Experimental and numerical investigations on flashing-induced instabilities in a single channel

During the start-up phase, natural circulation BWRs (NC-BWRs) need to be operated at low pressure conditions. Such conditions favor flashing-induced instabilities due to the large hydrostatic pressure drop induced by the tall chimney. Moreover, in novel NC-BWR designs the steam separation is performed in the steam separators which create large pressure drops at the chimney outlet, which effect on stability has not been investigated yet.In this work, flashing-induced oscillations occurring in a tall, bottom heated channel are numerically investigated by using a simple linear model with three regions and an accurate implementation for estimating the water properties. The model is used to investigate flashing-induced instabilities in a channel for different values of the core inlet friction value. The results are compared with experiments obtained by using the CIRCUS facility at the same conditions, showing a good agreement. In addition, the experiments on flashing-induced instabilities are presented in a novel manner allowing visualizing new details of the phenomenon numerical stability investigations on the effect of the friction distribution are also done. It is found that by increasing the total restriction in the channel the system is destabilized. In addition, the chimney outlet restriction has a stronger destabilizing effect than the core inlet restriction. A stable two-phase region is observed prior to the instabilities in the experiments and the numerical simulations which may help to pressurize the vessel of NC-BWRs and thus reducing the effects of flashing instabilities during start-up.     

Self-gravito-acoustic waves and their instabilities in a self-gravitating degenerate quantum plasma system

A self-gravitating degenerate quantum plasma (SGDQP) system containing degenerate electron and light nucleus species along with extremely low-dense heavy-nucleus species is considered. The existence of new degenerate pressure-driven self-gravito-acoustic (DPDSGA) waves in this SGDQP system is found, and their dispersion properties along with stable and unstable parametric regimes are identified. The DPDSGA waves emit from this SGDQP system due to the compression and rarefaction (and vice-versa) of the perturbed state of it. Its compression is due to the inward poll of degenerate electron and light nucleus species by the self-gravitational attractive pressures, whereas its rarefaction is due to the outward degenerate pressures exerted by the degenerate electron and light nucleus species. The DPDSGA waves are new because they completely disappear if the electron and light nucleus degeneracies are neglected. The DPDSGA waves exist in the SGDQP system that occurs in astrophysical compact objects like white dwarfs [H. M. Van Horn, Science 252 , 384 (1991); D. Koester, Astron. Astrophys. Rev. 11 , 33 (2002)].     

Channel confined active nematics

Active nematics is a popular model fluid for active matter. The popularity comes from the fact that several biological systems involving cells and cytoskeletal elements closely match active nematic fluid. Moreover, the theory of active nematics is amenable for analytical and computational developments. This review discusses different flow states and flow transitions exhibited by channel confined active nematics. The discussions based on experimental and theoretical investigations reveal the role of inherent hydrodynamic instabilities, the unique fluid properties, and the bounding geometry in dictating the behavior of active nematic fluids in channel confinements. The discussions also highlight the current and outstanding research questions in the field.     

A Critique of Recent Semi‐Classical Spin‐Half Quantum Plasma Theories

Certain recent semi‐classical theories of spin‐half quantum plasmas are examined with regard to their internal consistency, physical applicability and relevance to fusion, astrophysical and condensed matter plasmas. It is shown that the derivations and some of the results obtained in these theories are internally inconsistent and contradict well‐established principles of quantum and statistical mechanics, especially in their treatment of fermions and spin. Claims of large semi‐classical effects of spin magnetic moments that could dominate the plasma dynamics are found to be invalid both for single‐particles and collectively. Larmor moments dominate at high temperature while spin moments cancel due to Pauli blocking at low temperatures. Explicit numerical estimates from a variety of plasmas are provided to demonstrate that spin effects are indeed much smaller than many neglected classical effects. The analysis presented suggests that the aforementioned ‘Spin Quantum Hydrodynamic’ theories are not relevant to conventional laboratory or astrophysical plasmas. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Electron correlation in weakly coupled transition metal dimers: Bimetallocenylenes and bimetallocenes

The Hartree-Fock (HF) instabilities in a series of bimetallocenes (1) and bimetallocenylenes (2) with Fe, Co, Ni and Cr as 3d centers have been investigated by means of a semiempirical INDO Hamiltonian. The HF picture is only valid in the case of the iron dimers. Strong correlation effects are encountered in the Co, Ni and Cr complexes. The necessary conditions for singlet, non-singlet (triplet) and non-real variations of the HF orbitals are discussed in detail. Singlet fluctuations are the result of intraatomic angular correlation (short-range) at each 3d center. The violation of the spin symmetry corresponds to a long-range interaction between the transition metal centers. Only for MOs with large 3d _xz amplitudes there exists a channel for the interatomic spin decoupling. Consequences for polymetallocenes are shortly discussed.     

AUGMENTED LAGRANGIAN METHOD AND OPEN BOUNDARY CONDITIONS IN 2D SIMULATION OF POISEUILLE–BÉNARD CHANNEL FLOW

The main objective of this study is to compare the influence of different boundary conditions upon the incompressible Poiseuille –Bénard channel flow (PBCF) in a 2D rectangular duct heated from below. In a first technical part the algorithm used to carry out this work, based on the augmented Lagrangian method, is presented. The implementation details of the five different open boundary conditions (OBCs) and the periodic boundary conditions (PBCs) tested in the present paper are also given. The study is then carried out for 1800<Ra≤ 10,000, 0<Re≤10 and 0·67≤Pr≤6·4. The five selected OBCs, applied at the outlet of the computational domain, respectively express the following conditions: a square profile for the velocity (OBC1), mass conservation (OBC2), zero second derivative of the horizontal velocity component (OBC3), a mixed boundary condition combining Dirichlet and Neumann conditions (OBC4) and an Orlanski-type boundary condition (OBC5). A good estimation of the perturbation amplitude and of the length of the perturbed zone at the outlet boundary is proposed. It is shown that OBC5 causes very little perturbation in the recirculating flow compared with the other OBCs. © 1997 John Wiley & Sons, Ltd.     

Instabilities in viscoelastic flow past a square cavity

Elastic flow transitions in viscoelastic flow past a square cavity adjacent to a channel are reported. The critical conditions for the onset of flow transitions and the qualitative and quantitative characterization of the secondary flows generated by the instability have been examined using streakline photography and instantaneous pressure measurements. Cellular type of instabilities inside the cavity is observed for flow rates beyond a critical value. Small and large scale eddies are observed at high flow rates. The flow inside the cavity and in the channel upstream and downstream of the cavity becomes weakly time-dependent for high flow rates.     

Time dependent flow in equimolar micellar solutions: transient behaviour of the shear stress and first normal stress difference in shear induced structures coupled with flow instabilities

Recently we studied time dependent structural changes that are coupled with flow instabilities (Fischer 1998; Wheeler 1998; Fischer 2000). Within a stability analysis, a classification scheme for the feedback circuit of coupled shear-induced structure and flow instabilities was derived by Schmitt et al. (1995) and applied to our samples. Here, inhomogeneous flow layers of different concentration and viscosity are generated by shear-induced diffusion (spinodal demixing) and, as consequence, one no longer observes a homogeneous solution but a type of shear banding that is seen here for the first time. In this paper we present the behaviour of the first normal stress difference observed in the critical shear-rate regime where transient shear-induced structure is coupled with flow instability. Similar to the oscillations of the shear stresses (strain-controlled rheometer) one observes oscillations in the first normal stress difference. This behaviour indicates that elastic structures are built up and destroyed while the shear-induced structures occur and that the induced phase is more elastic than the initial one. Oscillations of shear stress and first normal stress difference are in phase and indicate that both phenomena are caused by the same mechanism. Received: 30 June 1999/Accepted: 14 December 1999     

Nonstandard finite difference schemes for reaction‐‐diffusion equations having linear advection

We extend previous work on nonstandard finite difference schemes for one‐space dimension, nonlinear reaction–diffusion PDEs to the case where linear advection is included. The use of a positivity condition allows the determination of a functional relation between the time and space step‐sizes, and provides schemes that are explicit. The Fisher equation is used to illustrate the method. © 2000 John Wiley & Sons, Inc. Numer Methods Partial Differential Eq 16: 361–364, 2000