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Die Deutschen jhrlich kommen Millionen Deutsche flirten oder Single ganz entspannt flirten. In Wien bis zum Alberthafen in Stuttgart. Influence of an axial magnetic field on the stability of convective flows between non-isothermal concentric spheres. Introduction to the Focus Issue: Chemo-Hydrodynamic Patterns and Instabilities. Pattern forming instabilities are often encountered in a wide variety of natural phenomena and technological applications, from self-organization in biological and chemical systems to oceanic or atmospheric circulation and heat and mass transport processes in engineering systems.

Spatio-temporal structures are ubiquitous in hydrodynamics where numerous different convective instabilities generate pattern formation and complex spatiotemporal dynamics, which have been much studied both theoretically and experimentally. In parallel, reaction-diffusion processes provide another large family of pattern forming instabilities and spatio-temporal structures which have been analyzed for several decades. At the intersection of these two fields, "chemo-hydrodynamic patterns and instabilities" resulting from the coupling of hydrodynamic and reaction-diffusion processes have been less studied.

The exploration of the new instability and symmetry-breaking scenarios emerging from the interplay between chemical reactions, diffusion and convective motions is a burgeoning field in which numerous exciting problems have emerged during the last few years. These problems range from fingering instabilities of chemical fronts and reactive fluid-fluid interfaces to the dynamics of reaction-diffusion systems in the presence of chaotic mixing.

The questions to be addressed are at the interface of hydrodynamics, chemistry, engineering or environmental sciences to name a few and, as a consequence, they have started to draw the attention of several communities including both the nonlinear chemical dynamics and hydrodynamics communities. The collection of papers gathered in this Focus Issue sheds new light on a wide range of phenomena in the general area of chemo-hydrodynamic patterns and instabilities. It also serves as an overview of the current research and state-of-the-art in the field. Chemo-Marangoni convection driven by an interfacial reaction: Pattern formation and kinetics.

A combined study devoted to chemo-Marangoni convection and the underlying kinetics is presented for a biphasic system in which surfactants are produced in situ by an interfacial reaction. The pattern formation studied in a Hele-Shaw cell in both microgravity and terrestrial environments initially shows an ensemble of chemo-Marangoni cells along a nearly planar interface. Soon, a crossover occurs to periodic large-scale interfacial deformations which coexist with the Marangoni cells.

This crossover can be correlated with the autocatalytic nature of the interfacial reaction identified in the kinetic studies.

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The drastic increase in the product concentration is associated with an enhanced aggregate-assisted transfer after the critical micellar concentration is approached. In this context, it was possible to conclusively explain the changes in the periodicity of the interfacial deformations depending on the reactant concentration ratio. Growth of a free dendrite in pure substances under modulated flow conditions. We study the free dendrite growth of a pure substance under the influence of a parallel flow, modulated in time using two different approaches, by means of two-dimensional phase-field simulations.

In a previous work amplified side-branching was shown to occur for velocity pulses of appropriate duration H. Materials Science and Engineering [1] which is extended for the case of modulated temperature. To efficiently detect the resonant pulse duration corresponding to the naturally preferred sidebranch frequency we make use of a technique in which the pulse duration is ramped during the growth of the dendrite. Geometrical features of the dendrites grown under the different modulation conditions are discussed.

Radial solidification of Al-Si alloys in the presence of a rotating magnetic field. We study the flow field during the radial solidification of an Al-7wt pct Si melt inside a cylindrical cavity with cooled side walls which is placed in a rotating magnetic field RMF. The solidification process is simulated by means of a continuum model which is implemented in a code which solves the axisymmetric Navier-Stokes equations. We first analyze the start-up of the buoyancy-driven convection driven by the radial heat flux in absence of the RMF.

We show that the initial large vortex breaks up into three smaller one. The resulting radial temperature profiles are compared with experimental data. Second, we study how the vortex structure is modified by the application of small RMF's. Lorentz-force-driven convection during copper magnetoelectrolysis in the presence of a supporting buoyancy force. During modulated gravity the front position undergoes periodic modulation with an accelerated front propagation under hyper-gravity together with a slowing down under low gravity.

Deeper insights into the correlation between grey-value changes in the experimental shadowgraph images and characteristic changes in the concentration profiles are obtained by a numerical simulation of the imaging process. In-situ velocity measurements were performed during potentiostatic Cu electrodeposition in different magnetic gradient fields VB. Astigmatism Particle Tracking Velocimetry was used to enable three-dimensional measurements of al three velocity components in a large part of the cell volume.

A complex three-dimensional fluid flow was observed. Thus, the magnetic field gradient force F-del B driven electrode-normal convection during deposition in high del B was experimentally proven and the interplay of F-del B and the Lorentz force could be analysed for different del B-setups. Moreover, in-situ measurements of the ion concentration during deposition and dissolution of Cu in a magnetic gradient field were performed by Mach-Zehnder interferometry.

Relaxation oscillations between Marangoni cells and double diffusive fingers in a reactive liquid—liquid system. We study a novel kind of coupling in chemo-hydrodynamic pattern formation driven by a neutralization reaction along a plane interface separating two immiscible liquid phases. The neutralization reaction, during which a surface-active carboxylic acid is converted into a surface-active salt, gives rise to numerous cycles of relaxation oscillations between a fast cellular Marangoni convection with parallel-acting density plumes and a slow finger convection. By means of particle image velocimetry the dynamics of the sub-structured Marangoni cells are characterized while their geometrical aspects are analysed using shadowgraphy.

Based on concentration-dependent density measurements and experiments with miscible solutions, the finger convection could be clearly identified as a double-diffusive phenomenon. Furthermore, the interaction of the sub-structured Marangoni cells with the density effects is examined.

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Flow oscillations driven by a rotating magnetic field in liquid metal columns with an upper free surface. The oscillatory flow instability in a liquid metal cylinder with a free upper surface, exposed to a rotating magnetic field RMF , is analyzed by numerical simulations of the axisymmetric Navier—Stokes equations. In parallel, the Taylor-number interval, where the flow oscillations occur, becomes narrower.

The instability is initiated near the free surface, where an oscillatory variation of both the size and the position of the upper vortex in the secondary flow can be observed accompanied by horizontal oscillations of the azimuthal velocity maximum at the free surface. The occurrence of the oscillatory flow regime depends sensitively on the cleanliness of the liquid metal surface.

Free dendrite growth under modulated flow in pure substances: The free dendrite growth in a pure substance, supposed to a parallel forced convection which is modulated in time, is studied using two-dimensional phase-field simulations. We show that the response of the tip velocity to the velocity pulses exhibits a first-order time delay. Most notably, when the pulse duration is chosen appropriately a resonant side-branching can be achieved.

This provides a way to manipulate the dendrite morphology and consequently its mechanical properties. How to obtain structured metal deposits from diamagnetic ions in magnetic gradient fields? While homogeneous Bi deposits were obtained from the former, structured Bi layers were derived from the latter. The structured deposits show an inverse correlation between deposit thickness and superimposed magnetic field gradient. Minima of film thickness are observed in regions of maximum magnetic gradients. This demonstration of magneto-electrochemical structuring by deposition of diamagnetic ions is discussed considering the acting magnetic forces.

Several possibilities explaining the structuring mechanism are presented. When two miscible solutions, each containing a reactive species, are put in contact in the gravity field, local variations in the density due to the reaction can induce convective motion and mixing. We characterize here both experimentally and theoretically such buoyancy-driven instabilities induced by the neutralization of a strong acid by a strong base in aqueous solutions. The diverse patterns obtained are shown to depend on the type of reactants used and on their relative concentrations.

They have their origin in a combination of classical hydrodynamic instabilities including differential diffusion of the solutes involved while temperature effects only play a marginal role. Influence of an axial magnetic field on the stability of spherical Couette flows with different gap widths. This paper deals with the linear instability analysis of the spherical Couette flow of an electrically conducting fluid in the presence of an axial magnetic field.

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The corresponding instability diagrams, i. This offers the possibility to stabilize the basic flow up to high Re by appropriately increasing Ha.

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Velocity measurements inside the concentration boundary layer during copper-magneto-electrolysis using a novel laser Doppler profile sensor. The evolution of the velocity boundary layer during the initial phase of copper electrolysis under the influence of a magnetic field is studied by using particle image velocimetry and a novel laser Doppler velocity profile sensor. It is shown that the Lorentz force-driven convection only dominates the velocity boundary layer during the early phase of electrolysis and induces a linear velocity profile near the cathode.

The linear relationship between the velocity gradient and Lorentz force is determined. With the onset of the opposing buoyancy-driven convection at the cathode, a duplex structure of the boundary layer appears. A wavelet and Zernike-polynomial-based shearing interferometry approach to analyse hydrodynamic instabilities at interfaces.

We presentahybridalgorithmtoanalysecomplexinterferogramswithsignificantfringe deformationswithoutana-priorigivenzero-phaseimagecontainingtheoptical aberrationoftheinstrument. Thealgorithmproposedemploysa wavelettransformationinparallelwithanapproximationofthephasefielddirectlyafter the experimentalcontainerisfilledapplyingZernikepolynomials. Thesubtraction of thesephaseaberrationscanbetracedbacktoacomplexmultiplicationinthewavelet space whichstronglyreducestheeffortofthephaseunwrappingusingtheGoldstein algorithm. Oscillatory Lorentz-force-driven flows during potentiostatic current oscillations in magnetic fields.

The velocity and refractive index fields during potentiostatic current oscillations of iron electrode in H2SO4 with applied magnetic fields were measured by a combination of particle image velocimetry and interferometry. Depending on the magnetic field orientation differently oriented swirling flows around the electrode were observed in the activation phase. The resulting secondary flows crucially affect the concentration boundary layer thickness at the iron electrode, which finally strongly influences the duration of the passivation phase.

We review recent experimental and theoretical findings on the behavior of surface tension dominated thermal convection in a thin fluid layer for Marangoni numbers significantly exceeding the threshold of the primary instability. Particular emphasis is placed on the description of a secondary instability which leads to a transformation of hexagonal convective cells into squares, referred to as the hexagon-square transition. Moreover, we explain the role of defects in the transition process and discuss some theoretical work aiming at the prediction of scaling laws for heat transport in the turbulent regime for low-Prandtl number fluids.

Contactless Mixing of Liquid Metals.

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This work is devoted to the numerical study of mixing two miscible liquid metals, e. The initial condition is a cylinder with the lower half filled with a melt that has two times the molecular viscosity of the liquid in the upper part of the cavity. Based on the mixture model of the two-fluid flow and the axisymmetric Navier—Stokes equations, the transient transport of momentum as well as the species for rotary and axial stirring is modeled.

The TMF stirring with forcing parameters that produced laminar flow offers a better mixing performance than the RMF stirring at the same magnetic field intensity. However, with an increase in magnetic forcing, the RMF and TMF lead to comparable mixing times for the same value of magnetic field induction. Numerical simulations showed that the discontinuous superposition of RMF and TMF—defined as the sequential switching on and off of both fields with a well-defined period—enhances the mixing in comparison with the non-superimposed case.

Hydrogen evolution under the influence of a magnetic field. The effect of a uniform magnetic field on the hydrogen evolution reaction HER during water electrolysis in 0. Irrespective of the magnetic field orientation with respect to the electrode surface, the desorption of hydrogen is enhanced by the presence of the magnetic field.

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This effect is displayed by a reduction of the mean bubble size as well as a narrower bubble size distribution in a magnetic field. Moreover, it is shown that in the presence of an external magnetic field the fractional bubble coverage is strongly retarded. As a consequence the current density is increased since more active sites are available for the reduction processes.

These effects are discussed with respect to the Lorentz force driven convection induced by a magnetic field. In order to resolve further the influence of a magnetic field applied in the perpendicular-to-electrode configuration, where the bulk Lorentz force is negligible, a numerical study has been performed. This revealed the mechanism of the improved desorption of a hydrogen bubble from the electrode surface. The numerical study has been validated by a model experiment.

Most importantly, it is clearly demonstrated that a magnetic field superposed during water decomposition is a very effective method to intensify hydrogen evolution processes, and it should be possible to significantly improve the energetic efficiency of the hydrogen production via water electrolysis in a magnetic field. On the decay of the Lorentz-force-driven convection in vertical concentration stratification during magnetoelectrolysis. We study the temporal development of the Lorentz-force-driven convection in an inhomogeneous magnetic field during copper electrolysis between two vertical electrodes.

Using a combination of interferometry and particle image velocimetry we show that the Lorentz-force-driven convection is canceled out even after a moderate duration of the electrolysis. By reversing the current, a re-excitation of the convection is possible which is, however, restricted to a time window of some seconds. The reasons for the decay and re-appearance of the MHD flow are discussed and explained by scaling analysis.

The impact of turbulent flow on the solidification of metal alloys driven by a rotating magnetic field. The weakly turbulent flow was modelled by means of direct numerical simulations in an axisymmetric approach, where the transient heat and mass transfer were simulated by means of a standard mixture model. Both types of solidification, columnar and equiaxed, were considered by the application of both a permeability and a hybrid model to treat the fluid flow in the mushy zone.

Use of time-modulated AC magnetic fields for melt flow control during unidirectional solidification. A new electromagnetic stirring approach using a combination of rotating RMF and travelling magnetic fields TMF is proposed, where both fields are applied subsequently in form of rectangular pulses. The strategy to utilise time-modulated RMF and TMF is aimed at overcoming the known deficiencies of conventional stirring, in particular flow-induced macrosegregation.

This paper considers the directional solidification of Al—Si alloys from a water cooled copper chill. The results demonstrate that melt agitation using modulated magnetic field offers a considerable potential for a well aimed modification of casting properties by an effective control of the flow field, but, this goal requires a well considered optimisation of the magnetic field parameters. Mit der Strategie der Nutzung zeitmodulierter Felder sollen diese bekannten Nachteile vermieden werden.

Eine erfolgreiche Anwendung erfordert eine dem jeweiligen Prozess angepasste Optimierung der Magnetfeldparameter. A new approach of electromagnetic stirring is presented using a combination of rotating RMF and travelling magnetic fields TMF , where both fields are applied subsequently in form of rectangular pulses. The strategy to utilise time modulated RMF and TMF is aimed at overcoming the known deficits of the conventional stirring, in particular the flow induced macrosegregation. This paper considers the directional solidification of Al-Si alloys from a water cooled copper chill.

The results demonstrate that the melt agitation using modulated magnetic field offers a considerable potential for a purposeful modification of casting properties by an effective control of the flow field, but, this goal requires a well balanced optimisation of the magnetic field parameters. Desorption of hydrogen from an electrode surface under influence of an external magnetic field - In-situ microscopic observations. The effect of a magnetic field in the perpendicular-to-electrode configuration on the hydrogen evolution reaction HER was investigated during the deposition of Co.

An enhanced desorption of hydrogen in the form of numerous small bubbles was found by combining potentiostatic investigations, coupled with an electrochemical quartz crystal microbalance EQCM , and simultaneous microscopic observations of the electrode surface. The results are consistent with the recently proposed qualitative model [J. We present a novel technique consisting in the combination of pulse plating and magnetoelectrolysis.

This technique is applied to copper electrolysis between two vertical copper-electrodes. The modulation of the current inside an inhomogeneous magnetic field generates a modulated Lorentz force driving an oscillating convection studied using particle image velocimetry.

The resulting changes in the concentration boundary layers are analysed by means of a Mach-Zehnder interferometer. Solidification of a binary metal alloy in a turbulent melt flow driven by AC magnetic fields.

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The weakly turbulent flow was modeled by means of direct numerical simulations in an axisymmetric approach, where the transient heat and mass transfer were simulated by means of a standard mixture model. The influence of a direct electric current on the growth of solutal dentrites. We study numerically the influence of an electric field on the dendritic growth during solidification of Al-4 wt. The numerical results showed that an electric discharging applied during the solidification leads to the appearance of a micro-pinch force due to the complex shape of dendrite and difference in electric conductivities between solid and liquid phases.

This pinch force is responsible for the motion of the interdendritic liquid in the form of a toroidal vortex near the dendrite tip. The fluid flow induced modifies the growth kinetics of solutal dendrites, namely, the sidebranching near the dendrite tip is enhanced and the growth of the main stem of the dendrite is suppressed.

The start-up of natural convection during copper electrolysis in the presence of an opposing Lorentz force. The start-up of natural convection in an inhomogeneous magnetic field was studied in a 0. A Mach-Zehnder interferometer was used for the measurements of the concentration while the velocity field was determined by particle image velocimetry PIV.

The main phenomenon reported is the growth of a bubble-like zone of less-concentrated solution at the bottom of the cathode. This zone rises along the cathode if buoyancy is sufficiently strong to compete with the opposing Lorentz force. The results are discussed in terms of a scaling analysis. Dendrite fragmentation by catastrophic elastic remelting. A new fragmentation mechanism of dendrite arms is proposed. The theoretical basis of this mechanism is a shift in the thermodynamical equilibrium at the solid—liquid interface due to the presence of elastic energy.

The resulting non-linear system of ordinary differential equations is integrated in time using a fully implicit scheme. It is demonstrated that there is a critical level of loading, exceeding which causes a catastrophic reduction in the neck cross section, leading to dendrite detachment. Periodic convective instability has been observed in a biphasic system during the complexation reaction of alkali picrate and dicyclohexanocrown-6 which undergoes mass transfer from the hexane phase into the aqueous phase.

The convection was visualized by means of precipitated crystals that are formed in both phases by the complexation reaction. The fluid motion was observed with an optical microscope and further analyzed with the particle image velocimetry PIV technique. The partition at the extraction of cesium into the organic phase was followed by means of the radioactive isotope Cs.

The type of the hydrodynamic instability is governed by the alkali metal expressed via its stability constants for the complex formed. More stable complexes trigger a higher precipitation, thereby favoring a Raleigh-Taylor instability. Complexes with a lower stability constant induce Marangoni cells which show a pulsating character in a cubic container.

Depending on the confinement of the experiment cell the fluid motion can also follow a back-and-forth movement. Possible mechanisms for the occurring oscillations are discussed. A novel Hele-Shaw cell design for the analysis of hydrodynamic instabilities in liquid—liquid systems. A novel Hele-Shaw cell design for accurate experiments in liquid—liquid systems with and without chemical reactions is hereby presented. It allows the formation of a stably pinned, plane liquid—liquid interface in less than one second with a minimum shear flow.

The underlying working principle and the performance for different solvent systems and gap widths are discussed. This article considers the nondirectional solidification of a binary-metal alloy in a cylindrical cavity, which is cooled along its outer vertical wall and the bottom. To study the influence of convection within the liquid phase on the final segregation, three cases are examined: The results show that convection in the form of multivortices caused by the thermosolutal buoyancy leads to macrosegregations in the form of V-channels.

The application of an external axial magnetic field alone suppresses the multivortex structure and, thus, the macrosegregation. Application to Solidification of Al-Si Alloys. The present study considers the solidification of an Al-7wtpct Si alloy under the influence of electromagnetic melt stirring using a rotating magnetic field RMF. The resulting flow structure in a cylindrical liquid metal column has been measured by isothermal experiments using the ternary alloy GaInSn. The solidification experiments performed with the Al-7wtpct Si alloy confirm our numerical predictions concerning the temperature field during solidification and the distribution of primary crystals and eutectic phase in the solidified samples.

The application of the RMF-PSAD regime at suitable frequencies of the reversals of the magnetic field direction f P delivers an equiaxed microstructure without macrosegregation. Flow Field in a Liquid Metal Column. The use of a pulsed, rotating magnetic field RMF is presented as an auspicious method for obtaining an intensive stirring and mixing in a pool of liquid metal; the RMF pulses within a sequence have been applied with a constant or alternating direction.

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The resulting flow structure in a cylindrical liquid metal column has been explored by numerical simulations and by model experiments, using the ternary alloy GaInSn. Ultrasonic Doppler velocimetry UDV has been used to determine profiles of the vertical velocity. Both the numerical results and the velocity measurements demonstrate the capability of the proposed stirring regimes for overcoming the limited mixing character of conventional rotary stirring.

The application of a time-modulated RMF offers considerable potential for providing an optimal flow pattern in a solidifying melt, for reasons of a well-aimed modification of casting properties. Spin-up and spin-down dynamics of a liquid metal driven by a single rotating magnetic field pulse. This paper presents a study concerning the transient dynamics of the flow field inside a liquid metal filling a finite cylindrical container: The flow is created by applying a rotating magnetic field RMF in the form of a single pulse.

The flow structure is governed by an impulsive spin-up from the rest state which is followed by a spin-down phase, with the fluid in a state of inertia. The pulse length has been found to have a distinct influence on the transient fluid flow. Two cases are considered: This phenomena is especially pronounced if the pulse length of the electromagnetic forcing corresponds to the so-called initial adjustment phase as defined by Nikrityuk, Ungarish, Eckert, Grundmann [P.