一种力-电协同驱动的细胞微流控培养腔理论模型

细胞培养液在微流控生物反应器中受到外界物理场（如压力梯度或者电场）作用流动而产生流体剪应力,并进一步刺激种子细胞调控其内部基因的表达,从而促进细胞的分化和生长,这个过程在自然生命组织内的微管中亦是如此。考虑到细胞培养微腔隙中液体流动行为很难实验量化测定,理论建模分析是目前可行的研究手段。因此建立了矩形截面的细胞微流控培养腔理论模型,将外部的物理驱动场（压力梯度与电场）与培养腔内液体的流速、切应力和流率联系起来,分别得到了压力梯度驱动（Pressure gradient driven,PGD）、电场驱动（Electric field driven,EFD）及力-电协同驱动（Pressure-electricity synergic driven,P-ESD）三种驱动方式下的液体流动理论模型。结果表明该理论模型与现有的实验结果基本一致,具体地:力-电协同作用下的解答为压力梯度驱动和电场驱动结果的叠加。细胞培养腔内的流体流速、剪应力及流率幅值均正比于外部物理场强幅值,但随着压力梯度驱动载荷频率的增大而减小,随着电场驱动频率的变化不明显。在压力梯度驱动作用下,细胞贴壁处的切应力随着腔高的增大而线性增大,流率则随着腔高的增大而非线性增大,而电场驱动下的结果不受腔高的影响。生理范围内的温度场变化对压力和电场驱动的结果影响不大。另外,在引起细胞响应的流体切应力水平,电场驱动能提供较大的切应力幅值而压力梯度驱动则能提供较大的流率幅值。该理论模型的建立为细胞微流控生物反应器实验系统的设计及参数优化提供理论参考,同时也为力-电刺激细胞生长、分化机理的研究的提供基础。      

一种骨小管中液体流动产生的流量及切应力模型

骨组织受力变形后其内部液体就会流动,同时在其微观结构——骨单元壁中扩散,并进一步产生一系列与骨液流动相关的物理效应,如流体剪切应力、流动电位等,这些物理效应被细胞感知并做出破骨或成骨等反应,来使骨适应外部载荷环境.鉴于骨组织产生的内部液体流动很难实验测定,理论模拟是目前的主要研究手段.基于骨单元的多孔弹性性质建立了骨小管内部液体的流动模型,该模型将骨单元所受的外部载荷与骨小管内部液体的压力、流速、流量和切应力联系起来,并进一步可以研究其力传导与力电传导机制.骨小管模型的建立分别基于中空和考虑哈弗液体的骨单元模型,并考虑了骨单元外壁的弹性约束和刚性位移约束两种边界条件.最终得到骨单元在外部轴向载荷作用下,骨小管内部液体的流量及流体切应力的解析解.结果表明:骨小管中的液体流量与流体切应力都正比于应变载荷幅值和频率,并由载荷的应变率决定.因此应变率可以作为控制流量和流体切应力的一种生理载荷因素.流量随着骨小管半径的增大而非线性增大,而流体切应力则随着骨小管半径的增大而线性增大.此外,在相同的载荷下,含哈弗液体的骨单元的模型中,骨小管中液体的流量和切应力均大于中空骨单元模型.     

The influence of rotation on turbulent flow and heat transfer in an annulus between independently rotating tubes

Experimental and numerical investigations of turbulent flow and heat transfer have been performed in a concentric annulus between independently rotating tubes. Numerical predictions, applying a Reynolds stress turbulence model, are compared with experimental fluid flow and heat transfer results for the case of a heated outer tube and an adiabatic inner tube. Compared to the above mentioned boundary conditions for the conservation equation of energy, differences in heat transfer in case of a heated inner tube and an adiabatic outer one, are examined by analysis, applying a mixing length turbulence model. Numerical investigations with both kinds of models about the influence of annulus radius ratio make evident that due to different superimpositions of centrifugal force and additional shear stress there is a wide variation of effects on fluid flow and heat transfer caused by the rotation of the inner and the outer tube.     

Numerical Simulation of Turbulent Flow in Concentric Annuli

In this paper we consider a fully developed turbulent flow in a round pipe with a small inner annulus. The diameter of the inner annulus is less than 10% of the diameter of the outer pipe. As a consequence, the surface area of the inner pipe compared to the outer pipe is small. The friction exerted by the wall on the flow is proportional to the surface area and the wall shear stress. Due to the small surface area of the inner annulus the additional stress on the flow due to the presence of the annulus may expected to be negligible. However, it will be shown that the inner annulus drastically changes the flow patterns and gives rise to unexpected scaling properties. In previous studies (Chung et al., Int J Heat Fluid Flow 23:426–440, 2002; Churchill and Chan, AIChE J 41:2513–2521, 1995) it was argued that radial position of the point of zero shear stress does not coincide with the radial location of the point of maximum axial velocity. In our direct numerical simulations we observe a coincidence of these points within the numerical accuracy of our model. It is shown that the velocity profile close to the inner annulus is logarithmic.     

On the origin of super-rotating layers in magnetohydrodynamic flows

An asymptotic analysis has been performed for the magnetohydrodynamic flow between perfectly conducting concentric cylindrical shells. The flow in the model geometry exhibits all the features which had been discovered in the past for the case of differentially rotating spherical shells considered in the context of geophysical analyses. For strong magnetic fields, the flow domain splits into distinct subregions and exhibits two different types of cores which are separated from each other by a tangent shear layer. The fluid in the inner core flows similar to a solid-body rotation and the outer core is entirely stagnant. With increasing magnetic fields the shear layer becomes thinner and, since the flow rate carried by the layer asymptotes to a finite value, the velocity in the layer increases as the layer thickness decreases. Moreover, the flux carried by the layer rotates in opposite direction compared with the rotation of the body. It is shown that the rotating jet is driven by the electric potential difference between the edges of the inner and the outer core.     

Effect of couple stresses on the flow in a constricted annulus

The flow of an incompressible couple stress fluid in an annulus with local constriction at the outer wall is considered. This configuration is intended as a simple model for studying blood flow in a stenosed artery when a catheter is inserted into it. The effects couple stress fluid parameters α and σ, height of the constriction (ε), and ratio of radii (k) on the impedance and wall shear stresses are studied graphically. Graphical results show that the resistance to the flow as well as the wall shear stress increases as the ratio of the radii increases and decreases as the couple stress fluid parameters increases.     

An interface crack with partially electrically conductive crack faces under antiplane mechanical and in-plane electric loadings

An interface crack in a bimaterial piezoelectric space under the action of antiplane mechanical and in-plane electric loadings is analyzed. One zone of the crack faces is electrically conductive while the other part is electrically permeable. All electro-mechanical values are presented using sectionally-analytic vector-functions and a combined Dirichlet-Riemann boundary value problem is formulated. An exact analytical solution of this problem is obtained. Simple analytical expressions for the shear stress, electric field and also for mechanical displacement jump of the crack faces are derived. These values are also presented graphically along the corresponding parts of the material interface. Singular points of the shear stress, electric field and electric displacement jump are found. Their intensity factors are determined as well. Intensity factors variations with respect to the external electric field and different ratios between the electrically conductive and electrically permeable crack face zones are also demonstrated.     

Layered model of electrorheological fluid under flow

We present a theoretical model of the behavior of a concentrated electrorheological fluid (ERF) which explicitly takes into account the effects of conductivity. The increase in shear viscosity under an electric field is due to a layered structure between the electrodes, made up of the remnants of particle chains adhering to the electrodes by electrostatic image forces, and a freely flowing liquid layer where all the shear flow is concentrated. This layered model can explain the variation of electric current with shear rate, as well as the rheological response of a dynamic yield stress proportional to the square of the applied electric field.     

Design and analysis of a smart composite pipe joint system integrated with piezoelectric layers under bending

Incorporating with the high electro-mechanical coupling performance of piezoelectric materials, design and analysis of an adhesively bonded smart composite pipe joint system were conducted. In this joint system, piezoelectric layers were integrated into the joint coupler in order to reduce stress concentration in the joint adhesive layer. To theoretically verify the composite action and efficiency of the integrated piezoelectric layers, an electro-mechanical model based on the first-order shear deformation theory was established. This model was able to clarify the energetic characteristics of the proposed joint system on the improvement in the joint strength, which was under the action of a bending moment at the joint ends. The state-space method was utilized to obtain the final analytical solutions, including the peel and shear stress distributions in the adhesive layer. Finally, some numerical examples were calculated to evaluate the effect of the detailed stacking sequence and size of the integrated piezoelectric layers on reducing the stress concentration in the adhesive layer as well as the applied electric fields. These numerical results validated the integrity of the developed adhesively bonded smart composite pipe joint system.     

Laminar,transitional and turbulent annular flow of drag-reducing polymer solutions

Mean and rms axial velocity-profile data obtained using laser Doppler anemometry are presented together with pressure-drop data for the flow through a concentric annulus (radius ratio κ = 0.506) of a Newtonian (a glycerine–water mixture) and non-Newtonian fluids—a semi-rigid shear-thinning polymer (a xanthan gum) and a polymer known to exhibit a yield stress (carbopol). A wider range of Reynolds numbers for the transitional flow regime is observed for the more shear-thinning fluids. In marked contrast to the Newtonian fluid, the higher shear stress on the inner wall compared to the outer wall does not lead to earlier transition for the non-Newtonian fluids where more turbulent activity is observed in the outer wall region. The mean axial velocity profiles show a slight shift (～5%) of the location of the maximum velocity towards the outer pipe wall within the transitional regime only for the Newtonian fluid.     

内管自转且公转时环管幂律流的受力分析

数值模拟了环管中内管偏心自转且公转时由轴向压力所驱动的幂律流体充分发展层流,分析了内管上的流体作用力。结果表明,内管偏心自转时流体作用力具有推动内管作和自转同向公转的效果。当只有外力矩驱动内管自转时,由于流体的作用,随内管线密度的不同,内管能达到的受力平衡态也不同:线密度较小时内管仅能在同心自转时达到受力平衡;线密度较大时内管能在作具有不变角速度和偏心率公转时达到受力平衡,且内管线密度越大,对应的受力平衡的公转的偏心率也越大。     

Characterization of yield stress fluids with concentric cylinder viscometers

The methods normally employed for shear rate calculations from concentric cylinder viscometer data generally are not applicable for fluids with a yield stress. In cylindrical systems with large radius ratios, as usually is the case with suspensions, the yield stress induces two possible flow regimes in the annulus. Unless the yield value is exceeded everywhere in the gap only part of the fluid can be sheared while the remaining region behaves like a solid plug. A correct calculation of the shear rate must take into account the presence of a variable effective gap width determined by the extent of the sheared layer. For time-independent yield stress fluids, a two-step procedure, which does not require any specific flow model, is proposed for analysing the experimental torque-speed data. Under the partially sheared condition, the shear rate can be computed exactly, whereas for the fully sheared flow the Krieger and Elrod approximation is satisfactory. The method is assessed by examining both semi-ideal data generated with a Casson fluid with known properties, and experimental data with an industrial suspension. A more complicated problem associated with characterization of time-dependent yield stress fluids is also identified and discussed. An approximate procedure is used to illustrate the dependence of the shear rate on time of shear in constant-speed experiments.     

Conjugate heat transfer within a concentric annulus filled with micropolar fluid

This paper investigates numerically the conjugate heat transfer in an annulus between two concentric cylinders. The annulus contains micropolar fluid and is heated isothermally from its inner wall. The effect of Rayleigh number, thickness of inner wall, inner wall-fluid thermal conductivity ratio, and material parameters of micropolar fluid on heat transfer rate within the annulus has been investigated. The study has shown that for low Rayleigh number regimes and for thermal conductivity of the inner wall greater than that of the fluid, the increase of inner wall thickness increases the heat transfer rate through the annulus and vice versa. While for convection dominating regimes Ra ≥ 10⁴ the increase of inner wall thickness decreases the heat transfer rate. Moreover, the study has shown that for fixed geometrical and flow parameters the heat transfer decreases in case of micropolar fluids in comparison with that of Newtonian fluids.     

MECHANOTRANSDUCTION OF THE CELL AND ITS PRIMARY CILIUM IN THE MICROFLUIDIC CHANNEL 1)

细胞处于复杂的生理环境之下,附着在细胞表面的初级纤毛被认为是重要的力学信号传感器,其与细胞的代谢、发育、分裂和增殖等生理活动密切相关.为了研究细胞及其初级纤毛在微流体环境下的力传导行为,本文建立了力-电协同驱动下的矩形微流控通道和含有多孔黏弹性属性的贴壁细胞有限元模型系统.考察了细胞的细胞质和细胞核在振荡层流下的应力、应变、孔隙压力和孔隙流速等力学信号响应,量化研究了初级纤毛作为细胞独特的力学感受器的生物力学行为. 结果表明:细胞在振荡层流下的力学响应表现出和外加力-电驱动载荷相同的震荡规律.渗透率是细胞多孔弹性力学行为的主要影响因素. 初级纤毛是细胞主要的力学感受器,细胞可以通过纤毛长度和直径调节其力学感受敏感性(应力影响区域),随着初级纤毛长度的增大, 其纤毛挠曲刚度减小, 但是敏感性增大.模型的建立为进一步研究微流体剪切作用下的细胞生长、分化等微观机理提供基础,同时也为检测细胞微结构器(纤毛等蛋白链)的力学性能提供了理论技术支持.     

Flow instabilities during annular displacement of one non-Newtonian fluid by another

This paper describes an experimental setup for axial laminar flow of liquids in the annulus between two eccentered cylinders. The design uses a conductivity method for measuring peak axial velocities around the annulus, and for the determination of displacement efficiency when displacing one fluid by another (displacement efficiency being defined as the ratio of volume of displaced fluid removed from the annulus, to the volume of the annulus, after a given number of annular volumes have been pumped). In an eccentric annulus, lower axial velocity in the narrow side produces “channeling” of the displacing fluid in the wide side and reduces the displacement efficiency. A positive density contrast between the two fluids can increase the efficiency by promoting azimuthal flow of the (denser) displacing fluid towards the narrow side. In this paper we report that gravity driven azimuthal flow is prone to severe instabilities which accelerate the displacement process but may leave behind an immobile strip of the displaced fluid in the narrow side.     

B平面上斜压波热力结构特性的实验研究

首次用转环实验模拟方法研究了β效应对斜压波热力结构的影响,发现β效应有抑制流动的水平混合和垂直混合的作用,使流动趋于正压;β平面上急流随高度降低而减弱,在急流的内外两侧各有一个无量纲温度值分布的突跃区,它们的空间结构与大气环流中的极锋锋区和北极锋锋区的结构相似。     

Experimental investigation of natural convection heat transfer in horizontal and inclined annular fluid layers

In the present study, an experimental investigation of heat transfer and fluid flow characteristics of buoyancy-driven flow in horizontal and inclined annuli bounded by concentric tubes has been carried out. The annulus inner surface is maintained at high temperature by applying heat flux to the inner tube while the annulus outer surface is maintained at low temperature by circulating cooling water at high mass flow rate around the outer tube. The experiments were carried out at a wide range of Rayleigh number (5 × 10⁴ < Ra < 5 × 10⁵) for different annulus gap widths (L/D _o = 0.23, 0.3, and 0.37) and different inclination of the annulus (α = 0°, 30° and 60°). The results showed that: (1) increasing the annulus gap width strongly increases the heat transfer rate, (2) the heat transfer rate slightly decreases with increasing the inclination of the annulus from the horizontal, and (3) increasing Ra increases the heat transfer rate for any L/D _o and at any inclination. Correlations of the heat transfer enhancement due to buoyancy driven flow in an annulus has been developed in terms of Ra, L/D _o and α. The prediction of the correlation has been compared with the present and previous data and fair agreement was found.     

Steady shear banding in complex fluids

Shear banding (SB) is manifested by the abrupt “demixing” of the flow into regions of high and low shear rate. In this paper, we first relate analytically the rheological parameters of the fluid with the range of shear rates and stresses of SB occurrence. For this, we accept that the origin of shear banding is constitutive, and adopt a non-linear viscoelastic expression able to accommodate the double-valuedness of the stress with flow intensity, under certain conditions. We then implement the model for the case of pressure driven flow through a cylindrical pipe; we derive approximate expressions for the velocity profile in the two-banded regions (core and outer annular), the overall throughput in the presence or absence of “spurt”, and the radial location limits of the shear rate discontinuity.     

RETRACTED ARTICLE: Flow of fractional Maxwell fluid between coaxial cylinders

This paper deals with the study of unsteady flow of a Maxwell fluid with fractional derivative model, between two infinite coaxial circular cylinders, using Laplace and finite Hankel transforms. The motion of the fluid is produced by the inner cylinder that, at time t = 0⁺, is subject to a time-dependent longitudinal shear stress. Velocity field and the adequate shear stress are presented under series form in terms of the generalized G and R functions. The solutions that have been obtained satisfy all imposed initial and boundary conditions. The corresponding solutions for ordinary Maxwell and Newtonian fluids are obtained as limiting cases of general solutions. Finally, the influence of the pertinent parameters on the fluid motion as well as a comparison between the three models is underlined by graphical illustrations.     

The behaviour of field-responsive fluids during shear start-up

The application of an external field (magnetic or electric) to suspensions of particles in a carrier liquid often causes a dramatic increase in the flow resistance. The transient stress response of these systems during the start-up of shear flow was studied as a function of the shear rate, using a system of carbonyl iron particles dispersed in paraffinic spindle oil under magnetic flux densities up to 0.57 T. It was found that initially the stress increased in proportion to the applied strain, reaching a plateau value at a characteristic strain of 0.2. Similar strain dependence of the transient stress behaviour was observed for shear rates spanning the range 0.01 s^–1 to 10 s^–1, suggesting that strain-governed deformation and rupture of the particle aggregates in the fluid was the main contribution to the response. In addition, the steady state flow curves of these fluids were obtained over the shear rate range 0.1 to 100 s^–1.