Statistical distributions in the folding of elastic structures

The behaviour of elastic structures undergoing large deformations is the result of the competition between confining conditions, self-avoidance and elasticity. This combination of multiple phenomena creates a geometrical frustration that leads to complex fold patterns. By studying the case of a rod confined isotropically into a disk, we show that the emergence of the complexity is associated with a well defined underlying statistical measure that determines the energy distribution of sub-elements,``branches'', of the rod. This result suggests that branches act as the ``microscopic'' degrees of freedom laying the foundations for a statistical mechanical theory of this athermal and amorphous system

‘‘Lozenge’’ contour plots in scattering from polymer networks

We present a consistent explanation for the appearance of “lozenge” shapes in contour plots of the two dimensional scattering intensity from stretched polymer networks. By explicitly averaging over quenched variables in a tube model, we show that lozenge patterns arise as a result of chain material that is not directly deformed by the stretch. We obtain excellent agreement with experimental data

The galaxy luminosity function of the Abell 496 cluster and its spatial variations

We investigate the LF in the very relaxed cluster Abell 496. Our analysis is based on deep images obtained at CFHT with MegaPrime/MegaCam in four bands (ugri) covering a 1x1 deg2 region, which is centered on the cluster Abell 496 and extends to near its virial radius. The LFs are estimated by statistically subtracting a reference field taken as the mean of the 4 Deep fields of the CFHTLS survey. Background contamination is minimized by cutting out galaxies redder than the observed Red Sequence in the g-i versus i colour-magnitude diagram. In Abell 496, the global LFs show a faint-end slope alpha=-1.55+/-0.06 and vary little with observing band. Without colour cuts, the LFs are much noisier but not significantly steeper. The faint-end slopes show a statistically significant steepening from alpha=-1.4+/-0.1 in the central region (extending to half a virial radius) to -1.8+/-0.1 in the Southern envelope of the cluster. Cosmic variance and uncertain star-galaxy separation are our main limiting factors in measuring the faint-end of the LFs. The large-scale environment of Abell 496, probed with the fairly complete 6dFGS catalogue, shows a statistically significant 36 Mpc long filament at PA=137 deg, but we do not find an enhanced LF along this axis. Our LFs do not display the large number of dwarf galaxies (alpha ~ -2) inferred by several authors, whose analyses may suffer from field contamination caused by inexistent or inadequate colour cuts. Alternatively, different clusters may have different faint-end slopes, but this is hard to reconcile with the wide range of slopes found for given clusters and for wide sets of clusters.Comment: Accepted in A&A. 13 pages, 11 figure

Frequency-adaptive control of a three-phase single-stage grid-connected photovoltaic system under grid voltage sags

The low-voltage ride-through service is carried out in this paper according to the voltage profile described by the IEC 61400-21 European normative when short-duration voltage sags happen, and some instantaneous reactive power is delivered to the grid in accordance with the Spanish grid code; the mandatory limitation of the amplitude of the three-phase inverter currents to its nominal value is carried out with a novel control strategy, in which a certain amount of instantaneous constant active power can also be delivered to the grid when small or moderate voltage sags happen. A Multiple second order generalized integrator frequency-locked loop synchronization algorithm is employed in order to estimate the system frequency without harmonic distortions, as well as to output the positive- and the negative- sequence of the {\alpha}\b{eta} quantities of the three-phase grid voltages when balanced and unbalanced voltage sags happen in a frequency-adaptive scheme. The current control is carried out in the stationary reference frame, which guarantees the cancellation of the harmonic distortions in the utility grid currents using a Harmonic compensation structure, and the implementation of a constant active power control in order to protect the DC link capacitor from thermal stresses avoiding the appearance of large harmonic distortions at twice the fundamental frequency in the DC link voltage. A case study of a three-phase single-stage grid-connected PV system with a maximum apparent power about 500 kVA is tested with several simulations using MATLAB/SIMULINK firstly, and secondly, with some experiments using the Controller hardware-in-the-loop (CHIL) simulation technique for several types of voltage sags in order to do the final validation of the control algorithms

Editorial: Grid Connection of Converters in Renewable Applications

Energy generated from renewable sources is fed into the grid by means of electronic power converters. These can be supervised at system (grid) level to coordinate all productions points together with storages and loads. Regulations impose power supply quality requirements regarding harmonics, grid fault response and low voltage ride through (LVRT). The progress of distributed generation presents challenges to converters such as island mode operation, voltage and frequency regulation, simulation, etc. New collaborative solutions for “more smart” microgrids must be included to improve power quality, reliability, service quality and duty. Wind turbines employing double-fed induction generators (DFIG) use two converters, one for the rotor side and one for the generator side. To improve the performance during severe grid failures, in Okedu and Barghash the advantages of using alternative configurations to the two-level converter, such as the parallel interleaved 2-level inverter, and the 3-level inverter, have been investigated. It has also been investigated to replace the classical dq-PLL with a new PLL, and to include a series dynamic braking resistor (SDBR) between the converters and the three-phase connections. Wind turbines must meet strict requirements, in terms of their behavior, in the event of grid failures, which are regulated by the LVRT regulations in each country. These regulations indicate, by means of voltage and time graphs, how long the wind turbines must remain connected depending on the depth of the faults. In addition, the limits of active and reactive power that can be exchanged during faults are established. The aim is to avoid cascading disconnections of wind turbines that would compromise the stability of the grid. In Okedu and Barghash, the effect of various elements in improving the behaviour of a DFIG against grid faults has been investigated. The first of these elements is the parameters of the IGBTs, concluding that the on-resistance has the greatest influence. The second is the use of a new PLL, and the third is the use of a SDBR during a grid failure. It was found that all of them could improve the performance of the generator in the event of a grid failure. When a wind turbine uses a permanent magnet synchronous generator (PMSG), 100% of the energy generated passes through both converters. In Okedu and Barghash, the control systems of the generator-side and grid-side converters have been considered; several scenarios regarding the turn on resistance of the IGBTs have been considered, and their behaviour during grid faults has been analysed. Generator performance has also been studied with and without the use of a DC-DC converter for overvoltage protection. The main generators used in wind turbines are DFIG and PMSG. In Okedu et al., a comparison of the behaviour of both wind turbines against grid faults has been carried out in various scenarios with different values of generator parameters. The number of converters, associated with renewable generation, connected to the grid has increased significantly lately. This can affect the dynamic response, especially during disturbances, but it can also provide new grid support functionalities if information on the oscillation characteristics is available. Through the use of artificial intelligence, in Baltas et al. the abilities to predict and damp electromechanical oscillations have been improved. With the constant increase in the number of wind turbines connected to the grid, it is very important to have the ability to maintain grid frequency control. In Okedu and Barghash, a work has been presented to stabilise the wind farm during periods of wind speed change by using capacitors connected via a DC-DC converter and a grid-connected DC-AC converter. It was concluded that higher values of the DC-DC converter time constant lead to better performances during load transients. A system including two steam turbines and two squirrel cage induction generators was used in the experiments. Microgrids bring power generation closer to the places of consumption to reduce the saturation of distribution lines. They consist of renewable generation, energy storage and fossil fuel generation. They have three levels of control, where the primary level is the closest to the converters, and the tertiary level, the most external and slowest, performs general monitoring functions. The paper (Buraimoh et al.) focuses on the secondary control functions related to grid failure performance. It proposes a distributed control between inverters and is based on fast detection techniques (fast Delayed Signal Cancellation, DSC), with the objective of a fast control of active and reactive power. A robust transition method between fault mode and normal mode is proposed. Accurate coordination and power sharing between distributed energy resources is achieved. Some energy conversion systems are so complex that they are very difficult to build and test in the laboratory. These include the study of high voltage direct current (HVDC) transmission when several modular multilevel converters (MMC) are involved together with DC grid failure protection elements. In Wang et al., a system including a simulated part (two digitally simulated MMC) and a physical part (two MMC) has been experimented with. The coupling between the two parts has been carried out by means of A/D and D/A converters and power amplifiers

Plastic Deformation of 2D Crumpled Wires

When a single long piece of elastic wire is injected trough channels into a confining two-dimensional cavity, a complex structure of hierarchical loops is formed. In the limit of maximum packing density, these structures are described by several scaling laws. In this paper it is investigated this packing process but using plastic wires which give origin to completely irreversible structures of different morphology. In particular, it is studied experimentally the plastic deformation from circular to oblate configurations of crumpled wires, obtained by the application of an axial strain. Among other things, it is shown that in spite of plasticity, irreversibility, and very large deformations, scaling is still observed.Comment: 5 pages, 6 figure

An optical view of the filament region of Abell 85

We compare the distribution of optically and Halpha (Ha) selected galaxies in the Southern half of the nearby Abell 85 (A85) cluster with the recently discovered X-ray filament (XRF). We search for galaxies where star formation (SF) may have been triggered by interactions with intracluster gas or tidal pressure due to the cluster potential when entering the cluster. Our analysis is based on images obtained with CFHT MegaPrime/MegaCam (1x1 deg2 field) in four bands (ugri) and ESO 2.2mWFI (38'x36' field) in a narrow band filter corresponding to the redshifted Halpha (Ha) line and in a broad R-band filter. The LFs are estimated by statistically subtracting a reference field. Background contamination is minimized by cutting out galaxies redder than the observed red sequence in the g-i vs. i colour-magnitude diagram. The galaxy distribution shows a significantly flattened cluster, whose principal axis is slightly offset from the XRF. The analysis of the broad band LFs shows that the filament region is well populated. The filament is also independently detected as a gravitationally bound structure by the Serna & Gerbal hierarchical method. 101 galaxies are detected in Ha, among which 23 have spectroscopic redshifts in the cluster, 2 have spectroscopic redshifts higher than the cluster and 58 have photometric redshifts that tend to indicate that they are background objects.The 23 galaxies with spectroscopic redshifts in the cluster are mostly concentrated in the South part of the cluster and along the filament. We find a number of galaxies showing evidence for SF in the XRF, and all our results are consistent with the previous hypothesis that the XRF in A85 is a gravitationally bound structure made of groups falling on to the main cluster.Comment: Accepted in A&A. 39 pages, 107 figures. Full resolution images available at ftp://ftp.iap.fr/pub/from_users/gam/A85

Statistical Mechanics of Two-dimensional Foams

The methods of statistical mechanics are applied to two-dimensional foams under macroscopic agitation. A new variable -- the total cell curvature -- is introduced, which plays the role of energy in conventional statistical thermodynamics. The probability distribution of the number of sides for a cell of given area is derived. This expression allows to correlate the distribution of sides ("topological disorder") to the distribution of sizes ("geometrical disorder") in a foam. The model predictions agree well with available experimental data

The cluster Abell 780: an optical view

The Abell 780 cluster, better known as the Hydra A cluster, has been thouroughly analyzed in X-rays. However, little is known on its optical properties. We derive the galaxy luminosity function (GLF) in this apparently relaxed cluster, and search for possible environmental effects by comparing the GLFs in various regions, and by looking at the galaxy distribution at large scale around Abell 780. Our study is based on optical images obtained with the ESO 2.2m telescope and WFI camera in the B and R bands, covering a total region of 67.22x32.94 arcmin^2, or 4.235x2.075 Mpc^2 for a cluster redshift of 0.0539. In a region of 500 kpc radius around the cluster centre, the GLF in the R band shows a double structure, with a broad and flat bright part and a flat faint end that can be fit by a power law with an index alpha=-0.85+-0.12 in the 20.25<R<21.75 interval. If we divide this 500 kpc radius region in North+South or East+West halves, we find no clear difference between the GLFs in these smaller regions. No obvious large scale structure is apparent within 5 Mpc from the cluster, based on galaxy redshifts and magnitudes collected from the NED database in a much larger region than that covered by our data, suggesting that there is no major infall of material in any preferential direction. However, the Serna-Gerbal method reveals the presence of a gravitationally bound structure of 27 galaxies, which includes the cD, and of a more strongly gravitationally bound structure of 14 galaxies. These optical results agree with the overall relaxed structure of Abell 780 previously derived from X-ray analyses.Comment: Accepted for publication in Astronomy & Astrophysic