Search

Today's available strong magnets allow to study distinct magnetic effects in various electrochemical systems. Lorentz forces and also magnetic gradient forces can be utilized to tailor convection and mass transfer in electrolytic cells, thereby influencing for example the morphology and the structure of the electrodeposits. The presentation will give an overview on recent results of numerical simulations and experimental findings in lab-scale electrochemical systems and will discuss potential applications. Whereas Lorentz forces are often known to enhance mass transfer, new results show that the convection pattern found in simple geometries can already be quite complex. A deeper understanding is required for improved applications. Recently, also magnetic gradient forces gained attention, e.g. for their potential in preparing micro-structured deposits. A detailed discussion of recent results will give further insight. Finally, new investigations of cyclic operation modes or time-dependent magnetic fields are presented which allow for a broad range of new phenomena.

Periodic addition of momentum by wall–parallel electromagnetic forces has a strong influence on the separated region of a stalled airfoil. As in the case of periodic blowing and suction, actuation frequency and momentum input are the main factors of influence. However, the control authority depends as well on the actuation wave form. This latter aspect is investigated in the present paper by means of time resolved particle image velocimetry data from the suction side of an inclined flat plate. The control effect is rated by the size of the remaining recirculation region in the time averaged flow fields. The controlled flow possesses typically a small number of relatively large vortices, which are believed to be related to the control mechanism. Consequently, the time resolved flow fields have been analyzed by proper orthogonal decomposition and continuous wavelet transform to extract dominant features of the flow.

Characterized by the interplay of Lorentz forces, flow and concentration distributions, magnetoelectrochemical processes are typically quite complex. We aim to show that flow field and concentration measurements by means of Particle Image Velocimetry (PIV) and the relatively new technique Background Oriented Schlieren (BOS) are a valuable aid in understanding the observed phenomena. Two examples, the flow field in a cylindrical cell and the concentration distribution near a circular millielectrode, will be discussed in detail.

For the present study time periodic wall parallel Lorentz forces have been used to excite the separated flow on the suction side of an inclined flat plate. Experiments for a Reynolds number of 10^4 and an angle of attack of a = 13° are reported. The controlled flow is characterised by a small number of relatively large scale vortices, which are related to the control mechanism. The influence of the main parameters, i.e. the excitation frequency, amplitude and wave form on the suction side flow structures was investigated by analysing time resolved particle image velocimetry (TR–PIV) measurements using continuous wavelet analysis for vortex detection and characterisation. Statistical analysis of the coherent structures of the flow was performed on a large amount of data samples.

Periodic addition of momentum by wall–parallel electromagnetic forces has a strong influence on the separated region of a stalled airfoil. As in the case of periodic blowing and suction, actuation frequency and momentum input are the main factors of influence. However, the control authority depends as well on the actuation wave form. This latter aspect is investigated in the present paper by means of time resolved particle image velocimetry data from the suction side of an inclined flat plate. The control effect is rated by the size of the remaining recirculation region in the time averaged flow fields. The controlled flow possesses typically a small number of relatively large vortices, which are believed to be related to the control mechanism. Consequently, the time resolved flow fields have been analyzed by proper orthogonal decomposition and continuous wavelet transform to extract dominant features of the flow.

Der in elektrochemischen Systemen in der Regel vorhandene Stromfluss führt zusammen mit einem magnetischen Feld zu Lorentzkraftverteilungen im Elektrolyten, die dessen Bewegungszustand modifizieren können. Dieser an sich einfache Effekt kann sich abhängig vom Aufbau der elektrochemischen Zellen in überraschend vielseitigen Strömungsformen äußern. Dies wird anhand von Beispielen demonstriert und im Hinblick auf aktuelle theoretische Vorstellungen diskutiert.

Periodic addition of momentum by wall–parallel electromagnetic forces has a strong influence on the separated region of a stalled airfoil. As in the case of periodic blowing and suction, actuation frequency and momentum input are the main factors of influence. However, the control authority depends as well on the actuation wave form. This latter aspect is investigated in the present paper by means of time resolved particle image velocimetry data from the suction side of an inclined flat plate. The control effect is rated by the size of the remaining recirculation region in the time averaged flow fields. The controlled flow possesses typically a small number of relatively large vortices, which are believed to be related to the control mechanism. Consequently, the time resolved flow fields have been analyzed by proper orthogonal decomposition and continuous wavelet transform to extract dominant features of the flow.

Lorentz forces originating from surface-mounted actuators of permanent magnets and electrodes in weakly conducting fluids like seawater provide a convenient tool for separation control at hydrofoils. A well-known actuator design of alternating stripes of permanent magnets and externally fed electrodes is considered which creates a mainly streamwise Lorentz force that is exponentially decaying in wall-normal direction. Separation control by steady forcing at the suction side and by oscillatory forcing near the leading edge of a symmetric foil is investigated numerically, mostly in the post-stall regime. The results are based on direct numerical simulations in the laminar flow regime in order to reveal basic control phenomena as well as on simulations using turbulence modelling at higher Reynolds numbers which are closer to possible naval application. By applying a strong enough steady control, separation can always be completely suppressed. The scaling behaviour of the maximum lift gain Delta C_L^{max} in the turbulent regime nicely agrees with experimental results. - Oscillatory forcing always has to compete with the natural shedding process, lock-in behavior may occur. Lift-optimum control for strong amplitudes is found in a frequency band around the natural shedding frequency. In terms of the momentum coefficient describing the control effort, appropriate excitation frequencies in relation to the natural vortex shedding frequency allow for a more effective lift control than steady forcing for small lift gains; for large lift enhancement the energetic effort seems to approach the level of steady control.

In two different cylindrical electrochemical cells, copper deposition and dissolution in the presence of a magnetic field mostly parallel to the electric field has been investigated. Particle image velocimetry measurements show that even under such a field configuration where Lorentz forces are often a priori neglected in fact they may dominate the flow. Further on, depending on the electrode radius a reversal of the secondary flow has been found. This feature can be explained by the different Lorentz force configurations calculated from the magnetic field and the primary current distributions. The components of the magnetic field have been determined by means of a traversed Hall probe as well as calculated by a commercial finite element package. Experimentally and numerically determined field distributions match very well. A good agreement between the measured and calculated velocity distributions has also been found, suggesting that in the present case the effect of the Lorentz force alone is sufficient to explain the magnetic field influence on the flow.

Concentration variations observed at circular electrodes with their axis parallel to a magnetic and normal to the gravitational field have previously been attributed elsewhere to the concentration gradient force only. The present paper aims to show that Lorentz force driven convection is a more likely explanation.

Time periodic Lorentz forces have been used to influence the separated flow on an inclined flat plate in deep stall at a Reynolds number of 10000. The influence of the control parameters effective momentum coefficient and excitation frequency as well as excitation wave form is discussed based on phase averaged PIV measurements. As expected, control authority depends strongly on momentum input and excitation frequency, but effects of the excitation wave form can be shown as well.

Electromagnetic, i.e. Lorentz forces can be used to control the flow of conducting fluids. This holds true even if the electrical conductivity is only weak, as in the case of electrolytes like sea-water. The applications of streamwise, wall parallel Lorentz forces to generic flow configurations will be exemplified with experimental data from flat plate boundary layers and separated flows around cylinders and hydrofoils. These results demonstrate the ability of stationary Lorentz forces to change the velocity profile in boundary layers and thereby to improve their stability properties, as well as to completely suppress separation. Time dependent Lorentz forces are used to mitigate the lift loss and drag increase of hydrofoils caused by flow separation. In this case, the versatility of Lorentz force actuators allows for detailed experimental investigations especially of the effects of excitation frequency and wave form. Finally, the impact of the low conductivity on the energy balance of Lorentz force actuators is discussed.

Lorentz forces originating from surface-mounted actuators of permanent magnets and electrodes in weakly conducting fluids like seawater can be used to control flow separation at hydrofoils [1]. As induction effects are typically weak, external currents have to be applied to achieve control. An alternating arrangement of electrodes and magnets is considered which creates a mainly streamwise Lorentz force that is exponentionally decaying in wall-normal direction. As spanwise influences are supposed to be small, the problem is treated as two-dimensional in the following. We investigate control by steady forcing at the suction side and by oscillatory forcing near the leading edge of symmetric foils, mostly in the post-stall regime. Similarities to the onventional method of oscillatory blowing for separation control do exist [2]. The numerical results presented cover direct numerical simulations based on a highly accurate spectral element method in the laminar flow regime in order reveal basic control phenomena as well as simulations using turbulence modelling at higher Reynolds numbers which are closer to possible naval application. Although strong-enough steady control is able to suppress separation completely, appropriate time-periodic control turns out to be more effective for finite lift-enhancement when comparing the energetical effort. Optimum control frequencies are found, typical flow structures are analyzed, and "lock-in" phenomena with the natural shedding process are discussed [3]. The scaling behaviour of the energetical effort is compared with recent experimental results [4]. References: 1. T.Weier, G.Gerbeth, G.Mutschke, O.Lielausis, G.Lammers, "Control of Flow Separation using electromagnetic Forces", Flow, Turbulence and Combustion 71 (2003) 5-17. 2. D.Greenblatt, I.J.Wygnanski, "The control of flow separation by periodic excitation", Prog. Aero. Sci. 36 (2000) 487-545. 3. G.Mutschke, G.Gerbeth, T.Albrecht, R.Grundmann, "Separation Control at Hydrofoils

Velocity measurements and shadowgraph visualizations for copper electrolysis under the influence of a magnetic field are reported. Experiments in a rectangular cell show the expected strong correlation between flow features and limiting current density. The flow can be understood as driven by the interplay of Lorentz forces and buoyancy. For a cylindrical cell with only slightly non-parallel electric and magnetic field lines, the presence and importance of the Lorentz force is demonstrated by velocity measurements.

Time periodic Lorentz forces have been used to influence the separated flow on an inclined flat plate in deep stall at a Reynolds number of 10000. The influence of the control parameters effective momentum coefficient and excitation frequency as well as excitation wave form is discussed based on phase averaged PIV measurements. As expected, control authority depends strongly on momentum input and excitation frequency, but effects of the excitation wave form can be shown as well.

Momentum and mass transfer during copper electrolysis in a small cell can be significantly influenced by Lorentz forces. Depending on the magnetic field configurations, a multitude of flow fields with an accordingly large range of mass transfer conditions can be arranged. The moderate magnetic field of simple permanent magnets placed behind the electrodes, although its action is limited to the vicinity of the electrodes, is able to promote convection in the whole cell. The interplay of Lorentz and buoyancy forces is substantial for the resulting flow structure.

Electromagnetic, i.e. Lorentz forces, may be used to influence the flow of electrically conducting fluids. The present paper investigates the application of time periodic Lorentz forces to the control of the suction side flow on a NACA 0015 hydrofoil. Experimental results, consisting of flow visualizations and force measurements, characterizing the control effect in the low Reynolds number range of 10^4 < Re < 10^5, are presented. A comparison of the forcing effect with stationary Lorentz forces on one hand and conventional oscillatory blowing on the other hand is given as well.

Introduction If a fluid is electrically conductive, its flow may be controlled using electromagnetic forces. Meanwhile, this technique is a recognized tool even on an industrial scale for handling highly conductive materials like liquid metals. However, also fluids of low electrical conductivity as considered in the present study, like sea--water and other electrolytes, permit electromagnetic flow control. Experimental results on the prevention of flow separation by means of a streamwise, wall parallel Lorentz force acting on the suction side of inclined flat plates and hydrofoils will be presented. Force Configuration The stripwise arrangement of permanent magnets and electrodes of alternating magnetization direction and polarity shown in Fig. 1 was proposed by Gailitis and Lielausis (1961). It generates a wall parallel force with a maximum value directly at the wall, decaying exponentially with the wall distance. Due to the low electrical conductivity, both electric and magnetic fields have to be applied to generate a Lorentz force of adequate strength. The width of the electrodes is equal to that of the magnets and determines the penetration of the force into the flow. The ratio of electromagnetic to inertial force can be characterized by the interaction parameter. In analogy to the terminology used in separation control by blowing, one may also define an electrohydrodynamic momentum coefficient. Results and Discussion A downstream directed force adds momentum to the flow and accelerates the fluid in the vicinity of the wall. This flow acceleration may be used to counteract the energy loss due to friction and adverse pressure gradients. Actually, a streamwise Lorentz force has already been successfully applied to control the flow around a circular cylinder in Weier et al. (1998). The two inserts in Fig. 2 show the flow around an inclined flat plate. In the lower left photograph, the unforced separated flow is visible, whereas the upper subfigure depicts the fully

The flow of electrically conducting fluids can be controlled by electromagnetic forces. Whereas this concept is already in industrial use in metallurgical or crystal growth technologies, its application to flows of electrolytes is less developed yet. For seawater flows around ship components, flow separation is an important phenomenon limiting the operation of, e.g., rudders or stabilizing foils. We present experimental and numerical results on the prevention of flow separation by means of a stream-wise, wall parallel Lorentz force acting on the suction side of inclined flat plates and hydrofoils. The use of steady as well as oscillatory Lorentz forces is considered. The results for oscillating Lorentz forces will be compared to recent literature results for periodic suction/blowing. Scaling relations for an extrapolation to application relevant Reynolds numbers will be given, and the related energy consumption will be analysed.

Introduction If a fluid is electrically conductive, its flow may be controlled using electromagnetic forces. Meanwhile, this technique is a recognized tool even on an industrial scale for handling highly conductive materials like liquid metals. However, also fluids of low electrical conductivity as considered in the present study, like sea--water and other electrolytes, permit electromagnetic flow control. Experimental results on the prevention of flow separation by means of a streamwise, wall parallel Lorentz force acting on the suction side of inclined flat plates and hydrofoils will be presented. Force Configuration The stripwise arrangement of permanent magnets and electrodes of alternating magnetization direction and polarity shown in Fig. 1 was proposed by Gailitis and Lielausis (1961). It generates a wall parallel force with a maximum value directly at the wall, decaying exponentially with the wall distance. Due to the low electrical conductivity, both electric and magnetic fields have to be applied to generate a Lorentz force of adequate strength. The width of the electrodes is equal to that of the magnets and determines the penetration of the force into the flow. The ratio of electromagnetic to inertial force can be characterized by the interaction parameter. In analogy to the terminology used in separation control by blowing, one may also define an electrohydrodynamic momentum coefficient. Results and Discussion A downstream directed force adds momentum to the flow and accelerates the fluid in the vicinity of the wall. This flow acceleration may be used to counteract the energy loss due to friction and adverse pressure gradients. Actually, a streamwise Lorentz force has already been successfully applied to control the flow around a circular cylinder in Weier et al. (1998). The two inserts in Fig. 2 show the flow around an inclined flat plate. In the lower left photograph, the unforced separated flow is visible, whereas the upper subfigure depicts the fully

Lorentz forces can be used to control the near wall flow of low conducting liquids like sea-water. To achieve force densities strong enough to modify the flow, both magnetic and electric fields have to be applied to the fluid. Here, wall parallel Lorentz forces in streamwise direction were used to influence the velocity profile of a flat plate boundary layer as well as the flow around a symmetric hydrofoil. Velocity measurements inside the boundary layer and direct force measurements are given for the flat plate. At moderate force strength, the mean velocity profile is characterized by a momentum thickness smaller than in the unforced case, whereas at high enough Hartmann numbers a wall jet develops. Additionally, a turbulent, but approximately non-growing boundary layer has been observed. The effect of a suction-side, streamwise Lorentz force on a NACA-0017-like hydrofoil is quantified by means of force balance measurements. Depending on the angle of attack, two different effects are observed. (1) At small angles of incidence, a moderate increase in lift due to additional circulation is observed. Simultaneously, a decrease in drag of the hydrofoil is caused by the momentum added. (2) At higher angles of attack, where the unforced hydrof oil would normally stall, a more pronounced lift increase and a corresponding drag reduction are observed due to separation prevention.

nicht vorhanden

Electromagnetic body forces, i.e. Lorentz forces, have been used to modify the boundary layer around a circular cylinder in cross flow. Depending on the polarity of the applied electric field different effects on the flow can be obtained. Lorentz forces directed with the mean flow are able to prevent the boundary layer from separation. Therefore flow separation as well as the von Karman vortex trail can be suppressed. When the momentum gain produced by the Lorentz forces in the boundary layer is high enough, thrust is produced, what results in a jet flow originated from the cylinders back side. If the Lorentz forces are directed opposite to the mean flow, the separation points are shifted towards the front stagnation point, the recirculation region broadens and the von Karman vortex trail is modified. The described technique gives a variety of opportunities to control the flow around the cylinder and the flow structure of the cylinder wake. Results from flow visualizations and numerical calculations are presented.

The hydromagnetic stability of the flow of an incompressible conducting fluid around a circular cyliner in a uniform magnetic field parallel to the mean flow is investigated with different approaches: Experiments for 1000 < Re < 9000 a simple analytical model numerical simulation for Re < 1000. Main goal of the investigations is to find the stability curce inthe (Re, N)-plane, and to distinguish between 2D and 3D instabilities. Experimental results will be presented based on hot-wire measurements in the down-stream cylinder wake. The suppression of the vortex street by increasing magnetic fields is clearly identified. Parallel to the disappearance of the typical Kármán frequency an increase of low frequency fluctuations is observed in the spectrum. This will be discussed in therms of theoretical predictions for long-wave MHD instabilities. More physical insight into the stability behaviour is obtained by a simple Kolmogorov flow modelling of the cylinder wake and the corresponding stability analysis. Theoretical results will be presented for the 2D case: critical Reynolds number, wave number and Strouhal number as function of the magnetic field. The critical wave number jumbs form the region of the typical Kármán value to very low values at a distinct magnetic field strength. The simple model is extended to include 3D instabilities in the flow. Limitations of the model and the comparison to experiments will be discussed. Finally the 2D case for Re < 1000 will be compared with results of a full numerical simulation.

Electromagnetic body forces, i.e. Lorentz forces, have been used to modify the boundary layer around a circular cylinder in cross flow. Depending on the polarity of the applied electric field different effects on the flow can be obtained. Lorentz forces directed with the mean flow are able to prevent the boundary layer from separation. Therefore flow separation as well as the von Karman vortex trail can be suppressed. When the momentum gain produced by the Lorentz forces in the boundary layer is high enough, thrust is produced, what results in a jet flow originated from the cylinders back side. If the Lorentz forces are directed opposite to the mean flow, the separation points are shifted towards the front stagnation point, the recirculation region broadens and the von Karman vortex trail is modified. The described technique gives a variety of opportunities to control the flow around the cylinder and the flow structure of the cylinder wake. Results from flow visualizations and numerical calculations are presented.