Microscale thermal fluid transport process in a microchannel integrated with arrays of temperature and pressure sensors

One of the most important components in a microfluidic system is the microchannel which involves complicated flow and transport process. This work presents microscale thermal fluid transport process inside a microchannel with a height of 37 μm. The channel can be heated on the bottom wall and is integrated with arrays of pressure and temperature sensors which can be used to measure and determine the local heat transfer and pressure drop. A more simplified model with modification of Young’s Modulus from the experimental test is used to design and fabricate the arrays of pressure sensors. Both the pressure sensors and the channel wall use polymer materials which greatly simplifies the fabrication process. In addition, the polymer materials have a very low thermal conductivity which significantly reduces the heat loss from the channel to the ambient that the local heat transfer can be accurately measured. The airflow in the microchannel can readily become compressible even at a very low Reynolds number condition. Therefore, simultaneous measurement of both the local pressure drop and the temperature on the heated wall are required to determine the local heat transfer. Comparison of the local heat transfer for a compressible airflow in microchannel is made with the theoretical prediction based on incompressible airflow in large scale channel. The comparison has clarified many of the conflicting results among different works.     

Computations of gas microflows using pressure correction method with Langmuir slip model

A Langmuir slip model combined with continuum-based compressible Navier-Stokes equations is proposed and implemented for the purpose of analyzing complex microscale gas flows. For our model, an efficient compressible pressure correction algorithm based on an unstructured grid is developed and modified to be applicable to low Reynolds number slip flows in microgeometries. Gaseous slip flows in a uniform microchannel and compressible flow at backward-facing step are computed for the assessment of the adequacy of the method. Separated flow in a T-shaped micro-manifold is also simulated for the Reynolds number ranging from 10 to 60. In the uniform microchannel flow, the pressure increases nonlinearly in Langmuir slip model as the Knudsen number increases, while it drops nonlinearly in Maxwell slip model. The results from Langmuir slip model have been found to be more compatible with physics. From all the simulation cases, nonlinear behavior owing to both compressibility and rarefaction clearly appears in terms of streamwise velocity, pressure profiles and even reattachment length in the separation-associated flows. These results show that the suggested pressure correction method along with the Langmuir slip model may effectively simulate complex microscale gas flows, thereby offering a sound theoretical and numerical basis and an inexpensive computation procedure.     

含激波、旋涡和声波的复杂多尺度流动的直接数值模拟

本文主要报道了我们近年来在银河并行机上采用五阶WENO格式所做的一系列直接数值模拟研究,主要包括激波与单旋涡相互作用、激波与旋涡对相互作用、激波与三维纵向旋涡的相互作用,以及可压缩各向同性湍流。研究的主要目的是揭示激波与旋涡间相互作用中的激波动力学特性、旋涡变形、旋涡破裂和声波的产生机理,以及湍流等多尺度复杂流动的流场结构和流动机理。研究表明,高阶WE-NO格式具有很好的分辨率和稳定性,是研究上述包含强间断与复杂流场结构的流动的理想数值方法。研究发现,激波与强旋涡相互作用具有多级特征,即激波与初始旋涡的相互作用、反射激波与变形旋涡的相互作用、小激波与变形旋涡的相互作用。激波与旋涡对相互作用中产生的声波包含两个区域:线性区和非线性区。在线性区,激波与旋涡对相互作用产生的声波是激波分别与每个旋涡单独作用产生的声波的线性叠加;而在非线性区则与激波和耦合旋涡对的作用有关。在激波与纵向旋涡的相互作用过程中,发现旋涡破裂区存在多螺旋结构。在高初始湍流马赫数的各向同性湍流脉动场中,也发现了广泛报导的"小激波"的存在,这是可压缩湍流有别于不可压缩湍流的显著结构特征。     

Numerical analysis on electroosmotic flows in a microchannel with rectangle-waved surface roughness using the Poisson–Nernst–Planck model

The present study has numerically investigated two-dimensional electroosmotic flows in a microchannel with dielectric walls of rectangle-waved surface roughness to understand the roughness effect. For the study, numerical simulations are performed by employing the Nernst–Planck equation for the ionic species and the Poisson equation for the electric potential, together with the traditional Navier–Stokes equation. Results show that the steady electroosmotic flow and ionic-species transport in a microscale channel are well predicted by the Poisson–Nernst–Planck model and depend significantly on the shape of surface roughness such as the amplitude and periodic length of wall wave. It is found that the fluid flows along the surface of waved wall without involving any flow separation because of the very strong normal component of EDL (electric double layer) electric field. The flow rate decreases exponentially with the amplitude of wall wave, whereas it increases linearly with the periodic length. It is mainly due to the fact that the external electric-potential distribution plays a crucial role in driving the electroosmotic flow through a microscale channel with surface roughness. Finally, the present results using the Poisson–Nernst–Planck model are compared with those using the traditional Poisson–Boltzmann model which may be valid in these scales.     

声表面波为能量源的微通道关闭研究

为控制微通道内微流体流向,提出了一种声表面波关闭微通道方法。在128°旋转Y切割X传播方向的LiNbO3压电基片上制作中心频率为27.5 MHz的叉指换能器,其激发的声表面波熔融聚二甲基硅氧烷微槽内固体石蜡,熔融后的石蜡由于毛细作用力沿微通道输运。当移去激发声表面波的电信号后,熔融石蜡固化并阻塞微通道,实现微通道关闭。以红色染料溶液为实验对象,对微通道进行关闭操作。结果表明,声表面波可以成功地实现微通道关闭操作,当电信号功率为31.7 dBm时,微通道关断时间约为5 min。本文工作对声表面波为驱动源的微阀研究具有一定的借鉴意义。     

Finite element analysis of long-period water waves

The finite element representation of the nonlinear equations governing the unsteady flow of the two-dimensional long-period shallow water wave is considered. The approximate solution assumes, that the flow is only a slight perturbation of an existing flow. With this assumption a finite element formulation in terms of discrete nodal values of velocity and water height is generated using Galerkin's method. The resulting matrix equation for an arbitrary triangular-based space-time element constitutes a set of linear algebraic equations solvable for nodal values of the flow variables. The topological properties of estuaries are treated and with the solution thus obtained, numerical results are shown for the North Sea.     

A two-port model for wave propagation along a long circular microchannel

A compact model for oscillatory flow in a long microchannel with a circular cross-section is derived from the linearised Navier–Stokes equations. The resulting two-port model includes the effects of viscosity due to rarefied gas in the slip flow regime, inertia, compressibility and losses due to heat exchange. Both an acoustic impedance T network and an acoustic admittance Π network are presented for implementation in system level and circuit simulation tools. Also, reduced T and Π networks with constant component values are given to be used in the low frequency region. They are useful in time domain simulations, too. To verify the analytical model, simulations with a harmonic finite element solver for acoustic viscous flow are performed for microchannels exploiting the axisymmetry. The simulation results with both open and closed outlet conditions are compared with the two-port model with excellent agreement. Contribution of the slip conditions and the accuracy of the simple model are demonstrated.     

Size-dependent microparticles separation through standing surface acoustic waves

This study presents a method that uses a standing surface acoustic wave (SSAW) to continuously separate particles in a size-gradient manner in a microchannel flow. The proposed method was applied to a colloidal suspension containing poly dispersed particles with three different sizes (1, 5, and 10 μm) but the same density and compressibility. Particle suspension was focused hydrodynamically at an entrance region, and particles were forced actively toward the side wall where SSAW-pressure nodes were generated by two interdigital transducers (IDTs) across the channel. The particles placed in the middle stream, in which the shear rate was minimized, were separated successfully in a size-gradient manner by acoustic force. In addition, this study further developed an analytical model to predict the displacement of particles in microchannel flow by considering viscous, acoustic, and diffusive forces. The predicted values of particle displacement showed excellent agreement with the experimental results, and diffusion was found to be important and not negligible. The advantage of this method is to minimize the shear rate on particles, which would be useful for potential applications of shear-dependent cells such as platelets.     

Liquid flow through a diverging microchannel

In this work, experiments and three-dimensional numerical calculations of fluid flow through diverging microchannels were carried out with the aim of bringing out differences between flow in uniform and nonuniform passages. Deionized water was used as the working fluid in the experiments where the effects of mass flow rate (8.33 × 10^?6 to 8.33 × 10^?5 kg/s), microchannel hydraulic diameter (118–177 µm), length (10–30 mm) and divergence angle (4°–16°) on pressure drop were studied. The results are analyzed in detail with the help of numerical data. The pressure drop exhibits a linear dependence on the mass flow rate, whereas it is inversely proportional to the divergence angle and square of the hydraulic diameter. The pressure drop increases anomalously at 16°, suggesting that flow reversal occurs between 12° and 16°, which agrees with the corresponding value at the conventional scale. For the purpose of predicting pressure drop using straight microchannel theory, an equivalent hydraulic diameter was defined. It is observed that the equivalent hydraulic diameter, located at one-third of the diverging microchannel length from the inlet, becomes mostly independent of the mass flow rate, microchannel hydraulic diameter, length and divergence angle. The pressure drop for a diverging microchannel becomes equal to an equivalent hydraulic diameter uniform cross-section microchannel, suggesting that conventional correlations for straight microchannels can also be applied to diverging microchannels. The data presented in this work are of fundamental importance and can help in optimization of diffuser design used for example in valveless micropumps.     

On a method for direct numerical simulation of shear layer/compression wave interaction for aeroacoustic investigations

Direct numerical simulation (DNS) of a spatially developing mixing layer was performed. The compressible three-dimensional Navier-Stokes equations were solved for pressure, velocities and entropy for this flow using a compact finite-difference scheme of sixth-order accuracy in space, combined with Runge-Kutta three-step time advancement. On one of the transverse boundaries of the box-shaped domain, a compression wave profile was imposed in pressure and velocity components via a wave decomposition of the governing equations, in order to study the interaction of an isolated weak shock wave entering the domain with the mixed subsonic/supersonic shear layer. This flow situation is found along the shear layer of supersonic, imperfectly expanded jets containing a shock cell structure. In the present work, an isolated compression-expansion structure constitutes the model problem. The domain setup and the boundary conditions were chosen such as to allow analysis of the sound field generated by the turbulent flow and the shock-turbulence interaction. The numerical method used to impose the boundary conditions and solve the compressible Navier-Stokes equations, and the choice of numerical parameters, are described in detail. Some results on the two-dimensional and three-dimensional flow field computed are presented as well.     

A numerical study of an electrothermal vortex enhanced micromixer

Temperature gradients aroused from the Joule heating in a non-uniform electrical field can induce inhomogeneities of electric conductivity and permittivity of the electrolyte, thus causing an electrothermal force that generates flow motion. A 2D numerical investigation of a micromixer, utilizing electrothermal effect to enhance its mixing efficiency, is proposed in this paper. Results for temperature and velocity distributions, as well as sample concentration distribution are obtained for an electrolyte solution in a microchannel with different pairs of electrodes under AC potentials with various frequencies. Numerical solutions were first carried out for one pair of electrodes, with a length of 10 μm separated by a gap of 10 μm, on one side wall of a microchannel having a length of 200 μm and a height of 50 μm. It is found that the electrothermal flow effect, in the frequency range for which Coulomb force is predominant, induces vortex motion near the electrodes, thus stirring the flow streams and enhancing its mixing efficiency. If more than one pair of electrodes is located on the opposite walls of the microchannel, the mixing efficiency depends on the AC potential applied pattern and the electrodes arrangement pattern. The distance between two pairs of electrodes on two opposite walls is then optimized numerically. Sample mixing efficiencies, using KCl solutions as the working fluid in microchannels with different number of electrodes pairs at optimal electrodes arrangement pattern, are also investigated. If root mean squared voltages of 10 V in an AC frequency range of 0.1–10 MHz are imposed on 16 pairs of electrodes separated at an optimal distance, the numerical results show that a mixing efficiency of 98% can be achieved at the end of the microchannel having a length of 700 μm and a height of 50 μm at Re = 0.01 Pe _C = 100, and Pe _T = 0.07. However, the mixing efficiency decreases sharply at a frequency higher than 10 MHz owing to the drastically decrease in the Coulomb force.     

The Data Analyses of a Vertical Storage Tank Using Finite Element SOFT Computing

With the rapid development of the petrochemical industry, the number of largescale oil storage tanks has increased significantly, and many storage tanks are located in potential seismic regions. It is very necessary to analyze seismic response of oil storage tanks since their damage in an earthquake can lead to serious disasters and losses. In this paper, three models of vertical cylindrical oil storage tank in different sizes, which are commonly used in practical engineering are established. The dynamic characteristics, sloshing wave height and hydrodynamic pressure of the oil tank considering the liquid-structure coupling effect are analyzed by using ADINA finite element software, which are compared with the result of the standard method. The close numerical values of both results have verified the correctness and reliability of finite element model. The analytic results show that liquid sloshing wave height is basically in direct proportion to ground motion peak acceleration, the standard method of portion sloshing wave height calculation is not conservative. The hydrodynamic pressure generated by liquid sloshing caused by ground motion is not negligible compared with the hydrostatic pressure. The tank radius and oil height have a significant effect on the numerical value of hydrodynamic pressure. The ratio of the hydrodynamic pressure and hydrostatic pressure, which is named hydraulic pressure increase coefficients, is related to the height, which given by the GB 50341-2014 code in China have a high reliability. The seismic performances of tank wall near the bottom needs to be enhanced and improved in the seismic design of the oil tank.     

一种用于生化传感检测的压电式行波微流泵的研究

为了减少生化传感器中样品的消耗,基于行波原理,本文设计了一种新型的压电式微流泵.首先,理论证明了行波的产生机理,并用流体仿真软件Fluent进行了验证;其次,设计并制作了锯齿沟道和直沟道两种结构的微流泵,在压电双晶片阵列的驱动下,测量了这两种微流泵在不同频率和电压下的特性.结果表明锯齿形沟道结构的压电式行波微流泵性能更...     

A numerical study on the fluid compressibility effects in strongly coupled fluid–solid interaction problems

Interactions between an incompressible fluid passing through a flexible tube and the elastic wall is one of the strongly coupled fluid–solid interaction (FSI) problems frequently studied in the literature due to its research importance and wide range of applications. Although incompressible fluid is a prevalent model in many simulation studies, the assumption of incompressibility may not be appropriate in strongly coupled FSI problems. This paper narrowly aims to study the effect of the fluid compressibility on the wave propagation and fluid–solid interactions in a flexible tube. A partitioned FSI solver is used which employs a finite volume-based fluid solver. For the sake of comparison, both traditional incompressible (ico) and weakly compressible (wco) fluid models are used in an Arbitrary Lagrangian–Eulerian (ALE) formulation and a PISO-like algorithm is used to solve the unsteady flow equations on a collocated mesh. The solid part is modeled as a simple hyperelastic material obeying the St-Venant constitutive relation. Computational results show that not only use of the weakly compressible fluid model makes the FSI solver in this case more efficient, but also the incompressible fluid model may produce largely unrealistic computational results. Therefore, the use of the weakly compressible fluid model is suggested for strongly coupled FSI problems involving seemingly incompressible fluids such as water especially in cases where wave propagation in the solid plays an important role.

     

Direct numerical simulation of transitional flow at high Mach number coupled with a thermal wall model

In transitional and turbulent high speed boundary-layer flows the wall thermal boundary conditions play an important role and in many cases an assumption of a constant temperature or a specified heat flux may not be appropriate for numerical simulations. In this paper we extend a formulation for direct numerical simulation of compressible flows to include a thin plate that is thermally fully coupled to the flow. Even without such thermal coupling compressible flows with shock waves and turbulence represent a challenge for numerical methods. In this paper we review the scaling properties of algorithms, based on explicit high-order finite differencing combined with shock capturing, that are suitable for dealing with such flows. An application is then considered in which an isolated roughness element is of sufficient height to trigger transition in the presence of acoustic forcing. With the thermal wall model included it is observed that the plate heats up sufficiently during the simulation for the transition process to be halted and the flow consequently re-laminarises.     

Microchannel miter bend effects on pressure equalization and vortex formation

Simulations have been carried out for water flow in a square microchannel with a miter bend. The simulation considered a pressure-driven flow in a channel-hydraulic diameter of 5 μm for series of Reynolds number (Re) range from 0.056 to 560, in order to investigate water flow at bends. The result shows formation of two vortices after the miter bend, which are more discernible above Re 5.6. The critical inlet velocity for the generation of vortex in this particular geometry occurs at 1 m/s. A simple energy mechanism is postulated to explain the vortex formation as well as core skew direction. The high pressure region at the outer wall before and after the bend is a major factor for vortex formation since the liquid needs to reduce the additional energy effected by the high pressure region. Navier–Stokes equation is utilized with a no slip boundary condition for a total microchannel centerline length of 795 μm which is sufficient to produce a laminar flow pattern at the outlet.     

不同宽高比的V型随行波表面减阻仿真研究

为获得内壁具有不同宽高比的V型随行波的管道模型的微观流场及减阻规律,使用计算机软件对各模型进行了不同速度下的一系列数值仿真计算。在建模过程中为减小计算量,采用了二维管道剖面代替三维管道模型的方法,并选用了更适于旋转流和近壁湍流的湍流RNG K-ε模型进行计算。仿真结果表明：在流动中,管道内壁随行波产生有利于壁面减阻的旋涡,且旋涡疏密与随行波的宽高比有关。相同高度条件下,减阻效果会随着随行波宽高比的变大而增强。结果为减阻技术的工程化应用提供了重要依据。     

The extension of incompressible flow solvers to the weakly compressible regime

Numerical simulation schemes for incompressible flows such as the Simple scheme are extended to weakly compressible fluid flow. A single time scale, multiple space scale asymptotic analysis is used to gain insight into the limit behavior of the compressible flow equations as the Mach number vanishes. Motivated by these results, multiple pressure variables (MPV) are introduced into the numerical framework. These account separately for thermodynamic effects, acoustic wave propagation and the balance of forces. Discretized analogues of the averaging and large scale differencing procedures known from multiple scales asymptotics allow accurate capturing of various physical phenomena that are operative on very different length scales. The MPV approach combines the explicit numerical computation of global compression from the boundary and the long wavelength acoustics on coarse grids with an implicit pressure or pressure correction equation that formally converges to the corresponding incompressible one when the Mach number tends to zero.     

Fluid physics around conductive deformable flaps within an induced-charge electrokinetically driven microsystem

The induced-charge electrokinetic motion of a conductive deformable flap (which is installed on the walls of a microchannel) is numerically studied in this article. The relationship between the flap orientation (i.e., vertical, horizontal and oblique positions) and its motion is studied. Stagnation point concept is used to explain the behavior of the flap at different situations. The stagnation point is defined as a point on the flap surface where the induced zeta potential is zero. Thus, the flow velocity at this point becomes zero, and the pressure gradient will be maximum. The direction of the flap motion is determined by the location of the stagnation point. As an example, here, it is shown that the obtuse conductive flap moves in the opposite direction of the flow field because in this case, the stagnation point is located on the back surface of the flap. Interaction of two vertical conductive flaps (located at different distances from each other) is also investigated in this paper. The results indicate that if both of the conductive flaps are fixed on the same microchannel wall, two vortices with opposite spin directions are generated between them. These vortices create a low-pressure zone through which the two flaps attract one another. However, when each flap is fixed on upper and lower microchannel walls, the two vortices with same spin directions are generated between the flaps. These two vortices merge and form a high-pressure zone through which two flaps repel each other.     

Solution to the Riemann problem in a one-dimensional magnetogasdynamic flow

The Riemann problem for a quasilinear hyperbolic system of equations governing the one-dimensional unsteady flow of an inviscid and perfectly conducting compressible fluid, subjected to a transverse magnetic field, is solved approximately. This class of equations includes as a special case the Euler equations of gasdynamics. It has been observed that in contrast to the gasdynamic case, the pressure varies across the contact discontinuity. The iterative procedure is used to find the densities between the left acoustic wave and the right contact discontinuity and between the right contact discontinuity and the right acoustic wave, respectively. All other quantities follow directly throughout the (x, t)-plane, except within rarefaction waves, where an extra iterative procedure is used along with a Gaussian quadrature rule to find particle velocity; indeed, the determination of the particle velocity involves numerical integration when the magneto-acoustic wave is a rarefaction wave. Lastly, we discuss numerical examples and study the solution influenced by the magnetic field.