Comparison of different methods to predict the mean flow velocity in step-pool channels

Steep mountain streams have irregular bed topography, where the mean flow velocity is heavily affected by the coarsest bed components and by their arrangement to form step pools, cascades, and rapids. According to literature findings the mean flow velocity is often related with water discharge, channel slope, and grain-size related variables through power relationships. Several approaches consider dimensionless hydraulic geometry terms to develop the analysis over a wide range of channel sizes and hydraulic conditions. The aim of this research is to test the performance of some literature formulas to directly compute the mean flow velocity (V) in step-pool sequences. The study area deals with two fish ladders located in the Vanoi torrent (Trento Province, Italy), which were built by mimicking the steppool morphology. Three reaches were selected to cover different channel slopes (2.6-10%). Data collection entailed three main phases: (1) topographical surveys, (2) granulometric analysis, and (3) flow discharge measurements (salt dilution method). Geometric and hydraulic variables were measured for the following step-pool cross sections: step head, pool center, and intermediate position between pool end next step. Particular attention has been reserved to determine the effective mean flow velocity over the whole path of each step pool sequence. The performance of different literature equations to predict V has been verified. The relations have been shared in three groups: dimensional (V), dimensionless with respect to the grain size (V*) or to a combination of grain size and slope (V**). In general, the V group of equations has produced the highest errors between computed and measured values. The dimensionless V*, V** groups have shown the best performance. In particular the V* equations, which use unit discharge and channel slope, have provided the better fitting, and the lowest root mean square error. The results highlight the difficult to estimate flow velocity in step-pool sequences, and the attitude of this channel-bed morphology to be highly dissipative. The good performance of some dimensionless equations to predict V could also support the hydraulic designer in case the ‘morphological rebuilding’ of mountain creeks is opportune. Further analyses are required to better understand the flow behavior in streams where very rough bed forms and hydraulic drops are the primary sources of flow energy dissipation

Viscosity and thermal conductivity effects at first-order phase transitions in heavy-ion collisions

Effects of viscosity and thermal conductivity on the dynamics of first-order phase transitions are studied. The nuclear gas-liquid and hadron-quark transitions in heavy-ion collisions are considered. We demonstrate that at non-zero thermal conductivity, $\kappa \neq 0$, onset of spinodal instabilities occurs on an isothermal spinodal line, whereas for $\kappa =0$ instabilities take place at lower temperatures, on an adiabatic spinodal.Comment: invited talk at 6th International Workshop on Critical Point and Onset of Deconfinment (CPOD2010), Dubna, August 22-28, 201

Isotopic composition of fragments in multifragmentation of very large nuclear systems: effects of the chemical equilibrium

Studies on the isospin of fragments resulting from the disassembly of highly excited large thermal-like nuclear emitting sources, formed in the ^{197}Au + ^{197}Au reaction at 35 MeV/nucleon beam energy, are presented. Two different decay systems (the quasiprojectile formed in midperipheral reactions and the unique source coming from the incomplete fusion of projectile and target in the most central collisions) were considered; these emitting sources have the same initial N/Z ratio and excitation energy (E^* ~= 5--6 MeV/nucleon), but different size. Their charge yields and isotopic content of the fragments show different distributions. It is observed that the neutron content of intermediate mass fragments increases with the size of the source. These evidences are consistent with chemical equilibrium reached in the systems. This fact is confirmed by the analysis with the statistical multifragmentation model.Comment: 9 pages, 4 ps figure

Assortativity Decreases the Robustness of Interdependent Networks

It was recently recognized that interdependencies among different networks can play a crucial role in triggering cascading failures and hence system-wide disasters. A recent model shows how pairs of interdependent networks can exhibit an abrupt percolation transition as failures accumulate. We report on the effects of topology on failure propagation for a model system consisting of two interdependent networks. We find that the internal node correlations in each of the two interdependent networks significantly changes the critical density of failures that triggers the total disruption of the two-network system. Specifically, we find that the assortativity (i.e. the likelihood of nodes with similar degree to be connected) within a single network decreases the robustness of the entire system. The results of this study on the influence of assortativity may provide insights into ways of improving the robustness of network architecture, and thus enhances the level of protection of critical infrastructures

Critical Temperature for the Nuclear Liquid-Gas Phase Transition

The charge distribution of the intermediate mass fragments produced in p (8.1 GeV) + Au collisions is analyzed in the framework of the statistical multifragmentation model with the critical temperature for the nuclear liquid-gas phase transition $T_c$ as a free parameter. It is found that $T_c=20\pm3$ MeV (90% CL).Comment: 4 pages, 3 figures, published in Phys. Rev.

The new IMGC-02 transportable absolute gravimeter: measurement apparatus and applications in geophysics and volcanology

The research carried out at the Istituto Nazionale di Ricerca Metrologica (formerly Istituto di Metrologia «G. Colonnetti») aiming to develop a transportable ballistic absolute gravimeter ended with a new version of the instrument, called the IMGC-02. It uses laser interferometry to measure the symmetrical free rising and falling motion of a test mass in the gravity field. Providing the same accuracy achieved with previous versions, the instrumental improvements mainly concern size, weight, data processing algorithms and operational simplicity. An uncertainty of 9 μGal (1 μGal=1×10–8 m·s−2) can be achieved within a single observation session, lasting about 12 h, while the time series of several observation sessions show a reproducibility of 4 μGal. At this level, gravity measurements provide useful information in Geophysics and Volcanology. A wide set of dynamic phenomena, i.e. seismicity and volcanic activity, can produce temporal gravity changes, often quite small, with an amplitude ranging from a few to hundreds of microgals. Therefore the IMGC absolute gravimeter has been employed since 1986 in surveying the Italian active volcanoes. A brief history of the gravimeter and the description of the new apparatus, together with the main results of ongoing applications in Geophysics and Volcanology are presented

Analysis of fragment yield ratios in the nuclear phase transition

The critical phenomena of the liquid-gas phase transition has been investigated in the reactions 78,86Kr+58,64Ni at beam energy of 35 MeV/nucleon using the Landau free energy approach with isospin asymmetry as an order parameter. Fits to the free energy of fragments showed three minima suggesting the system to be in the regime of a first order phase transition. The relation m =-{\partial}F/{\partial}H, which defines the order parameter and its conjugate field H, has been experimentally verified from the linear dependence of the mirror nuclei yield ratio data, on the isospin asymmetry of the source. The slope parameter, which is a measure of the distance from a critical temperature, showed a systematic decrease with increasing excitation energy of the source. Within the framework of the Landau free energy approach, isoscaling provided similar results as obtained from the analysis of mirror nuclei yield ratio data. We show that the external field is primarily related to the minimum of the free energy, which implies a modification of the source concentration \Delta used in isospin studies