1,721,622 research outputs found
Violent disturbance and fragmentation of free surfaces
No full textA multi-phase fluid method has been adopted to model the behaviour of fragmenting interfaces. The flow field is described through the solution of the Navier-Stokes equations with an approximate projection method. The interface separating the two phases is captured by a {em level-set} function. The interface dynamics and its modelling are the main topics addressed in the present numerical study. High gradients of density, viscosity, pressure and velocity are localized at the interface. Therefore attention has to be paid to the discretization of the equations in that area. Here, an original variable coefficients ENO scheme and a redefined reinitalizaiton procedure for the Level set function led to higher accuracy. An exponential smoothing of the density and the split of the Poisson equations for the pressure terms improved the stability properties of the solver. The resulting scheme has been extensively verified and validated through canonical problems, where the method showed good capability of handling: a) high deformation of the interface with breaking and air entrainment; b) generation and evolution of vorticity and c) its interaction with the interface. Dedicated experiments have been performed for the case of a surface piercing plate in forward motion. Flow visualizations and velocity field measurements were carried out and compared with the numerical results. The globally satisfactory agreements allowed for a synergistic use of the numerical and experimental tools within a parametric analysis. The influence of the Froude number and of the plate inclination have been investigated. The former highlighted the role of the post-breaking phenomena in the definition of the different regimes of interaction between vorticity and free surface. The latter highlighted the influence of the inclination on the occurrence of breaking and on the dynamics of the vorticty released. In particular, very large positive and negative inclinations of the plate prevent energetic breaking
Underwater explosions near marine structures: a Dynamics Fluid Structure Domain-Decomposition strategy
Full text linkHistorically underwater explosions (UEs) have been investigated for their huge military relevance, but they remain an important issue also for civil marine applications. As an example, UEs can occur near or on oil-gas plants due to severe environmental conditions or human errors and this has substantial consequences for the production. Preliminary information about time scales of the phenomenon and knowledge about possible strategies on how to limit the consequences of its interaction with close-by structures is crucial to make the proper decisions. A numerical investigation would in general require a 3D compressible (at least) two-phase hydro-dynamic solver strongly coupled with a suitable model of the involved structure. Because the CPU-time requirements are still too high for reliable and feasible predictions, a Domain Decomposition (DD) strategy has been proposed by Colicchio et al. (2013) and Colicchio et al. (2014) and applied to a fully coupled fluid-structure analysis by Colicchio et al. (2015). Here, the DD is further extended as Dynamic DD (DDD) to overcome limits of applicability in time. The dynamic strategy proposed is not limited to UEs problems and to the two coupled solvers involved. When examining the UE interaction with a marine structure, like a surface ship, one can distinguish basically two stages: the first one, with important compressible effects and local fluid-structure interactions; the second one, with global consequences for the structure, possibly involving large deformations and damages as well as free-surface waves generation. The present research focuses on the first stage but the proposed DDD strategy can be adopted also for the second stage.publishedVersionPublished by University of Michiga
Interictal epileptic activity during sleep: a stereo-EEG study in patients with partial epilepsy
No full textCerebral electrical activity was recorded through chronic stereotactically implanted electrodes in 19 epileptic patients suffering from different types of severe and medically refractory partial seizures and who were considered for surgical treatment. 213 brain sites, in all cerebral lobes, in neocortical as well as in archicortical structures, were explored. The behaviour of the interictal spiking across wakefulness and nocturnal physiological sleep was analysed, using automatic elaboration. (i) Spike rate is affected by the occurrence of sleep and by the passage from one sleep phase to another. The degree and direction of the phenomenon differ remarkably in the various patients and, in the same patient, in the different cerebral sites explored. Generally, interictal spiking increases at the beginning of sleep, reaches its maximum during the deep non-REM phases and returns to a level slightly lower than that in wakefulness during REM. (ii) The nocturnal spike rate is hardly influenced by spike location. In most cases, however, the variations recorded during sleep are more significant in the frontal regions than elsewhere. (iii) Spike rate across wakefulness and sleep is affected by the local level of epileptogenicity: spiking variations are less in the most epileptogenic cerebral zone (identified by the origin of the seizure discharges and by the disappearance of seizures following its surgical removal) than elsewhere. The physio-pathological meaning and the diagnostic value of these findings, and particularly of the peculiar stability or autonomy of the electrical epileptic activity of the most epileptogenic cerebral zone, is discussed
Resection surgery for partial epilepsy. Relation of surgical outcome with some aspects of the epileptogenic process and surgical approach
No full textIn spite of the progressive improvement of the results of resective surgery for epilepsy, the number of not significantly benefited patients remains high. An attempt was made to find out a relation between outcome and some aspects of the pathophysiological organization of the epileptogenic process and of the surgical procedure. Chi-square and logistic regression statistic analyses were utilized. The study was retrospectively performed on 138 surgically treated patients having a minimum follow-up of three years. Three classes of surgical outcome were considered: completely seizure free (including aura; 86 cases, 62.3%), significant seizure reduction (31 cases, 22.5%), and no significant improvement (21 cases, 15.2%). What follows was brought into evidence by the study. 1) On the diagnostic side, the spatial arrangement (focal, unilateral, multifocal) of both the interictal and the ictal epileptic electrocerebral activities are significantly associated with the surgical outcome. Their relative impact on outcome is related to the presence of a structural lesion: when a lesion is documented, the interictal activity has the higher value: vice versa, when no lesion is apparent, the role of the ictal activity is prevalent. However, the presence, as well as the nature of the lesion, per se, are not significantly associated with outcome. 2) On the surgical side, the extent of resection of both the structural lesion and of the epileptogenic zone are highly associated with the surgical result; the extent of lesion resection prevails on that of the epileptogenic zone. The type of surgical approach (hemispherectomy: 17 cases; temporal lobectomy: 67 cases; extratemporal resection: 54 cases) has no significant relation to the outcome. The value and the limits of the results obtained are discussed
Short and middle-latency Median Nerve (MN) SEPs recorded by depth electrodes in human pre-SMA and SMA-proper
No full textOBJECTIVE: To analyse waveforms, latencies and amplitudes of Median Nerve (MN) SEPs recorded by stereotactically electrodes implanted in the SMA of 14 epileptic patients (9 in pre-SMA,3 in SMA-proper, 2 in both) in order to evaluate which short and middle-latency SEPs are generated in this area and which could be the physiological relevance of these responses. METHODS: Short and middle-latency MN SEPs were recorded by chronically implanted electrodes in the fronto-temporal cortex and in particular in the mesial frontal region of 14 drug-resistant epileptic patients. MN stimulations of 100 micros were delivered by skin electrodes at the wrist; stimulus intensity was adjusted slightly above the motor threshold. RESULTS: The main result of this study is that middle-latency SEPs were originated in pre-SMA but not in SMA-proper as demonstrated by both referential and bipolar recordings. In particular off-line computed bipolar traces between neighbouring contacts implanted in the pre-SMA and in the frontal external regions showed a phase reversal at the deepest contacts located in pre-SMA. Conversely, bipolar recordings between neighbouring contacts implanted in the SMA-proper and in the frontal external regions showed inversion recovery at more superficial contacts, implanted in area 6. Finally, we confirmed that no short-latency MN SEP (and in particular the N30) is originated in the whole SMA. CONCLUSIONS: Among premotor areas, somatosensory inputs seem to reach pre-SMA and area 6 but not SMA-proper. SIGNIFICANCE: This study assessed that no scalp SEP in the first 100 ms after MN stimulus could be generated in SMA-proper
[Research on the epileptogenic complex]
No full textThe great importance of a correct definition of the lesional-functional epileptogenic complex for the surgical treatment of the epileptic patient is stressed. The means utilizable to reach the knowledge of the topographic organizatif the latters are descri0ed and discussed in detail: the scalp EEG, the examinations based on the EEG effects of endocarotid injection of barbiturates and convulsants, the EEG during nocturnal sleep, the direct recording of electrocerebral activity from the cortical surface (electro-corticography) and from deep cerebral structures (stereoelectro-encephalography). The relative importance of the informations provided by the electrical activity of "lesional" type, by the interictal epileptic activity and by the ictal one is analyzed. The utilization of methodologies of automatic elaboration of the electrocerebral signals is reminded. Finally, the different modes of application of the means of analysis mentioned above in the different epileptic patients are discussed and exemplified
Underwater explosions near marine structures: a Dynamics Fluid Structure Domain-Decomposition strategy
No full textHistorically underwater explosions (UEs) have been investigated for their huge military relevance, but they remain an important issue also for civil marine applications. As an example, UEs can occur near or on oil-gas plants due to severe environmental conditions or human errors and this has substantial consequences for the production. Preliminary information about time scales of the phenomenon and knowledge about possible strategies on how to limit the consequences of its interaction with close-by structures is crucial to make the proper decisions. A numerical investigation would in general require a 3D compressible (at least) two-phase hydro-dynamic solver strongly coupled with a suitable model of the involved structure. Because the CPU-time requirements are still too high for reliable and feasible predictions, a Domain Decomposition (DD) strategy has been proposed by Colicchio et al. (2013) and Colicchio et al. (2014) and applied to a fully coupled fluid-structure analysis by Colicchio et al. (2015). Here, the DD is further extended as Dynamic DD (DDD) to overcome limits of applicability in time. The dynamic strategy proposed is not limited to UEs problems and to the two coupled solvers involved. When examining the UE interaction with a marine structure, like a surface ship, one can distinguish basically two stages: the first one, with important compressible effects and local fluid-structure interactions; the second one, with global consequences for the structure, possibly involving large deformations and damages as well as free-surface waves generation. The present research focuses on the first stage but the proposed DDD strategy can be adopted also for the second stage
Study of the electrocerebral activity in partial epilepsies: its role for surgical treatment
No full textPartial epilepsies, at least 90 percent of all epileptic syndromes, are the result of a localized abnormal brain hyperexcitability due to organic brain disease. The combination of the causal brain pathology and of the abnormally hyperexcitable neuronal population form what we have called the "lesional-functional epileptogenic complex
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