Numerical simulation of the von Kármán sodium dynamo experiment

We present hydrodynamic and magnetohydrodynamic (MHD) simulations of liquid sodium flows in the von Kármán sodium (VKS) set-up. The counter-rotating impellers made of soft iron that were used in the successful 2006 experiment are represented by means of a pseudo-penalty method. Hydrodynamic simulations are performed at high kinetic Reynolds numbers using a large eddy simulation technique. The results compare well with the experimental data: the flow is laminar and steady or slightly fluctuating at small angular frequencies; small scales fill the bulk and a Kolmogorov-like spectrum is obtained at large angular frequencies. Near the tips of the blades the flow is expelled and takes the form of intense helical vortices. The equatorial shear layer acquires a wavy shape due to three coherent co-rotating radial vortices as observed in hydrodynamic experiments. MHD computations are performed: at fixed kinetic Reynolds number, increasing the magnetic permeability of the impellers reduces the critical magnetic Reynolds number for dynamo action; at fixed magnetic permeability, increasing the kinetic Reynolds number also decreases the dynamo threshold. Our results support the conjecture that the critical magnetic Reynolds number tends to a constant as the kinetic Reynolds number tends to infinity. The resulting dynamo is a mostly axisymmetric axial dipole with an azimuthal component concentrated near the impellers as observed in the VKS experiment. A speculative mechanism for dynamo action in the VKS experiment is proposed

Optimal joint routing and link scheduling for real-time traffic in TDMA Wireless Mesh Networks

We investigate the problem of joint routing and link scheduling in Time-Division Multiple Access (TDMA) Wireless Mesh Networks (WMNs) carrying real-time traffic. We propose a framework that always computes a feasible solution (i.e. a set of paths and link activations) if there exists one, by optimally solving a mixed integer-non linear problem. Such solution can be computed in minutes or tens thereof for e.g. grids of up to 4x4 nodes. We also propose heuristics based on Lagrangian decomposition to compute suboptimal solutions considerably faster and/or for larger WMNs, up to about 50 nodes. We show that the heuristic solutions are near-optimal, and we exploit them to investigate the optimal placement of one or more gateways from a delay bound perspective

Direct numerical simulation of the axial dipolar dynamo in the Von Kármán Sodium experiment

For the first time, a direct numerical simulation of the incompressible, fully nonlinear, magnetohydrodynamic (MHD) equations for the Von Kármán Sodium (VKS) experiment is presented with the two counter-rotating impellers realistically represented. Dynamo thresholds are obtained for various magnetic permeabilities of the impellers and it is observed that the threshold decreases as the magnetic permeability increases. Hydrodynamic results compare well with experimental data in the same range of kinetic Reynolds numbers: at small impeller rotation frequency, the flow is steady; at larger frequency, the fluctuating flow is characterized by small scales and helical vortices localized between the blades. MHD computations show that two distinct magnetic families compete at small kinetic Reynolds number and these two families merge at larger kinetic Reynolds number. In both cases, using ferromagnetic material for the impellers decreases the dynamo threshold and enhances the axisymmetric component of the magnetic field: the resulting dynamo is a mostly axisymmetric axial dipole with an azimuthal component concentrated in the impellers as observed in the VKS experiment

Population trends of the fan mussel Pinna nobilis from Portofino MPA (Ligurian Sea, Western Mediterranean Sea) before and after a mass mortality event and a catastrophic storm

Two Pinna nobilis populations thriving inside the borders of the Portofino Marine Protected Area (MPA) (Ligurian Sea, western Mediterranean Sea) were monitored before (2012) and after (September 2018) a dire mass mortality event that, since September 2016, spread through the whole Mediterranean Sea. In Portofino MPA, recorded mortality rates reached values of 91.29% and 43.94% in the two populations. The presence of a Haplosporidium protozoan parasite, considered to be the main cause of the mortality episodes, was confirmed from histological evidence: sporocysts and plasmodia were observed in all the tubules of the digestive glands of the collected specimens. Moreover, a catastrophic storm hit the Ligurian coasts at the end of October 2018, causing considerable damages both below and above the surface; a new survey conducted in November 2018 showed the complete annihilation of the two studied populations, as a probable combination of the continued parasite infections and the mechanical impacts caused by the storm. Finally, in June 2020 the sites were monitored again looking for traces of recovery, but no new specimens were recorded, indicating that P. nobilis became virtually absent from the MPA