Laminar Flow

One major drawback of the eddy viscosity subgrid-scale stress models used in large-eddy simulations is their inability to represent correctly with a single universal constant different turbulent fields in rotating or sheared flows, near solid walls, or in transitional regimes. In the present work a new eddy viscosity model is presented which alleviates many of these drawbacks. The model coefficient is computed dynamically as the calculation progresses rather than input apriori. The model is based on an algebraic identity between the subgrid-scale stresses at two different filtered levels and the resolved turbulent stresses. The subgrid-scale stresses obtained using the proposed model vanish in laminar flow and at a solid boundary, and have the correct asymptotic behavior in the near-wall region of a turbulent boundary layer. The results of large-eddy simulations of transitional and turbulent channel flow that use the proposed model are in good agreement with the direct simulation data.

This review describes the pattering of proteins and cells using a non-photolithographic microfabrication technology, which we call &soft lithography' because it consists of a set of related techniques, each of which uses stamps or channels fabricated in an elastomeric (&soft') material for pattern transfer. The review covers three soft lithographic techniques: microcontact printing, patterning using micro#uidic channels, and laminar #ow patterning. These soft lithographic techniques are inexpensive, are procedurally simple, and can be used to pattern a variety of planar and non-planar substrates. Their successful application does not require stringent regulation of the laboratory environment, and they can be used to pattern surfaces with delicate ligands. They provide control over both the surface chemistry and the cellular environment. We discuss both the procedures for patterning based on these soft lithographic techniques, and their applications in biosensor technology, in tissue engineering, and for fundamental studies in cell biology.

Direct numerical simulations of unsteady channel flow were performed at low to moderate Reynolds numbers on computational boxes chosen small enough so that the flow consists of a doubly periodic (in x and z) array of identical structures. The goal is to isolate the basic flow unit, to study its morphology and dynamics, and to evaluate its contribution to turbulence in fully developed channels. For boxes wider than approximately 100 wall units in the spanwise direction, the flow is turbulent and the low-order turbulence statistics are in good agreement with experiments in the near-wall region. For a narrow range of widths below that threshold, the flow near only one wall remains turbulent, but its statistics are still in fairly good agreement with experimental data when scaled with the local wall stress. For narrower boxes only laminar solutions are found. In all cases, the elementary box contains a single low-velocity streak, consisting of a longitudinal strip on which a thin layer ...

General discussion is given of the structure and dynamics of wrinkled fronts of premixed flames in turbulent and laminar flow fields. Recent theoretical analyses and experimental results leading to complete description of the coupling between diffusive transport processes, the hydrodynamical phenomena associated with gas expansion, complex chemical kinetics and acceleration of gravity are presented in detail. The following topics are considered: stability and flammability limits of planar fronts, cellular flames, flame stretch, turbulent and self-turbulizing flames, hydrodynamic interactions between weakly turbulent gas flows and wrinkled flame fronts, molecular diffusion effects of intermediate species involved in chain reactions. The main objective of this paper is to provide a review for non-specialists of recent developments in flame theory, in sufficient detail to give the reader a comprehensive introduction to the field. CONTENTS Nomenclature * The sound velocity is of same order of magnitude as the mean free path times the collision rate. * This is only true for ordinary fuels that have a large Lewis number than oxygen.

The method of lattice Boltzmann equation (LBE) is a kinetic-based approach for fluid flow computations. This method has been successfully applied to the multi-phase and multi-component flows. To extend the application of LBE to high Reynolds number incompressible flows, some critical issues need to be addressed, noticeably flexible spatial resolution, boundary treatments for curved solid wall, dispersion and mode of relaxation, and turbulence model. Recent developments in these aspects are highlighted in this paper. These efforts include the study of force evaluation methods, the development of multi-block methods which provide a means to satisfy different resolution requirement in the near wall region and the far field and reduce the memory requirement and computational time, the progress in constructing the second-order boundary condition for curved solid wall, and the analyses of the single-relaxation-time and multiple-relaxation-time models in LBE. These efforts have lead to successful applications of the LBE method to the simulation of incompressible laminar flows and demonstrated the potential of applying the LBE method to higher Reynolds flows. The progress in developing thermal and compressible LBE models and the applications of LBE method in multi-phase flows, multi-component flows, particulate suspensions, turbulent flow, and micro-flows are reviewed.

In this study, the effect of a synthetic jet on the heat transfer of flow over a flat plate is investigated experimentally. The experimental study is consist of a heater made of copper plate having constant heat flux located in the wind tunnel, and including a synthetic jet actuator injected into the stream by the entrance of plate. The synthetic jet is created by a piston-cylinder mechanism. In the investigations, the Reynolds number in the main stream, the frequency and amplitude of the actuator are changed while the geometry and Prandtl number remain constant for all cases and the effect of these parameters on the convective heat transfer is analyzed. The experiments are carried out at six different frequencies and four different amplitudes. To explain the heat transfer mechanism, the flow visualization is performed by using the smoke-wire method, and the instantaneous flow images are obtained. The experimental results reveal that there is a disruptive effect on the hydrodynamic boundary layer of the synthetic jet actuator. The obtained results are given as dimensionless parameters. It is observed that, the cycle-averaged Nusselt number increases with the increase of both Womersley number (Wo) and dimensionless amplitude (A o ).

The initiation of turbulent non-premixed combustion of gaseous fuels through autoignition and through spark ignition is reviewed, motivated by the increasing relevance of these phenomena for new combustion technologies. The fundamentals of the associated turbulent-chemistry interactions are emphasized. Background information from corresponding laminar flow problems, relevant turbulent combustion modelling approaches, and the ignition of turbulent sprays are included. For both autoignition and spark ignition, examination of the reaction zones in mixture fraction space is revealing. We review experimental and numerical data on the stochastic nature of the emergence of autoignition kernels and of the creation of kernels and subsequent flame establishment following spark ignition, aiming to reveal the particular facet of the turbulence causing the stochasticity. In contrast to fullyfledged turbulent combustion where the effects of turbulence on the reaction are reasonably wellestablished, at least qualitatively, here the turbulence can cause trends that are not straightforward. Autoignition occurs usually away from stoichiometry at a ''most reactive mixture fraction'', which can be approximately determined from homogeneous or laminar flow autoignition calculations, and at locations in the turbulent flow with low scalar dissipation. Such locations may be the cores of vortices. Once autoignition has occurred at a time that is mostly affected by the history of the conditional scalar dissipation, the relative magnitudes of convection, diffusion and reaction can affect the stabilisation height of flames in sprays or jets. Modelling efforts based on the Conditional Moment Closure, advanced flamelet approaches, and the transported PDF method seem suitable for capturing many, but not yet all, of the trends observed in DNS or experiment. Further experiments and DNS of realistic fuels and at conditions demonstrating chemical complexities must be performed to examine more fully the effects of scalar dissipation and its fluctuations on pre-ignition reaction zones. The statistics of the first appearance of autoignition in transient problems and its connection with the mixing field must also be studied. Ignition from a localised spark has a stochastic character that depends on the mixture fraction sampled at the spark location and duration and the local scalar dissipation. The success or not of the subsequent flame depends on the development of turbulent edge or stratified flames. Only preliminary data exist on the propagation speed of such flames and on their quenching. A lot remains to be done on turbulent edge flame propagation in unreacted and partially-reacted mixtures with inhomogeneities, turbulent flame propagation in non-uniformly dispersed droplet mists, and the transient stabilisation process of recirculating flames. The nature of the flame generation process at very short timescales, i.e. before any appreciable propagation, by sparking in inhomogeneous mixtures needs also to be examined. The development of high repetition rate diagnostics, for single-and two-phase flows, and the development of modelling approaches capturing both premixed and non-premixed reaction zones in gaseous and spray combustion are necessary.

The focal pattern of atherosclerotic lesions in arterial vessels suggests that local blood flow patterns are important factors in atherosclerosis. Although disturbed flows in the branches and curved regions are proatherogenic, laminar flows in the straight parts are atheroprotective. Results from in vitro studies on cultured vascular endothelial cells with the use of flow channels suggest that integrins and the associated RhoA small GTPase play important roles in the mechanotransduction mechanism by which shear stress is converted to cascades of molecular signaling to modulate gene expression. By interacting dynamically with extracellular matrix proteins, the mechanosensitive integrins activate RhoA and many signaling molecules in the focal adhesions and cytoplasm. Through such mechanotransduction mechanisms, laminar shear stress upregulates genes involved in antiapoptosis, cell cycle arrest, morphological remodeling, and NO production, thus contributing to the atheroprotective effe...

Frictionless flows with finite vorticity are usually made determinate by the imposition of boundary conditions specifying the distribution of vorticity ' at infinity '. No such boundary conditions are available in the case of flows with closed streamlines, and the velocity distributions in regions where viscous forces are small (the Reynolds number of the flow being assumed large) cannot be made determinate by considerations of the fluid as inviscid. It is shown that if the motion is to be exactly steady there is an integral condition, arising from the existence of viscous forces, which must be satisfied by the vorticity distribution no matter how small the viscosity may be. This condition states that the contribution from viscous forces to the rate of change of circulation round any streamline must be identically zero. (In cases in which the vortex lines are also closed, there is a similar condition concerning the circulation round vortex lines.)

This paper discusses a novel microfluidic fuel cell concept that utilizes the occurrence of multi-stream laminar flow at the microscale to keep the fuel and oxidant streams separated yet in diffusional contact. The system consists of a Y-shaped microfluidic channel in which two liquid streams containing fuel and oxidant merge and continue to flow laminarly in parallel between two catalyst-covered electrodes on opposing walls without turbulent mixing. Preliminary results indicate that this novel fuel cell concept may lead to the development of efficient room temperature power sources of microscopic dimensions that are comparable or better in performance than conventional polymer-electrolyte-membrane based microfuel cells that typically operate between 60 and 80 • C.

Little is known about the influence of substrate-bound gradients on neuronal development, since it has been difficult to fabricate gradients over the distances typically required for biological studies (a few hundred micrometers). This article demonstrates a generally applicable technique for the fabrication of substratebound gradients of proteins with complex shapes, using laminar flows in microchannels. Gradients that range from pure laminin to pure BSA were formed in solution by using a network of microchannels, and these proteins were allowed to adsorb onto a homogeneous layer of poly-L-lysine. Rat hippocampal neurons were cultivated on these substrate-bound gradients. Analysis of optical images of these neurons showed that axon specification is oriented in the direction of increasing surface density of laminin. Linear gradients in laminin adsorbed from a gradient in solution having a slope of ١[laminin] > about 0.06 g (ml⅐m) ؊1 (defined by dividing the change of concentration of laminin in solution over the distance of the gradient) orient axon specification, whereas those with ١[laminin] < about 0.06 g (ml⅐m) ؊1 have no effect. microfluidics ͉ neuronal polarity ͉ hippocampal G radients of chemoattractant and chemorepellent substances play central roles in controlling the development of the brain (1). Although the influence of gradients of soluble substances on neuronal behavior has been studied extensively and has been used to unravel the molecular and cellular mechanisms of axon guidance (2), much less is known about gradients of substrate-bound substances (3). Generating consistent substrate-bound gradients over the distances required for biological studies (a few hundred micrometers) has been difficult, and this difficulty hindered the investigation of the role of immobilized gradients in neuronal development (e.g., establishment of cellular polarity and axon guidance) and cell migration. This article demonstrates a generally applicable technique for the fabrication of substrate-bound gradients that uses laminar flow of fluids in microchannels; we have described the microfluidic systems used to form these gradients (4). By generating gradients in the concentration of proteins in solution and allowing them to adsorb on a surface, we fabricated substratebound gradients that range from pure laminin to pure BSA immobilized on a uniform layer of poly-L-lysine (PLL) over distances of a few hundred micrometers. We demonstrate that axon specification of rat hippocampal neurons cultivated on these gradients is oriented in the direction of increasing concentration of laminin. We demonstrate that gradients of extracellular molecules adsorbed on the surface are capable of orienting the polarity of cultured hippocampal neurons.

Drop formation at a capillary tip in laminar ow is investigated experimentally. The disperse phase is injected via a needle into another co-owing immiscible uid. Two di erent drop formation mechanisms are distinguished: Either the drops are formed close to the capillary tip-dripping-or they break up from an extended liquid jet-jetting. The e ect of the process and material parameters on the drop formation depends on the breakup mechanism and has to be investigated for each ow domain separately. In this study, we focus on dripping. The drop breakup is a ected by the ow dynamics of both the disperse and the continuous phase. Consequently, we investigate the e ect of ow rates, uid viscosities and interfacial tension on the droplet size and observe the dynamics of satellite drop generation. Whereas the fundamentals of disperse uid injection via a capillary into an ambient uid have been investigated extensively, the focus of this article is on providing a comprehensive experimental data set for proving the applicability of this technique as a dispersing tool. It is shown that drop formation at a capillary tip into a co-owing ambient liquid represents a promising technique for the production of monodisperse droplets where the droplet size is controlled externally by the ow strength of the continuous phase. The breakup dynamics changes signiÿcantly at the transition point from dripping to jetting. Consequently, the transition point between the ow domains represents an important operating point. In this article, dripping is demarcated from jetting by studying the in uence of the various material and process parameters on the transition point. ?

The quantity h(x, t) = u" o~ is the helicity density of the flow. Both h and are pseudoscalar quantities, i.e. they change sign under change from a right-handed to a left-handed frame of reference (parity transformation). It is important therefore to specify the frame that is used; we shall always use a right-handed Cartesian (or orthogonal curvilinear) frame unless otherwise stated. The simplest prototype) helical flow u = U+½f~ ^ x where U and f~ are constants. Then curl 0066-4189/92/0115-0281 $02.00 281 www.annualreviews.org/aronline Annual Reviews Annu. Rev. Fluid. Mech. 1992.24:281-312. Downloaded from arjournals.annualreviews.org by CAMBRIDGE UNIVERSITY on 02/11/05. For personal use only. www.moffatt.tc .annualreviews.org/aronline nual Reviews Annu. Rev. Fluid. Mech. 1992.24:281-312. Downloaded from arjournals.annualreviews.org by CAMBRIDGE UNIVERSITY on 02/11/05. For personal use only.

Nonlinearity in finite-Reynolds-number flow results in particle migration transverse to fluid streamlines, producing the well-known ''tubular pinch effect'' in cylindrical pipes. Here we investigate these nonlinear effects in highly confined systems where the particle size approaches the channel dimensions. Experimental and numerical results reveal distinctive dynamics, including complex scaling of lift forces with channel and particle geometry. The unique behavior described in this Letter has broad implications for confined particulate flows.

Surface-based binding assays are often influenced by the transport of analyte to the sensor surface. Using simulated data sets, we test a simple two-compartment model to see if its description of transport and binding is sufficient to accurately analyze BIACORE data. First we present a computer model that can generate realistic BIACORE data. This model calculates the laminar flow of analyte within the flow cell, its diffusion both perpendicular and parallel to the sensor surface, and the reversible chemical reaction between analyte and immobilized reactant. We use this computer model to generate binding data under a variety of conditions. An analysis of these data sets with the two-compartment model demonstrates that good estimates of the intrinsic reaction rate constants are recovered even when mass transport influences the binding reaction. We also discuss the conditions under which the two-compartment model can be used to determine the diffusion coefficient of the analyte. Our results illustrate that this model can significantly extend the range of association rate constants that can be accurately determined from BIACORE.

Computer-controlled microfluidics would advance many types of cellular assays and microscale tissue engineering studies wherever spatiotemporal changes in fluidics need to be defined. However, this goal has been elusive because of the limited availability of integrated, programmable pumps and valves. This paper demonstrates how a refreshable Braille display, with its grid of 320 vertically moving pins, can power integrated pumps and valves through localized deformations of channel networks within elastic silicone rubber. The resulting computerized fluidic control is able to switch among: (i) rapid and efficient mixing between streams, (ii) multiple laminar flows with minimal mixing between streams, and (iii) segmented plug-flow of immiscible fluids within the same channel architecture. The same control method is used to precisely seed cells, compartmentalize them into distinct subpopulations through channel reconfiguration, and culture each cell subpopulation for up to 3 weeks under perfusion. These reliable microscale cell cultures showed gradients of cellular behavior from C2C12 myoblasts along channel lengths, as well as differences in cell density of undifferentiated myoblasts and differentiation patterns, both programmable through different flow rates of serumcontaining media. This technology will allow future microscale tissue or cell studies to be more accessible, especially for highthroughput, complex, and long-term experiments. The microfluidic actuation method described is versatile and computer programmable, yet simple, well packaged, and portable enough for personal use.

The dispersion of a solute in the flow of a Casson fluid in an annulus is studied. The generalized dispersion model is employed to study the dispersion process. The effective diffusion coefficient, which describes the whole dispersion process in terms of a simple diffusion process, is obtained as a function of time, in addition to its dependence on the yield stress of the fluid and on the annular gap between the two cylinders. It is observed that the dispersion coefficient changes very rapidly for small values of time and becomes essentially constant as time takes large values. In non–Newtonian fluids the steady state is reached at earlier instants of time when compared to the Newtonian case and the time taken to reach the steady state is seen to depend on the values of the yield stress. It is observed that a decrease in the annular gap inhibits the dispersion process for all times both in Newtonian as well as in non–Newtonian fluids. When the yield stress is 0.05, depending upon the size of the annular gap (0.9–0.7) the reduction factor in the dispersion coefficient varies in the range 0.58–0.08. The application of this study for understanding the dispersion of an indicator in a catheterized artery is discussed.

This paper describes a miniaturized, integrated, microfluidic device that can pull molecules and living cells bound to magnetic particles from one laminar flow path to another by applying a local magnetic field gradient, and thus selectively remove them from flowing biological fluids without any wash steps. To accomplish this, a microfabricated high-gradient magnetic field concentrator (HGMC) was integrated at one side of a microfluidic channel with two inlets and outlets. When magnetic micro- or nano-particles were introduced into one flow path, they remained limited to that flow stream. In contrast, when the HGMC was magnetized, the magnetic beads were efficiently pulled from the initial flow path into the collection stream, thereby cleansing the original fluid. Using this microdevice, living E. coli bacteria bound to magnetic nanoparticles were efficiently removed from flowing solutions containing densities of red blood cells similar to that found in blood. Because this microdevice allows large numbers of beads and cells to be sorted simultaneously, has no capacity limit, and does not lose separation efficiency as particles are removed, it may be especially useful for separations from blood or other clinical samples. This on-chip HGMC-microfluidic separator technology may potentially allow cell separations to be carried out in the field outside of hospitals and clinical laboratories.

Similarity solutions of the laminar boundary-layer equations describing heat and flow in a quiescent fluid driven by a stretched surface subject to suction or injection are obtained herein. The surface is moving with a power-law velocity distribution, and its temperature has a power-law variation. The effect of various governing parameters, such as Prandtl number Pr, temperature exponent n, velocity exponent m, and the injection parameter d, which determine the temperature profiles and heat transfer coefficient are studied. Three boundary conditions of uniform temperature, variable temperature, and uniform heat flux at the surface have been investigated. The effect of decreasing d is found to be significant, particularly for high Prandtl numbers.

Experimental results have been obtained for single-phase forced convection in deep rectangular microchannels. Two con®gurations were tested, a single channel system and a multiple channel system. The two systems are identical except for the lack of extended surfaces in the single channel system. In the case of the multiple channel system the channels are 251 lm wide and the channel walls are 119 lm thick. In both systems the channels are approximately 1000 lm deep and de®ne a projected area of 2.5 cm´2.5 cm. All tests were performed with deionized water as the working¯uid, where the Reynolds number ranged from 173 to 12 900. The experimentally obtained local Nusselt number agrees reasonably well with classical developing channel¯ow theory. Furthermore, the results show that, in terms of¯ow and heat transfer characteristics, our microchannel system designed for developing laminar¯ow outperforms the comparable single channel system designed for turbulent¯ow. Ó . 0142-727X/99/$ ± see front matter Ó 1999 Elsevier Science Inc. All rights reserved. PII: S 0 1 4 2 -7 2 7 X ( 9 8 ) 1 0 0 5 5 -3

Excited state lifetimes have been measured for the A-states of CH, OH, and NO in a number of low-pressure, premixed, laminar flow methane flames. From these lifetimes, collisional quenching rates were determined as a function of height above the burner and thus as a function of flame temperature and composition. The results were compared with values calculated using a model of the flame chemistry to predict collider mole fractions, together with parameterizations of quenching rate coefficients for each collider. Measured OH and NO quenching rates agree well with those calculated from these quenching rate coefficients and modeled flame composition data. This indicates that collisional quenching corrections for laser-induced fluorescence measurements can be calculated from knowledge of major species mole fractions and gas temperature. Predicted quenching rates for CH range from agreement with measured values to 27% higher than measured values. This discrepancies suggest insufficient knowledge of high temperature quenching by H 2 O and N 2 .

Effect of copper-water nanofluid has been studied as a cooling medium to simulate the heat transfer behaviour in a two-dimensional (infinite depth) horizontal rectangular duct, where top and bottom walls are two isothermal symmetric heat sources. The governing continuity, momentum and energy equations for a laminar flow are being discretized using a finite volume approach using a power law profile approximation and has been solved iteratively, through alternate direction implicit, using the SIMPLER algorithm. The thermal conductivity of nanofluid has been determined by model proposed by Patel et al. Study has been conducted considering the fluid as Newtonian as well as non-Newtonian for a wide range of Reynolds number (Re = 5 to 1500) and solid volume fraction (0.00 φ 0.050). It has been observed that the heat transfer augmentation is possible using nanofluid in comparison to conventional fluids for both the cases. The rate of heat transfer increases with the increase in flow as well as increase in solid volume fraction of the nanofluid. Unlike natural convection the increase in heat transfer is almost same for both the cases.

This communication reports the design, assembly, and performance of an air-breathing laminar flow-based microfluidic fuel cell. Micro fuel cells have long been recognized as promising high energy density power sources for portable applications. Many

A three-dimensional laminar flow and heat transfer with two different nanofluids, Al 2 O 3 and CuO, in an ethylene glycol and water mixture circulating through the flat tubes of an automobile radiator have been numerically studied to evaluate their superiority over the base fluid. New correlations for viscosity and thermal conductivity of nanofluids as a function of particle volumetric concentration and temperature developed from the experiments have been used in this paper. Numerical results from the present simulation were first validated for the flow of water by comparing the friction factor and the Nusselt number in flat tubes, for which accurate results are available in the literature. Next, the model was applied to study the peripheral variations of shear stress and convective heat transfer coefficient, both showing higher magnitudes in the flat regions of the tube. Convective heat transfer coefficient in the developing and developed regions along the flat tubes with the nanofluid flow showed marked improvement over the base fluid. Results for the local and the average friction factor and convective heat transfer coefficient show an increase with increasing particle volumetric concentration of the nanofluids. Quantitative results of the increase of the heat transfer coefficient and the friction factor with increasing volumetric concentrations of nanofluids at various Reynolds numbers are presented. The pressure loss increases with increasing particle volumetric concentrations of nanofluids; however, due to the reduced volumetric flow needed for the same amount of heat transfer, the required pumping power diminishes.

A microfluidic platform for the construction of microscale components and autonomous systems is presented. The platform combines liquid-phase photopolymerization, lithography, and laminar flow to allow the creation of complex and autonomous microfluidic systems. The fabrication of channels, actuators, valves, sensors, and systems is demonstrated. Construction times can be as short as 10 min, providing ultrarapid prototyping of microfluidic systems.

This report describes the manipulation of light in waveguides that comprise a liquid core and a liquid cladding (liq͞liq waveguide). These waveguides are dynamic: Their structure and function depend on a continuous, laminar flow of the core and cladding liquids. Because they are dynamic, they can be reconfigured and adapted continuously in ways that are not possible with solid-state waveguides. The liquids are introduced into the channels of a microfluidic network designed to sandwich the flowing core liquid between flowing slabs of the cladding fluid. At low and moderate Reynolds numbers, flow is laminar, and the liq͞liq interfaces are optically smooth. Small irregularities in the solid walls of the channels do not propagate into these interfaces, and liq͞liq waveguides therefore exhibit low optical loss because of scattering. Manipulating the rate of flow and the composition of the liquids tunes the characteristics of these optical systems.

The e}ects of the electric double layer near the solidÐliquid interface and the~ow induced electrokinetic _eld on the pressure!driven~ow and heat transfer through a rectangular microchannel are analyzed in this work[ The electric double layer _eld in the cross!section of rectangular microchannels is determined by solving a non!linear\ two!dimensional PoissonÐBoltzmann equation[ A body force caused by the electric double _eld and the~ow!induced electrokinetic _eld is considered in the equation of motion[ For steady!state\ fully!developed laminar~ows\ both the velocity and the temperature _elds in a rectangular microchannel are determined for various conditions[ The~ow and heat transfer characteristics with:without consideration of the electrokinetic e}ects are evaluated[ The results clearly show that\ for aqueous solutions of low ionic concentrations and a solid surface of high zeta potential\ the liquid~ow and heat transfer in rectangular microchannels are signi_cantly in~uenced by the presence of the electric double layer _eld and the induced electrokinetic~ow[ Þ 0887 Elsevier Science Ltd[ All rights reserved[ Corresponding author[ Tel[ ] 990 392 381 8623^fax ] 990 392 381 1199^e!mail ] dongqing[liÝualberta[ca

This work describes a novel bioreactor system for the cultivation of animal, insect, and plant cells using wave agitation induced by a rocking motion. This agitation system provides good nutrient distribution, off-bottom suspension, and excellent oxygen transfer without damaging fluid shear or gas bubbles. Unlike other cell culture systems, such as spinners, hollow-fiber bioreactors, and roller bottles, scale-up is simple, and has been demonstrated up to 100 L of culture volume. The bioreactor is disposable, and therefore requires no cleaning or sterilization. Additions and sampling are possible without the need for a laminar flow cabinet. The unit can be placed in an incubator requiring minimal instrumentation. These features dramatically lower the purchase cost, and operating expenses of this laboratory/pilot scale cell cultivation system. Results are presented for various model systems: 1) recombinant NS0 cells in suspension; 2) adenovirus production using human 293 cells in suspension; 3) Sf9 insect cell/baculovirus system; and 4) human 293 cells on microcarrier. These examples show the general suitability of the system for cells in suspension, anchorage-dependent culture, and virus production in research and GMP applications.

Based on Newtonian fluid dynamic relations, a model is constructed to describe laser-induced mass transport in thin films of polymers containing isomerizable azobenzene chromophores, in which surface profile diffraction gratings can be inscribed with an interference pattern of coherent light. The Navier-Stokes equations for laminar flow of a viscous fluid are developed to relate velocity components in the film to pressure gradients in the polymer film, by definition of boundary layer conditions. This general laminar flow model is applicable to the formation of surface gratings through a variety of mechanisms. Considering the mechanism of an isomerization-driven free volume expansion to produce internal pressure gradients, a specific model is developed to describe polymer flow resulting from laser-induced isomerization of the bulky chromophores. This yields an expression relating the time evolution of the surface gratings to properties which could be varied experimentally, such as those of the irradiating light, inscription geometry, and bulk polymer, which incorporates no arbitrary fitting parameters or integration constants. In the general model, the rate of grating inscription is predicted to vary directly with the intensity of the inscription laser, to vary inversely with the molecular weight of the polymer below the limit of entanglement, and to scale with the third power of the initial thickness of the film. Considering an isomerization pressure mechanism, the model predicts the rate of grating inscription to further vary with the free volume requirements of the induced geometric transformation of the dye molecules, and the polarization state of the inscription laser. Predictions from the model were tested against the results of experiments to vary these parameters, and are shown to be in good agreement.

The present paper focuses on the analysis of two-and three-dimensional flow past a circular cylinder in different laminar flow regimes. In this simulation, an implicit pressure-based finite volume method is used for time-accurate computation of incompressible flow using second order accurate convective flux discretisation schemes. The computation results are validated against measurement data for mean surface pressure, skin friction coefficients, the size and strength of the recirculating wake for the steady flow regime and also for the Strouhal frequency of vortex shedding and the mean and RMS amplitude of the fluctuating aerodynamic coefficients for the unsteady periodic flow regime. The complex three dimensional flow structure of the cylinder wake is also reasonably captured by the present prediction procedure.

The ability of oxyhemoglobin to inhibit nitric oxide (NO)-dependent activation of soluble guanylate cyclase and vasodilation provided some of the earliest experimental evidence that NO was the endothelium-derived relaxing factor (EDRF). The chemical behavior of this dioxygenation reaction, producing nearly diffusion limited and irreversible NO scavenging, presents a major paradox in vascular biology: The proximity of large amounts of oxyhemoglobin (10 mmol/L) to the endothelium should severely limit paracrine NO diffusion from endothelium to smooth muscle. However, several physical factors are now known to mitigate NO scavenging by red blood cell encapsulated hemoglobin. These include diffusional boundaries around the erythrocyte and a red blood cell free zone along the endothelium in laminar flowing blood, which reduce reaction rates between NO and red cell hemoglobin by 100-to 600-fold. Beyond these mechanisms that reduce NO scavenging by hemoglobin within the red cell, 2 additional mechanisms have been proposed suggesting that NO can be stored in the red blood cell either as nitrite or as an S-nitrosothiol (S-nitroso-hemoglobin). The latter controversial hypothesis contends that NO is stabilized, transported, and delivered by intra-molecular NO group transfers between the heme iron and ␤-93 cysteine to form S-nitrosohemoglobin (SNO-Hb), followed by hypoxia-dependent delivery of the S-nitrosothiol in a process that links regional oxygen deficits with S-nitrosothiol-mediated vasodilation. Although this model has generated a field of research examining the potential endocrine properties of intravascular NO molecules, including S-nitrosothiols, nitrite, and nitrated lipids, a number of mechanistic elements of the theory have been challenged. Recent data from several groups suggest that the nitrite anion (NO 2 Ϫ ) may represent the major intravascular NO storage molecule whose transduction to NO is made possible through an allosterically controlled nitrite reductase reaction with the heme moiety of hemoglobin. As subsequently understood, the hypoxic generation of NO from nitrite is likely to prove important in many aspects of physiology, pathophysiology, and therapeutics. (Arterioscler Thromb Vasc Biol. 2006;26:697-705.) Key Words: nitric oxide Ⅲ nitrite Ⅲ hemoglobin Ⅲ vasodilation Ⅲ red blood cell N itric oxide (NO) is the endothelium-derived relaxing factor that modulates vascular tone by activating soluble guanylyl cyclase (sGC) in smooth muscle. 1-4 It is now appreciated that NO is produced in endothelial cells by the endothelial NO synthase enzyme and participates in many aspects of normal vascular physiology, including tonic vasodilation and inhibition of platelet activation and endothelial adhesion molecule expression. Endothelial dysfunction, characterized by reduced bioavailability of endothelial derived NO, is a central mechanistic feature of coronary artery disease and its risk factors, including diabetes, hypertension, smoking, and obesity. Excessive NO production from inducible NO synthase is a central mechanism of septic shock. Novel therapeutic strategies based on increasing or decreasing native concentrations of NO are undergoing investigation or have already translated into clinical practice.

The appearance of chaotic particle trajectories in steady, laminar, incompressible flow through a twisted pipe of circular cross-section is demonstrated using standard dynamical systems diagnostics and a model flow based on Dean's perturbation solutions. A study is performed to determine the parameters that control fluid stirring in this mixing device that has no moving parts. Insight into the chaotic dynamics are provided by a simple one-dimensional map of the pipe boundary onto itself. The results of numerical experiments illustrating the stretching of material lines, stirring of blobs of material, and the three-dimensional trajectories of fluid particles are presented. Finally, enhanced longitudinal particle dispersal due to the coupling between chaos in the transverse direction and the non-uniform longitudinal transport of particles is shown.

The onset of slip motion at fluid-solid boundaries is investigated as a function of the reflectivity of the solid wall by means of a mesoscopic lattice Boltzmann model. Substantial slip flow is observed for reflectivity values below a critical threshold. It is shown that this slip flow may significantly affect the conversion efficiency of catalytic microchannels.

We previously reported that laminar flow activates peroxisome proliferator-activated receptor ␥ (PPAR␥) in vascular endothelial cells in a ligand-dependent manner that involves phospholipase A2 and cytochrome P450 epoxygenases. In this study, we investigated whether epoxyeicosatrienoic acids (EETs), the catalytic products of cytochrome P450 epoxygenases, are PPAR␥ ligands. Competition and direct binding assays revealed that EETs bind to the ligandbinding domain of PPAR␥ with Kd in the M range. In the presence of adamantyl-ureido-dodecanoic acid (AUDA), a soluble epoxide hydrolase (sEH)-specific inhibitor, EETs increased PPAR␥ transcription activity in endothelial cells and 3T3-L1 preadipocytes. Inclusion of AUDA in the perfusing media enhanced, but overexpression of sEH reduced, the laminar flow-induced PPAR␥ activity. Furthermore, laminar flow augmented cellular levels of EETs but decreased sEH at the levels of mRNA, protein, and activity. Blocking PPAR␥ by GW9662 abolished the EET͞AUDA-mediated antiinflammatory effect, which indicates that PPAR␥ is an effector of EETs. endothelial cells ͉ shear stress