A hybrid model for electroosmotic flows in microchannels induced by internal electrodes

Internal electrodes, adjacent to insulating walls at defined zeta potential, lead to a non-continuous potential distribution at the wall. Hence, simplified treatment appears problematic due to the singularity of the electrical field strength. To avoid this difficulty, we develop a hybrid model, which solves the electrical problem, including a resolution of the EDL, while the flow problem is solved in the fluid bulk only. We apply this hybrid model to investigate the position of internal electrodes with regard to their influence onto the flow field, driven by electroosmosis in a modular rectangular microchannel. (© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Experimental investigation of the electrokinetic flow in microchannels with internal electrodes

We investigate the electrokinetic flow in microchannels with internal electrodes. Experiments and numerical simulations are performed. The micro–particle–image velocimetry method is used to measure two–dimesional, two–component velocity fields over the complete height of the microchannel. Based on this measurements, the third velocity component, which cannot be measured directly, is calculated by an integration of the continuity equation. Due to the fact that microparticles, used for the μPIV are electrically non-neutral leads to the problem that these particles experience electrophoretic forces. That means that the particle movement appears to be a superposition of electroosmotic and electrophorectic effects. To verify the influence of electrophoretic effects on the microparticles, additional numerical calculations are made. (© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Fully–coupled model for electrokinetic flow and transport in microchannels

We investigate the electrokinetic flow and mass transport in microchannels. Therefore, mathematical models of the electrical, fluid-mechanical and chemical processes are established. Within the electrical double layer (EDL), approximative analytical solutions can be found and matched asymptotically to the numerical (FEM) solution of the channel core. The results of the simulations show a strong coupling between the processes. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

The influence of a DC electric field on the von Kármán vortex street in the wake of a confined cylinder

We investigate the influence of a DC electric field on the flow around and in the wake of a confined cylinder by means of numerical simulations. Our results indicate that even very small electrical perturbations have significant impact on the settling time of the lift coefficient. Moreover, the oscillations of the lift coefficient of pure pressure-driven and pressure-driven flow with induced electrical field are in anti-phase. (© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effect of an external force on the by-pass transition mechanism in internal flows electrical double layer effect in mini and micro channels

The effect of the electric double layer (EDL) on the bypass transition mechanism is explored through direct numerical simulations. The electrokinetic effects destabilize rapidly the flow when the local disturbance and/or the Reynolds number are respectively strong and large enough to overcome the transient growth regime. It is found that a weak perturbation quickly leads to the transition through bypass and non-linear interactions under the EDL effect, while an order of magnitude larger disturbance is incapable to destabilize the macro-scale flow. The EDL develops some new transitional wall structures during the bypass process. It is concluded that electrostatic effects can be efficiently used to enhance the mixing and heat transfer in microchannels, providing that the diffuse layer is large enough.     

On the control of layered flow of a viscous incompressible fluid within MHD

The control of a viscous incompressible flow within MHD is considered. The phenomenon of internal heat release is explored as applied to a layered plane flow in which the kinetic energy dissipates strictly into heat. The influence exerted by the electromagnetic field of the flowing fluid on the surrounding medium is examined, and methods for flow control in the context of digital deposit field technology are considered.     

The Trailing Edge Problem For Mixed Convection Flow Past a Horizontal Plate

The influence of buoyancy onto the boundary‐layer flow past a horizontal plate aligned parallel to a uniform free stream is characterized by the buoyancy parameter K = Gr/Re^5/2 where Gr and Re are the Grashof and Reynolds number, respectively. An asymptotiy analysis of the complete flow field including potential flow, boundary layer, wake and interaction region is given for small buoyancy parameters and large Reynolds numbers in the distinguished limit KRe^1/4 = O(1). (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Acceleration of Plasma in Coaxial Channels with Preshaped Electrodes and Longitudinal Magnetic Field

The paper presents a mathematical model and results of numerical simulation of axisymmetric plasma flows in nozzle-type channels formed by two coaxial electrodes. Transonic accelerated flows, which are of interest for the development of plasma accelerators, are considered. The mathematical apparatus of the models are two-dimensional MHD problems solved numerically, the steady-state solutions of which are obtained in the process of relaxation. Some characteristics of the flow in narrow tubes between close trajectories are considered in the quasi-one-dimensional approximation. The main attention is paid to the influence of the longitudinal magnetic field and the curvilinear geometry of the electrodes on the properties of accelerated flows.     

A numerical study of flows driven by a rotating magnetic field in a square container

The influence of the geometry on the magnetically driven flow is studied by means of numerical simulations. Low–frequency, low–induction and low–interaction conditions are assumed. The rotating magnetic field (RMF) gives rise to a time–independent magnetic body force, computed via the electrical potential equation and Ohm's law and a time–dependent part that is neglected due to the low interaction parameter. Flow results of the cylindrical and square container are compared with respect to the magnetic body force, time–averaged velocity fields, first flow instabilities and Reynolds stress tensors. The dependency of the maximal velocity magnitude and the intensity of the magnetic induction is identical in axisymmetric and non–axisymmetric containers and in good agreement with Davidson's theory. However, significant differences are recognized, for instance, in the distribution of the Reynolds stress tensors. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Impact of permeabilization and pH effects in the electrochemical treatment of tumors: Experiments and simulations

Electrochemical treatment is used in the local control of solid tumors in preclinical and clinical studies. We study pH fronts, permeabilization, electrical field and concentration of four chemical species in a tissue under electrochemical treatment by means of in vitro and in silico modeling. The in vitro model uses a piece of potato (solanum tuberosum L.) as analogy of tumor tissue, due to the characteristics of the potato tissue and the way in which it reacts against pH and potential. The in silico model solves the two-dimensional Nernst-Planck equations for ionic transport in a four-ion electrolyte. Modeled ions demonstrate the diffusive regime of ionic transport and show the little influence of the low electric field applied on this phenomenon. This work evidences that extreme pH fronts affect the permeabilization of the tissue and consequently its destruction. We also evaluate different shapes of electrode arrays by means of simulated and in vitro models. We concluded that the higher area of necrotic tissue was achieved for the greater separation between electrodes.     

A Numerical Method for Conformal Mapping

A method is developed for constructing the conformal map ofa distorted region onto a rectangle. A discrete Fourier transformis used to map the boundary of the region onto the boundaryof the rectangle; the resulting equations may be solved usinga fast Fourier transform algorithm. The map for internal pointsmay then be constructed using a standard Laplace equation solver.The method is computationally competitive, and is applicableto field problems, for instance in fluid mechanics.     

A Finite Element Algorithm for Electromechanical Contact Problems

In the last hundred years a lot of work is done in describing and measuring the influence of the pressure on the resistance and wear in electrical contacts. But up to now there exists a lack of knowledge in predicting and optimizing the behavior of electrical contacts with numerical simulation tools considering the pressure dependency. The present work provides a new constitutive model for the contact interface in the case of current flow and a new friction law including electrical wear phenomena. Additionally numerical investigations are made to compare the numerical results with experimental data. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Finite element method solution of electrically driven magnetohydrodynamic flow

The magnetohydrodynamic (MHD) flow in a rectangular duct is investigated for the case when the flow is driven by the current produced by electrodes, placed one in each of the walls of the duct where the applied magnetic field is perpendicular. The flow is steady, laminar and the fluid is incompressible, viscous and electrically conducting. A stabilized finite element with the residual-free bubble (RFB) functions is used for solving the governing equations. The finite element method employing the RFB functions is capable of resolving high gradients near the layer regions without refining the mesh. Thus, it is possible to obtain solutions consistent with the physical configuration of the problem even for high values of the Hartmann number. Before employing the bubble functions in the global problem, we have to find them inside each element by means of a local problem. This is achieved by approximating the bubble functions by a nonstandard finite element method based on the local problem. Equivelocity and current lines are drawn to show the well-known behaviours of the MHD flow. Those are the boundary layer formation close to the insulated walls for increasing values of the Hartmann number and the layers emanating from the endpoints of the electrodes. The changes in direction and intensity with respect to the values of wall inductance are also depicted in terms of level curves for both the velocity and the induced magnetic field.     

A finite element formulation of a viscoelastic model for dielectric elastomers

Smart materials by definition, are solids, fluids or gases which react independently on changing external conditions and modify one or more properties without external stimuli. Sensu lato an external energy can produce the reaction, such as stress, temperature, moisture, pH, magnetic or electric fields. The distinguishing characteristic for electroactive polymers (EAP) is, that they react with a deformation by the application of an electrical field. This contribution presents a nonlinear electro-viscoelastic model for dielectric elastomers and its finite element implementation. This type of smart materials belong to the group of EAP's and consists out of soft elastomer between compliant and conducting electrodes. (© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Comparison of the Solution of the Riemann Problem for Inviscid/Viscous Flows

The aim of this paper is to present the influence of the method used to solve the convective fluxes on the transonic internal flow field. The governing equations are discretized using an upwind method based on different solutions of the Riemann problem, flux vector splitting (FVS) or flux difference splitting schemes (FDS). Turbulence effects are simulated by means of the low‐Reynolds‐number k – ϵ and the SST (Shear‐Stress‐Transport) turbulence models.     

Numerical Modeling of Finite Nonlinear Visco­Electroelasticity

Polymeric electroelastic materials, so called electroactive Polymers (EAP) own the characteristic to deform by the application of an electrical field. Dielectric Elastomer (DE) actuators, belong to the group of electroactive Polymers. This type of actuator consists of soft elastomer between compliant and conducting electrodes. For the elastomer often silicones and acrylic elastomers are used. This material show in general a visco-hyperelastic behavior. Therefore, models which describe large strains and viscoelasticity are required for the calculation of such actuators. (© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Numerical investigation into a liquid displacing a gas in thin porous layers

To examine the filling process in a lithium-ion battery, a numerical model to characterize the displacing flow of a liquid in air-filled pores of thin heterogeneous porous materials is elaborated. The investigation is based on the volume-averaged Navier-Stokes equations for small Reynolds numbers, using a volume-of-fluid method to cover the multiphase flow. The flow is investigated with respect to the wall effect and to capillary action within the porous matrix. On the one hand, model experiments with similar particle-size distributions as in the battery layers are conducted to extract the porosity as function of the wall distance. On the other hand, experiments with the three different porous layers of the battery are performed to receive mean values for the most important properties related to the two-phase flow. Results for the displacement flow in parts of the battery are presented and discussed, showing a considerable influence of the modeled effects onto the flow characteristics. (© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Numerical investigation of the effect of helix angle and leaf margin on the flow pattern and the performance of the axial flow cyclone separator

A two-stage turbulence model based on the RNG κ–ε model combined with the Reynolds stress model is developed in this paper to analyze the gas flow in an axial flow cyclone separator. Five representative simulation cases are obtained by changing the helix angle and leaf margins of the cyclone. The pressure field and velocity field of the five cases are simulated, and then the effects of helix angle and leaf margins on the internal flow field of the cyclone are analyzed. When the continuum fluid (air) flow is relatively convergent, the discrete particle phase is added into the continuous phase and the gas-solid two-phase flow is simulated. One-way coupling method is used to solve the two-phase flow and a stochastic trajectory model is implemented for simulation of the particle phase. Finally, the pressure drop and separation efficiency of one case are measured and compare quantitatively well with the numerical results, which validates the reliability and accuracy of the simulation method based on the two-stage turbulence model.