Vahid Nassehi

A finite volume model for the hydrodynamics of combined free and porous flow in sub-surface regions

Abstract In this paper, a finite volume model for the 3-D analysis of sub-surface water flow is described. The model is applied to simulate groundwater hydrodynamics in domains representing combined free flow and porous sections. The free flow section is isotropic, while the porous section is assumed to be a saturated and anisotropic medium with a constant porosity. The Navier–Stokes equations are used to simulate the free flow regime, while the porous flow is modelled by the Darcy equation. The governing equations of motion for each sub-domain are linked through the imposition of suitable matching conditions at the free/porous flow interface. At the exit from the domain, ‘no boundary condition’ is imposed to avoid forcing of any artificial condition on the flow system. This is guided by previously published work that recommends this approach for obtaining realistic simulations for complex flow systems. Results presented in this paper show that depending on pressure distribution, underground flow circulation inside the porous domain may occur. It is also shown that the direction of the porous flow may reverse at the interface, with the flow front reversal and the circulation centre moving away from their initial positions with time.

Modelling Transdermal Drug Delivery Using Microneedles: Effect of Geometry on Drug Transport Behaviour

Transdermal drug delivery using microneedles (MNs) depends on the rate of drug transport through the viable epidermis. Therefore, minimising the distance between the drug-loaded surface and the microcirculation in the dermis where the drug is absorbed into the body is significant in improving drug delivery efficiency. A quantifiable relationship between MN design parameters and skin diffusion properties is therefore desirable, which is what this study aims to achieve. A framework is presented to quantitatively determine the effects of design parameters on drug diffusion through skin, where the effects of compressive strain on skin due to insertion of MN are considered. The model is then used to analyse scenarios of practical importance. For all scenarios analysed, predicted steady-state flux was found to be lower when effect of MN strain on diffusion coefficient was accounted for. For example, simulations results indicated increasing tip radius from 5 to 20 µm and flux increased from 6.56 × 10(-6) to 7.02 × 10(-6) mol/(m(2) s) for constant diffusion coefficient. However, if the effect of strain on diffusion coefficient is considered, the calculated flux increases from 5.30 × 10(-6) to a peak value of 5.32 × 10(-6) mol/(m(2) s) (at 10 µm) and decreases to 5.29 × 10(-6) mol/(m(2) s). This paper contributes by reporting a framework to relate MN geometry to permeability with inclusion of the possible effects the MN design may pose on the diffusion coefficient.

NUMERICAL ANALYSIS OF MEDIUM COMPRESSION AND LOSSES IN FILTRATION AREA IN PLEATED MEMBRANE CARTRIDGE FILTERS

A computer model has been developed to simulate the fluid flow in pleated filter cartridges. This model has been used to evaluate the performance and design of pleated cartridge membrane filters. The effects of medium compression, pleat deformation, and pleat crowding are analyzed. At higher flow rates due to the exerted fluid pressure the medium is deformed, which leads to a reduction in the material permeability. Further, due to pleating and bending there is a loss in effective filtration area. The combined effects of compression and reduction in filtration area cause deviations from Darcys law. To interpret such deviations, permeability models based on the data obtained from the flat sheets of the filter material used in cartridge fabrication have been developed. The incorporation of the permeability model within the main hydrodynamic model determines the percentage loss in filtration area, percentage medium compression, and the pressure drop across the filters. Results of this study have been presented for fiberglass medium. The simulated results have been compared against experimental industrial data for purposes of model validation. The developed simulation tool offers a robust, cost-effective, and user-friendly design and analysis tool for pleated cartridge membrane filters, which can be easily used by engineers in industry.

Finite element modelling of free surface viscoelastic flows with particular application to rubber mixing

Internal mixers are used extensively in industry for mixing the components of rubber compounds. In these operations, in order to achieve effective mixing, the mixer chamber is always partially filled. This inevitably results in the appearance of multiple free surfaces in flow fields inside rubber mixer chambers. Mathematical modelling of such a flow regime is not a simple task and requires a great deal of effort. Traditional free surface flow-modelling techniques, which are mainly based on the use of volume-of-fluid or pseudo-density approaches in an Eulerian framework, are not flexible enough to cope with this problem. In this paper we describe a new method for the numerical modelling of free surface flows. In this method the pseudo-density approach is extended to a special Lagrangian framework along the trajectories of the fluid particles. We show that the developed scheme can very effectively simulate viscoelastic free surface flows encountered in rubber-mixing processes.

Calcium cation interactions with polysaccharides and proteins in wastewater UF membrane fouling

The role of divalent calcium cations in bio-flocculation and their interactions with wastewater components such as biopolymers (proteins, polysaccharides, nucleic acids and lipids) are crucial in membrane fouling. In this work, the interactions between calcium cations and sodium alginate (a model polysaccharide) and meat extract (a complex protein mixture) were investigated. Dead-end stirred cell and crossflow ultrafiltration experimental matrices were carried out. The alginate–calcium interaction was found to be a more severe foulant of the membrane than the meat-extract–calcium. The alginate–calcium membrane fouling was almost completely irreversible, while 40% of the membrane fouling was reversible with the meat-extract–calcium filtration experiments. Membrane fouling by alginate and meat extract was greater when the two coexisted in the feed stream than when they existed as individual components.

Case-Based Reasoning for Estuarine Model Design

Estuaries are complex natural water systems. Their behaviour depend on many factors, which are possible to analyse only by adopting different study approaches. The physical processes within estuaries, such as floods and pollutant dispersion, are generally investigated through computer modelling. In this paper the application of case-based reasoning technology to support the design of estuarine models is described. The system aims to provide a nonexpert user in modelling with the necessary guidance for selecting a model that matches his goal and the nature of the problem to be solved. The system is based on three components: a case-based reasoning scheme, a genetic algorithm and a library of numerical estuarine models. An example based on the Upper Milford Haven estuary (UK) is used to demonstrate the efficacy of the systems structure for supporting estuarine model design.

Simulation of Free Surface Flow in Partially Filled Internal Mixers

Abstract Internal mixers used in rubber processing and other industries are always partially filled. This results in the establishment of multiple random free surfaces in the flow field generated inside these mixers. Therefore successful mathematical modelling of internal mixing process depends on the development of efficient techniques for the reliable simulation of complex free surface flows. Various interface tracking and boundary capturing methods have been used in the past to model this kind of flow regimes. In particular, in recent years the well-known volume of fluid (VOF) method has been frequently used to model a variety of processes involving free surface and moving boundary flows. Both Eulerian and Lagrangian frameworks can be adopted in the VOF scheme to simulate free surface regimes. Under realistic flow conditions, however, the straightforward application of the technique in both frameworks may yield inaccurate results unless elaborate solution strategies are used to avoid errors. In many cases the use of such elaborate schemes requires excessive computational costs and effort or the solution scheme becomes complex and inflexible. In this paper we describe a relatively simple free surface tracking method based on the application of the VOF method in an Eulerian framework. In this scheme the flow field inside a partially filled internal mixer is treated as a two-phase system consisting of incompressible and compressible phases. The sections filled with the fluid which is being mixed are always regarded as an incompressible phase. The parts which are filled with air (or voids in some applications) form the second phase in the present two-phase flow analysis. The latter phase is treated as a compressible or an incompressible fluid (or pseudo fluid in the case of voids) depending on the value of the pressure calculated at each time step for the sections which contain it. We show that free surface flow of highly viscous fluids in partially filled internal mixers can be very successfully simulated by this method.

Computational methods in the management of hydro-environmental systems

Contents: 1: Introduction to Hydro-Environmental Systems 2: Mathematical Models 3: Computer Simulator Architecture 4: River Networks 5: Tidal System 6: Underground Systems 7: Combined Flow in Surface and Subsurface Systems 8: Hydroinformatics

Finite Element Analysis of Mixing in Partially Filled Twin Blade Internal Mixers

Abstract Twin blade internal mixers are the main devices used in rubber processing industry to mix natural and synthetic rubbers with carbon black and other compounding ingredients. To facilitate mixing, these internal mixers are always partially filled. Consequently, the flow regime that becomes established inside the mixer is a multiple random free surface flow with moving external boundaries. The stress field associated with this flow regime is usually very uneven. In addition, the flow stream is continuously divided and rejoined by the action of the counter rotating blades. The combination of these complex effects results in the break down, dispersion and distribution of carbon black agglomerates within the rubber matrix. The efficiency of the entire process mainly depends on the geometry of the mixer blades and the operating conditions. Predictive computer modelling offers a very convenient method for the quantitative analysis of mixing process and can be used to design more efficient internal mixers. However, the successful modelling of a complex process such as rubber mixing requires the development and use of sophisticated mathematical algorithms that can take into account its main characteristics. In this respect, a major difficulty is the imposition of the transient boundary conditions in a continuously varying flow domain. In the present paper we describe a robust method which can very effectively resolve this problem. This method is based on the combination of Lagrangian and Eulerian approaches for the modelling of moving boundary flows. We have used this scheme to simulate flow and mixing in the cross-sectional plane of the blades of a tangential rotor internal mixer.

Finite element modelling of underground water flow in low permeability lands

The present work deals with three-dimensional finite element analysis of water flow in saturated underground domains. This type of flow is a common feature of contaminants mobility under the ground and hence development of efficient models for such regimes is of significant environmental importance. The choice of the model equations mainly depends on the permeability of porous medium. For low permeability cases, Darcys equation provides the best choice whilst in higher permeability cases, the Brinkman equation must be used. Here, the use of Darcy equation provides a more realistic representation for underground flow. Therefore, first, we consider the validation of the Darcy equation for isotropic, homogenous, saturated porous media. The model is subsequently extended to the analysis of an underground domain in which the flow is partly obstructed by the presence of an impermeable barrier. The model is shown to yield theoretically expected results for both domains and is hence a reliable predictive tool.