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Search Results: 1 - 10 of 191368 matches for " D. Shalybkov "
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The stability of axisymmetric Taylor-Couette flow in hydromagnetics
G. Ruediger,D. Shalybkov
Physics , 2001, DOI: 10.1103/PhysRevE.66.016307
Abstract: The linear marginal instability of an axisymmetric MHD Taylor-Couette flow of infinite vertical extension is considered. The dependence of the flow stability on magnetic Prandtl number, Pm, and gap-width between rotating cylinders is investigated. There is an unstability for flows with resting outer cylinder even without a magnetic field. Nevertheless, there are solutions with smaller critical Reynolds numbers for certain (weak) magnetic fields. These solutions, however, exist for not too small Pm or large enough gap. For hydrodynamically stable flow, we found that a magnetic field amplitude always exsits where the critical Reynolds number is minimal. In all calculated cases the minimal critical Reynolds numbers are running with 1/Pm for small Pm and the critical Reynolds numbers exceed value of 10^6 for Pm of sodium (10^{-5}) or gallium (10^{-6}).
Jet launching theory and the vertical structure of a magnetically-dominated thin accretion disc
G. Rudiger,D. A. Shalybkov
Physics , 2001,
Abstract: The presence of an imposed external magnetic field may drastically influence the structure of thin accretion discs. The magnetic field energy is here assumed to be in balance with the thermal energy of the accretion flow. The vertical magnetic field, its toroidal component B^tor at the disc surface (due to different rotation rates between disc and its magnetosphere), the turbulent magnetic Prandtl number and the viscosity-alpha are the key parameters of our model. Inside the corotation radius for rather small B^tor the resulting inclination angle i of the magnetic field lines to the disc surface normal can exceed the critical value 30^\circ (required to launch cold jets) even for small magnetic Prandtl numbers of order unity. The self-consistent consideration of both magnetic field and accretion flow demonstrates a weak dependence of the inclination (``dragging'') angle on the magnetic Prandtl number for given surface density but a strong dependence on the toroidal field component at the disc surface. A magnetic disc is thicker than a nonmagnetic one for typical parameter values. The accretion rate can be strongly amplified by large B^tor and small magnetic Prandtl number. On the other hand, for given accretion rate the magnetised disc is less massive than the standard-alpha disc. The surface values of the toroidal magnetic fields which are necessary to induce considerably high values for the inclination angle are much smaller than expected and are of order 10^-3 of the imposed vertical field. As the innermost part of the disc produces the largest B^tor, the largest radial inclination can be expected also there. The idea is therefore supported that the cold jets are launched only in the central disc area.
The linear MHD Taylor-Couette instability for liquid sodium
G. Rüdiger,M. Schultz,D. Shalybkov
Physics , 2002, DOI: 10.1103/PhysRevE.67.046312
Abstract: The linear stability of MHD Taylor-Couette flow of infinite vertical extension is considered for liquid sodium with its small magnetic Prandtl number Pm of order of 10^-5 and an uniform axial magnetic field. The sign of the constant a in the basic rotation law \Omega=a+b/R^2 strongly influences the presented results. It is negative for resting outer cylinder. The main point here is that the subcritical excitation which occurs for large Pm disappears for small Pm. This is the reason that the existence of the magnetorotational instability remained unknown over decades. For rotating outer cylinder the limiting case a=0 plays an exceptional role. The hydromagnetic instability exists with certain Reynolds numbers at certain Hartmann numbers of the magnetic field. These Reynolds numbers exactly scale with Pm^-1/2 resulting in moderate values of order 10^4 for Pm=10^-5. However, already for the smallest positive value of a the Reynolds numbers start to scale as 1/Pm leading to much higher values of order 10^6 for Pm=10^-5. Also nonaxisymmetric modes have been considered. With vacuum boundary conditions their excitation is always more difficult than the excitation of axisymmetric modes; we never observed a crossover of the lines of marginal stability. For conducting walls, however, such crossovers exist for both resting and rotating outer cylinders, and this might be essential for future dynamo experiments.
MHD Instability in cylindric Taylor-Couette flow
G. Rüdiger,D. A. Shalybkov
Physics , 2001,
Abstract: The linear marginal instability of an axisymmetric MHD Taylor-Couette flow of infinite vertical extension is considered. For flows with a resting outer cylinder there is a well-known characteristic Reynolds number even without magnetic field but for sufficiently weak magnetic fields there are solutions with smaller Reynolds numbers so that a characteristic minimum exists. The minimum only exists, however, for not too small magnetic Prandtl numbers. For small magnetic Prandtl numbers one only finds the typical magnetic suppression of the instability. We are here particularly interested in the case where the outer cylinder rotates so fast that the Rayleigh criterion for hydrodynamic stability is fulfilled. We find that for given magnetic Prandtl number now always a magnetic field amplitude exists where the characteristic Reynolds number is minimal. These critical values are computed for different magnetic Prandtl numbers. In all cases the Reynolds numbers are running with 1/Pm so that for the small magnetic Prandtl numbers of sodium (10^-5) or gallium (10^-6) the critical Reynolds numbers exceed values of 10^6 or 10^, resp.
Hydrodynamic stability in accretion disks under the combined influence of shear and density stratification
G. Rüdiger,R. Arlt,D. Shalybkov
Physics , 2002, DOI: 10.1051/0004-6361:20020853
Abstract: The hydrodynamic stability of accretion disks is considered. The particular question is whether the combined action of a (stable) vertical density stratification and a (stable) radial differential rotation gives rise to a new instability for nonaxisymmetric modes of disturbances. The existence of such an instability is not suggested by the well-known Solberg-Hoiland criterion. It is also not suggested by a local analysis for disturbances in general stratifications of entropy and angular momentum which is presented in our Section 2 confirming the results of the Solberg-Hoiland criterion also for nonaxisymmetric modes within the frame of ideal hydrodynamics but only in the frame of a short-wave approximation for small m. As a necessary condition for stability we find that only conservative external forces are allowed to influence the stable disk. As magnetic forces are never conservative, linear disk instabilities should only exist in the magnetohydrodynamical regime which indeed contains the magnetorotational instability as a much-promising candidate. To overcome some of the used approximations in a numerical approach,the equations of the compressible adiabatic hydrodynamics are integrated imposing initial nonaxisymmetric velocity perturbations with m=1 to m=200. Only solutions with decaying kinetic energy are found. The system always settles in a vertical equilibrium stratification according to pressure balance with the gravitational potential of the central object. keywords: accretion disks -- hydrodynamic instabilities -- turbulence
Nonaxisymmetric patterns in the linear theory of MHD Taylor-Couette instability
D. A. Shalybkov,G. Rüdiger,M. Schultz
Physics , 2002, DOI: 10.1051/0004-6361:20021284
Abstract: The linear stability of MHD Taylor-Couette flow of infinite vertical extension is considered for various magnetic Prandtl numbers Pm. The calculations are performed for a wide gap container with \hat\eta=0.5 with an axial uniform magnetic field excluding counterrotating cylinders. For both hydrodynamically stable and unstable flows the magnetorotational instability produces characteristic minima of the Reynolds number for certain (low) magnetic field amplitudes and Pm> 0.01. For Pm <~ 1 there is a characteristic magnetic field amplitude beyond which the instability sets in in form of nonaxisymmetric spirals with the azimuthal number m=1. Obviously, the magnetic field is able to excite nonaxisymmetric configurations despite of the tendency of differential rotation to favor axisymmetric magnetic fields which is known from the dynamo theory. If Pm is too big or too small, however, the axisymmetric mode with m=0 appears to be the most unstable one possessing the lowest Reynolds numbers -- as it is also true for hydrodynamic Taylor-Couette flow or for very weak fields. That the most unstable mode for modest Pm proves to be nonaxisymmetric must be considered as a strong indication for the possibility of dynamo processes in connection with the magnetorotational instability.
Electrical Conductivity of Collapsed Multilayer Graphene Tubes  [PDF]
D. Mendoza
World Journal of Nano Science and Engineering (WJNSE) , 2012, DOI: 10.4236/wjnse.2012.22009
Abstract: Synthesis of multilayer graphene on copper wires by a chemical vapor deposition method is reported. After copper etching, the multilayer tube collapses forming stripes of graphitic films, their electrical conductance as a function of temperature indicate a semiconductor-like behavior. Using the multilayer graphene stripes, a cross junction is built and owing to its electrical behavior we propose that a tunneling process exists in the device.
Porous Carbon Grown by Chemical Vapor Deposition on Copper Substrates  [PDF]
D. Mendoza
Journal of Materials Science and Chemical Engineering (MSCE) , 2015, DOI: 10.4236/msce.2015.38003
Abstract: Amorphous porous carbon was synthesized by chemical vapor deposition on copper substrates. The average size of the pores is around 1.2 microns with some small pores decorating the big ones. Lamellar samples of this carbonaceous material can be separated from the copper support and may be useful as electrode due to its low electrical resistivity of the order of 0.4 Ωcm.
Application of Enzyme Extracted from Aloe vera Plant in Chemical Pretreatment of Cotton Knitted Textile to Reduce Pollution Load  [PDF]
D. Jothi
World Journal of Engineering and Technology (WJET) , 2015, DOI: 10.4236/wjet.2015.33B007
Abstract:

Nowadays, highly alkaline chemicals like caustic soda, soda ash, silicate, acetic acid and soaping agents are used for scouring to remove the non-cellulosic impurities from the cotton. Using 30 - 40 gm/Kg on weight of the fabric results in destruction of cotton structure. Intensive rinsing and more acid is needed for reutilization of cotton, which enlarges the volume of effluent. Furthermore, these hazards chemicals result in increase in COD, BOD and TDS in waste water. These chemicals also attack the cellulose leading to heavy strength loss and weight loss in the fabric. The net result is low quality control and polluted environment with high usage of energy, time, chemical and water. Aloe vera presents the finest commercial opportunity in various industrial sectors among the various plants. Also, most of the countries are gifted with the unique geographical features that are essential for cultivation of Aloe vera. Yet, none of the country has realized and reaped the full potential of such plants in various industrial applications. The reason is simple: lack of the requisite expertise in extraction of various enzymes present in aloe plant. Fortunately, the technology is now accessible to make use of enzyme in textile application. In this research an attempt has been made to make use of lipase enzyme extracted from aloe plant in textile chemical pre- treatment process. In the present research work, an attempt was made to develop bio scouring of 100% cotton knitted fabric with lipase enzyme extracted from Aloe deberena plant at various concentration (1%, 2% and 3%) at various temperature (40?C, 60?C and 70?C) for a period of 30 minutes, 60 minutes and 90 minutes. The properties of bio scoured fabrics are compared with these of conventional scoured one. Encouraging results in terms of dye uptake, dye levelness, wash fastness, light fastness and rubbing fastness are obtained in case of bio scouring fabric dyed with dark reactive colors. Further, it reduces volume of effluent as well as COD, TDS and pH. It saves a substantial thermal energy 50% and electrical energy 40%. Bio scouring waste water has 40% - 50% less COD and 60% less TDS as compared to conventional scouring waste water.

Hyporheic Zone Hydrochemistry of the Mine-Polluted River  [PDF]
D. Ciszewski
Journal of Geoscience and Environment Protection (GEP) , 2015, DOI: 10.4236/gep.2015.310008
Abstract:

Intensity of stream waters mixing with groundwaters and lateral extent of these processes in the hyporheic zone were investigated in a near-bank sandbar and an adjacent floodplain through the comparison of groundwaters and stream water chemistry of the Bia?a Przemsza River in southern Poland. The stream waters were polluted by the discharge of mine waters from “Boles?aw” lead and zinc mine. The investigated waters were several times more mineralized than the natural spring waters of the river valley. The concentration of: potassium, sodium, and the pH, as well as cadmium, lead, and zinc decreased in the hyporheic zone towards the stream bank, whereas conductance, calcium, magnesium, sulphates, as well as silica contents were the highest on the floodplain, diminishing towards the stream. The changes observed in the chemical composition of groundwaters were apparent in mixing stream waters below the depth of 2 m with shallow groundwaters draining the valley slope. Hyporheic mixing also takes place in the 10-meter-wide, marginal zone of the sandbar, whereas in the 5-meter-wide stream-side zone of the sandbar groundwaters represent weakly transformed stream water.

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