60 research outputs found
Fluid dynamics on ultrastatic spacetimes and dual black holes
We show that the classification of shearless and incompressible stationary
fluid flows on ultrastatic manifolds is equivalent to classifying the
isometries of the spatial sections. For a flow on R x S this leaves only
one possibility, since on the 2-sphere all Killing fields are conjugate to each
other, and it is well-known that the gravity dual of such a (conformal) fluid
is the spherical KNAdS black hole. On the other hand, in R x H the
situation is more complicated, since the isometry group of H admits
elliptic, parabolic and hyperbolic elements. One might thus ask what the
gravity duals of the flows corresponding to these three different cases are.
Answering this question is one of the scopes of this paper. In particular we
identify the black hole dual to a fluid that is purely translating on the
hyperbolic plane. Although this lies within the Carter-Plebanski (CP) class, it
has never been studied in the literature before, and represents thus in
principle a new black hole solution in AdS. For a rigidly rotating fluid in
R x H (holographically dual to the hyperbolic KNAdS solution), there is
a certain radius where the velocity reaches the speed of light, and thus the
fluid can cover only the region within this radius. Quite remarkably, it turns
out that the boundary of the hyperbolic KNAdS black hole is conformal to
exactly that part of R x H in which the fluid velocity does not exceed the
speed of light. We extend these results to establish a precise mapping between
possible flows on ultrastatic spacetimes (with constant curvature spatial
sections) and the parameter space of the CP solution. Finally, we show that the
alternative description of the hyperbolic KNAdS black hole in terms of
fluid mechanics on R x S or on flat space is dynamical and consists of a
contracting or expanding vortex.Comment: 43 pages, many figures. v2: Final version to appear in JHE
Moduli of sheaves, quiver moduli, and stability
The goal of this thesis is to provide a modern interpretation and an extension of the classical
works of the 1970s and 1980s constructing moduli spaces of vector bundles and coherent sheaves
on projective spaces by means of "linear data", that is spaces of matrices modulo a linear group
action. These works culminated with the description by Dr\ue9zet and Le Potier of the moduli spaces
of Gieseker-semistable sheaves on P2 as what are called today quiver moduli spaces. We show
that this can be naturally understood and generalized using the language of derived categories and
stability structures on them. In particular, we obtain analogous explicit constructions for moduli
of sheaves on P1xP1, and we investigate these moduli spaces using the theory of quiver moduli
Fatigue strength of corroded bolted connection
This note summarizes some recent investigation results on the behavior of corroded steel bolted joints under uniaxial fatigue loading. Fatigue test specimens, were made up using S355 structural steel plates joined together with preloaded M12 bolts of class 10.9 with a geometry that corresponds to the Δσ = 112 MPa EC3 detail category. The accelerated corrosion process was accomplished using an electrolyte consisting of an aqueous 5% NaCl solution whereby the specimens were treated. In particular, during the corrosion process specimens were repeatedly immersed for 2 minutes in the electrolyte and then removed keeping them 60 minutes long in free air at 35 °C. An atmospheric corrosion in marine-industrial environment is wellrepresented through corrosion test. Fatigue loading tests and surface morphology measurement of uncorroded and corroded specimens were performed and the results were compared
numerical analyses of corroded bolted connections
Abstract The interaction between fatigue and corrosion is the phenomena that is called Corrosion Fatigue (CF) and regarding steel structures subjected to cyclic loads. The S-N curves proposed by the main International Standards for fatigue life assessment do not take into account the state of degradation of the detail. For this reason, in this paper, a local approach is used to determine the fatigue life of corroded bolted joint with preload high strength bolts. In particular, fatigue life estimates are presented using the strain life method based on numerical analysis conducted on the joint, assuming that the crack nucleation phase is predominant in the whole fatigue life. The models used to simulate bolted joint are implemented using solid and contact elements and the geometry is realized taking into account the geometric imperfections produced by pitting corrosion. These imperfections were measured by surface surveys with a 3D profilometer. In conclusion, the results of the numerical analysis conducted on corroded joint model were compared with the experimental results obtained from cyclical tests
Surgical Options In Oroantral Fistula Treatment
Oral fistula (OAF) is a pathological communication between the oral cavity and maxillary sinus which has its origin either from iatrogenic complications or from dental infections, osteomyelitis, radiation therapy or trauma. OAF closures can be achieved using different flaps which show both advantages and limitations. Therefore they all need careful consideration in order to select the best approach depending on the situation. The most widely employed flaps are of three types: vestibular flap, palatal flap and buccal fat pad Flap(BFP). The authors present three cases of OAF with the different techniques. It is suggested that the buccal flap is best applied in the case of large fistulas located in the anterior region, the palatal flap is suitable to correct premolar defects and the BFP flap for wide posterior OAFs
Multiple teeth replacement with endosseous one-piece yttrium-stabilized zirconia dental implants
Objectives: The purpose of this study is to clinically and radiographically evaluate survival and success rate of multiple
zirconia dental implants positioned in each patient during a follow-up period of at least 12 months up to 48 months.
Study
Design: Eight patients were treated for multiple edentulism with 29 zirconia dental implants. All implants
received immediate temporary restorations and 6 months after surgery were definitively restored. 6 months to 4
years after implant insertion, a clinical-radiographic evaluation was performed in order to estimate peri-implant
tissues health and peri-implant marginal bone loss.
Results: Survival rate within follow-up period was therefore 100%. The average marginal
bone
loss (MBL)
from
baseline to 6
months was
+1.375±0.388 mm;
from
6 months
to 1 year
was
+0.22±0.598 mm; from
1 year
to 2
years
was
-0.368±0.387 mm;
from 2 years to
3 years
was
-0.0669±0.425 mm; from 3 years to 4 years +0.048±0.262
mm. The mean marginal bone loss at 4 years from the implants insertion was +1.208 mm.
Conclusions: According to several studies, when using a radiographic criterion for implant success, marginal bone
loss below 0.9-1.6 mm during the first year in function can be considered acceptable. In our work, radiographic
measurements of MBL showed values not exceeding 1.6 mm during the first year of loading and also 1 year
up to 4 years after surgery further marginal bone loss was minimal and not significant. This peri-implant bone
preservation may be associated to the absence of micro-gap between fixture and abutment since zirconia dental
implants are one-piece implant. Moreover, zirconia is characterized by high biocompatibility and it accumulates
significantly fewer bacteria than titanium
Polyphased contractions inside the Sicily Channel Rifting Zone: new evidence from seismic reflection profiles analysis and geodynamic implications
The Sicily Channel, located in the foreland area of the African plate, is a very interesting geological area in the Western-Central Mediterranean, as it has undergone different tectonic processes because of its proximity to the convergence zone with the European plate. Extension and opening of a rift zone (Sicily Channel Rift Zone, including the Pantelleria, Malta, and Linosa grabens) occurred in the lower plate of the subduction zone marked by the Gela Thrust System and the Calabrian Accretionary Wedge, respectively located south and south-east of Sicily, and the Maghrebian chain to the west. We analyzed geological and geophysical data, such as variable penetration seismic reflection profiles integrated with borehole data; these allowed us to investigate subsurface structures down to the crust-mantle boundary. The crustal profile shows a Moho deepening down to 11.8 s/(TWT) under the Gela Thrust System and going up to 8 s/(TWT) under the Linosa Graben. Moreover, the presence of several hyperbolae zones and signal anomalies have been linked to a rise of deep fluids associated with mantle uplift and, upward, to magmatic intrusions. The sub-surface also shows evidence of a N-S oriented zone, from the Gela Thrust System to the Malta and Linosa grabens, which has undergone contractional tectonic events superimposed on previous extensional structures. Throughout this area, from the Early-Middle Miocene to the Early Pliocene, an extensional event occurred in association with the slab roll-back of the African Plate. In this phase, several volcanic intrusions concentrated near the grabens’ rims suggest a relation between the extension, the Moho rising, and the magmatic manifestations. Afterward, a compressional event in the Madrepore and Malta Grabens was registered. This event has been correlated to the advance of the Gela Thrust Front, which, according to literature bio-chronostratigraphic analysis, had three stages of advancement in Zanclean, Piacenzian and Chibanian. Furthermore, a recent contractional event caused the folding of the seafloor in the central part of the Malta Graben. This latter phase has been related to a potential change in the subduction polarity. These results provide new insights into the regional kinematic setting of the Sicily Channel, suggesting that strain located within the African Plate can be explained through the overlapping of both intra-plate (localized asthenospheric rise) and inter-plate (compression transmitted from surrounding mountain belts) processes ongoing between Europa and Africa. Indeed, the Sicily Channel structural setting resulted from the interplay of the rollback of the African slab, the consequent changes in the asthenospheric flow that caused extension and local magmatic intrusions, and the active subduction front and its potential polarity reversal that caused local and polyphased compressional pulses
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