the velocity needed to escape the gravitational force. In this paper a new control method based on Feedback Local Optimality Principle, named FLOP is applied to the rocket landing problem. The FLOP belongs to a special class of optimal controllers, developed by the mechatronic and vehicle dynamics lab of Sapienza, named Variational Feedback Controllers - VFC, that are part of an ongoing research and are recently applied in different field: nonlinear system [5], marine and terrestrial autonomous vehicles [6,7,8], multi agents interactions and vibration control [9, 10]. The paper is devoted to show the robustness of the nonlinear controlled system, comparing the performances with the LQR, one of the most acknowledged methods in optimal control
Color Screening, Casimir Scaling, and Domain Structure in G(2) and SU(N) Gauge Theories
We argue that screening of higher-representation color charges by gluons
implies a domain structure in the vacuum state of non-abelian gauge theories,
with the color magnetic flux in each domain quantized in units corresponding to
the gauge group center. Casimir scaling of string tensions at intermediate
distances results from random spatial variations in the color magnetic flux
within each domain. The exceptional G(2) gauge group is an example rather than
an exception to this picture, although for G(2) there is only one type of
vacuum domain, corresponding to the single element of the gauge group center.
We present some numerical results for G(2) intermediate string tensions and
Polyakov lines, as well as results for certain gauge-dependent projected
quantities. In this context, we discuss critically the idea of projecting link
variables to a subgroup of the gauge group. It is argued that such projections
are useful only when the representation-dependence of the string tension, at
some distance scale, is given by the representation of the subgroup.Comment: 24 pages, 14 figures; v2: references added; v3: published version
containing some additional introductory discussio
Hybrid sol-gel coatings containing clay nanoparticles for corrosion protection of mild steel
The development of a new environment-friendly anticorrosive coating for mild steel substrate is afforded in this work. The combined use of cerium, as a self-healing agent, and clay nanoparticles, as improvers of the barrier properties, was considered to the development of new anticorrosive sol-gel coatings. Nanostructured hybrid films were synthesized by the sol-gel route from tetraethoxysilane (TEOS) and 3-glicidoxipropyl-trimetoxisilano (GPTMS) using laminar nanoclays (Laponite Na+0.7[Si8Mg5.5Li0.3H4O24]−0.7) to improve mechanical and barrier properties, and Ce(NO3)3?6H2O as a supplier of Ce(III) to provide an inhibiting effect in the event of coating failure. Carbon steel plates, AISI 1010, were used as substrates. Prior to the application of the coating, samples were treated with a phosphoric acid 2% v/v in order to improve coating adherence.In order to evaluate cerium effect, electrochemical behaviour of films containing Laponite and cerium salts (TGL-Ce) were compared with films containing only Laponite (TGL) by means of potentiodynamic polarization tests and electrochemical impedance spectroscopy (EIS) measurements using a 0.35 wt% NaCl solution. Microstructural characterization and surface analysis of substrates and sol-gel coatings were performed by optical microscopy and by XPS techniques. The use of nanoclays allowed to achieve a significant improvement of the anticorrosive behaviour of the cerium doped coating at the same time that enhances the physical integrity of the coatings under immersion tests.Fil: Santana, Ianina Cecilia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones en Ciencia y Tecnología de Materiales. Universidad Nacional de Mar del Plata. Facultad de Ingeniería. Instituto de Investigaciones en Ciencia y Tecnología de Materiales; ArgentinaFil: Pepe, Andrés. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones en Ciencia y Tecnología de Materiales. Universidad Nacional de Mar del Plata. Facultad de Ingeniería. Instituto de Investigaciones en Ciencia y Tecnología de Materiales; ArgentinaFil: Schreiner, Wido H.. Universidade Federal do Paraná; BrasilFil: Pellice, Sergio Antonio. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones en Ciencia y Tecnología de Materiales. Universidad Nacional de Mar del Plata. Facultad de Ingeniería. Instituto de Investigaciones en Ciencia y Tecnología de Materiales; ArgentinaFil: Ceré, Silvia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones en Ciencia y Tecnología de Materiales. Universidad Nacional de Mar del Plata. Facultad de Ingeniería. Instituto de Investigaciones en Ciencia y Tecnología de Materiales; Argentin
Unification of bulk and interface electroresistive switching in oxide systems
We demonstrate that the physical mechanism behind electroresistive switching
in oxide Schottky systems is electroformation, as in insulating oxides.
Negative resistance shown by the hysteretic current-voltage curves proves that
impact ionization is at the origin of the switching. Analyses of the
capacitance-voltage and conductance-voltage curves through a simple model show
that an atomic rearrangement is involved in the process. Switching in these
systems is a bulk effect, not strictly confined at the interface but at the
charge space region.Comment: 4 pages, 3 figures, accepted in PR
VOLCANICLASTIC DEPOSITION AND MIGRATION OF BASIN DEPOCENTRES AFTER THE ERUPTION OF THE NEAPOLITAN YELLOW TUFF: THE POZZUOLY BAY (NAPLES, ITALY)
The Campi Flegrei is an active caldera located on the coastal
zone of SW Italy (Fig. 1), close to the town of Naples Bay, that has
been characterized by explosive activity and unrest throughout the
Late Quaternary. This area represents a very active segment of the
Eastern Tyrrhenian margin and may be regarded as an ideal
laboratory to understand the mechanisms of caldera dynamics and
the interplay between volcanism, tectonics and sedimentary
processes along a continental back-arc margin. Recent research at
Campi Flegrei has shown that a significant part of the offshore
volcaniclastic products and structures, the late-stage geodynamic
evolution of the inner caldera resurgence and the stratal geometry
of the caldera fill are still poorly known. Particularly, highresolution
seismic data highlight the presence of an intra-caldera
resurgent dome in the inner sector of the Pozzuoli bay that
underwent significant uplift/subsidence after the eruption of the
NYT (Sacchi et al., this volume). In this study we integrate geophysical data of different
resolution/penetration obtained from high-resolution reflection
seismic profiles (Chirp and Sparker sources) partly calibrated by
gravity core in order to provide a 3D depositional model of density
flows and migration of basin depocentres for the Pozzuoli Bay
after the eruption of the NYT.
The new data document the occurrence of two distinct layers
of resediments, mostly represented by density flow deposits,
separated by an interval of hemipelagic sediments. The two density
flow units display a remarkable difference in their thickness and
internal geometry. Across the bay, the lower unit is ca 5m thick in
the western sector and reaches its maximum of ca 10 m in the
central sector while it is absent towards the east. The upper unit, on
the contrary, displays the minimum thickness of 10m close to the
central sector of the bay and increases up to ca 16 and 12m in the
western and eastern sector of the bay, respectively. The variation in
thickness of the density flow deposits appears to be related with
the amount of sediments available. The upper density flow
deposits is also internally more chaotic respect to the lower one,
suggesting higher energy and/or turbulence (Fig. 2).
Sections and isopach maps clearly illustrate that the basin
depocentre topography is not fixed at one position but migrates
southwards in time (Fig. 3). Based on the above observations, we suggest that the uplift of
the resurgent dome and subsidence of the southern sector, occurred
after the eruption of the Neapolitan Yellow Tuff, acted as a major
control in the increase of the sea-floor gradient in the Pozzuoli bay.
This may have triggered in turn, the deposition of gravity flow
deposits along with a progressive migration of basin depocentres
through time
TW Hydrae: evidence of stellar spots instead of a Hot Jupiter
TW Hydrae shows significant radial-velocity variations in the optical regime.
They have been attributed to a 10 Jupiter Mass planet orbiting the star at 0.04
AU. In this work, we have tested whether the observed RV variations can be
caused by stellar spots. We have also analyzed new optical and infrared data to
confirm the signal of the planet companion. We fitted the RV variations of TW
Hya using a cool spot model. Our model shows that a cold spot covering 7% of
the stellar surface and located at a latitude of 54 deg can reproduce the
reported RV variations. The model also predicts a bisector semi-amplitude
variation <10 m/s, which is less than the errors of the RV measurements
discussed in an earlier publication. The analysis of new optical RV data, with
typical errors of 10 m/s, shows a larger RV amplitude that varies depending on
the correlation mask used. A slight correlation between the RV variation and
the bisector is also observed, although not at a very significant level. The
infrared H-band RV curve is almost flat, showing a small variation (<35 m/s)
that is not consistent with the optical orbit. All these results support the
spot scenario rather than the presence of a hot Jupiter around TW Hya.Comment: accepted for publication in A&
Laparoscopic-assisted percutaneous endoscopic gastrostomy: Insertion of a skin-level device using a tear-away sheath
Background: This study describes our experience with the placement of a skin-level gastrostomy device (MIC-KEY) in a single procedure. Methods: We identified infants, children and young adults who underwent laparoscopicassisted percutaneous endoscopic gastrostomy (LAPEG) tube insertion between October 2009 and June 2013. The steps of this procedure include upper endoscopy, single-port laparoscopy, gastropexy via percutaneous T-fasteners and placement of a skin-level gastrostomy device (MIC-KEY) using a push technique with a tearaway sheath. Results: We included 92 patients in our study. Mean age was 3.7 years (range 3 wk- 5 yr), and mean weight was 11.2 (range 2.8-54) kg. Median procedural time was 20 (range 12-76) minutes. Total median duration for the most recent 25 procedures was lower than that of the first 25 (62 v. 79 min, p = 0.004). There were no intraoperative complications or conversions to open surgery. Postoperative complications were observed in 6 (6.5%) patients. Three retained T-fasteners were assessed endoscopically (n = 1) or removed via local excision (n = 2). Two patients experienced early dislodged feeding tubes that were replaced via interventional radiology (n = 1) or repeat LAPEG (n = 1). There was also 1 intra-abdominal fluid collection that was drained percutaneously but ultimately required a laparotomy and washout. There were no major complications in the most recent 50 procedures. Conclusion: Our results suggest that LAPEG is a safe, minimally invasive procedure for infants, children and young adults. This approach allows for immediate use of a skin-level gastrostomy device without the need for postoperative tube exchanges
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