Simulation of transport processes in the Couette flow problem under incomplete accommodation of the tangential momentum of gas molecules by channel walls

The Couette flow problem has been used as an example for the proposed analytical method for calculating macro parameters of gas in channels, the thickness of which is comparable with the mean free path of gas molecules by means of simple numerical procedures. As a basic equation, the linearized BGK (Bhatnagar-Gross-Krook) model of the Boltzmann kinetic equation is used, and the boundary condition on the channel walls is taken to be the model of a specular-diffuse reflection. For different values of the channel thickness and the coefficient of accommodation of the tangential momentum of gas molecules by the channel walls, the profiles of the gas mass velocity in the channel have been constructed and the values of the nonzero component of the viscous stress tensor and the gas mass flow per unit channel width have been calculated. A comparison with similar results published in the press has been made.     

Numerical approximation of flow induced vibrations of channel walls

In this paper the numerical method for solution of an aeroelastic model describing the interactions of air flow with vocal folds is described. The flow is modelled by the incompressible Navier–Stokes equations spatially discretized with the aid of the stabilized finite element method. The motion of the computational domain is treated with the aid of Arbitrary Lagrangian–Eulerian method. The structure motion is described by an equivalent system with two degrees of freedom governed by a system of ordinary differential equations and discretized in time with the aid of an implicit multistep method and strongly coupled with the flow model. The influence of inlet/outlet boundary conditions is studied. The numerical analysis is performed and compared to the related results from literature.     

Charge control of parallel-plate, electrostatic actuators and the tip-in instability

Controlling the charge, rather than the voltage, on a parallel-plate, electrostatic actuator theoretically permits stable operation for all deflections. Practically, we show that, using charge control, the maximum stable deflection is limited by 1) charge pull-in, in which the actuator snaps due to the presence of parasitic capacitance and 2) tip-in, in which the rotation mode becomes unstable. This work presents a circuit that controls the amount of charge on a parallel-plate, electrostatic actuator. This circuit reduces the sensitivity to parasitic capacitance, so that tip-in is the limiting instability. A small-signal model of the actuator is developed and used to determine the circuit bandwidth and gain requirements for stable deflections. Four different parallel-plate actuators have been designed and tested to verify the charge control technique as well as to verify charge pull-in, tip-in, and the bandwidth requirements. One design travels 83% of the gap before tip-in. Another design can only travel 20% of the gap before tip-in, regardless of whether voltage control or charge control is used.     

Simulation of unsteady compressible flow in a channel with vibrating walls - Influence of the frequency

This study deals with the numerical solution of a 2D unsteady flow of a compressible viscous fluid in a channel for low inlet airflow velocity. The unsteadiness of the flow is caused by a prescribed periodic motion of a part of the channel wall with large amplitudes, nearly closing the channel during oscillations. The channel is a simplified model of the glottal space in the human vocal tract and the flow can represent a model of airflow coming from the trachea, through the glottal region with periodically vibrating vocal folds to the human vocal tract.The flow is described by the system of Navier–Stokes equations for laminar flows. The numerical solution is implemented using the finite volume method (FVM) and the predictor–corrector MacCormack scheme with Jameson artificial viscosity using a grid of quadrilateral cells. Due to the motion of the grid, the basic system of conservation laws is considered in the Arbitrary Lagrangian–Eulerian (ALE) form.The authors present the numerical simulations of flow fields in the channel, acquired from a program developed exclusively for this purpose. The numerical results for unsteady flows in the channel are presented for inlet Mach number M_∞ = 0.012, Reynolds number Re_∞ = 4.5 × 10³ and the wall motion frequency 20 and 100 Hz.     

Deposition of suspensions in the entrance of a parallel-plate channel with gravity effect

The deposition of aerosols near the inlet region of a horizontal parallel channel (for a distance of 7 channel widths) was investigated by solving numerically the governing equations for both the fluid and particulate phases with boundary layer assumptions. Surface adhesion between the channel walls and the aerosol particles along with the gravitational influence was varied.This analysis took into consideration the simultaneous development of both the fluid and particle phases. The deposition for the present analysis for high surface adhesion was found to be greater for $\text{X ≥ 2}$ than the deposition obtained by Ingham for Plug flow who used only the diffusion equation with zero particulate density at the wall.     

Spin coating of hydrophilic polymeric films for enhanced centrifugal flow control by serial siphoning

In this paper, we implement rotational flow control on a polymeric microfluidic “lab-on-a-disc” platform by combining serial siphoning and capillary valving for sequential release of a set of on-board stored liquid reagents into a common (assay) channel. The functionality of this integrated, multi-step, multi-reagent centrifugal assay platform critically depends on the capability to establish very reproducible, capillary-driven priming of the innately only weakly hydrophilic siphon microchannels made from common poly(methyl methacrylate) (PMMA) substrates. Due to the relatively high contact angle of the native PMMA substrate, it was practically impossible to ensure sequential release of on-board stored reagents using the capillary-driven serial siphon valves. In this work, we demonstrate that spin-coated hydrophilic films of poly(vinyl alcohol) (PVA) and (hydroxypropyl)methyl cellulose (HPMC) provide stable contact angles on PMMA substrates for more than 60 days. The deposited films were characterized using contact angle measurements, surface energy calculations and X-ray photoelectron spectroscopy spectra. The PVA and HPMC films reduced the water contact angle of the PMMA substrate from 68° to 22° and 27° while increasing their surface energies from 47 to 62 and 57 mN m^?1, respectively. On the centrifugal microfluidic platform, the films were validated to enable the effective and reproducible priming of the serial siphon microchannels at low rotational frequencies while ensuring that the in-line capillary valves are not opened until their respective burst frequencies are passed. Furthermore, the biocompatibility of the proposed surface modification method was examined, and the platform was used to run a sandwich immunoassay for the detection of human immunoglobulin G, and its performance was proven to be comparable to dynamic coating using surfactants.     

不同气液流量下喷射器内液滴特性分析

单重态氧是氧碘化学激光的能量来源,其反应生成效率直接影响COIL输出的激光功率和光束质量.单重态氧的反应生成效率取决于气液接触面积,喷射式单重态氧生成器的气液接触面积与喷射器内的液滴分布有关.本文利用商业计算流体力学软件.Fluent 6.3,对喷射式单重态氧发生器内不同气液流量情况下的液滴直径、速度分布规律进行模拟.模拟过程主动流体为水,被动流体为氯气和氦气的混合气体,通过模拟得出了不同气液流量情况下,喷射器内液滴的直径、速度分布情况.在液相流率不变的情况下,随气相流率的增大,喷射器内的液滴平均直径减小,平均速度增大,直径方差和速度方差减小,液滴直径和速度分布趋于集中,且变化趋势随气相流率的增大而减缓;当气相流率不变时,随液相流率的增大.液滴平均直径减小,平均速度增大,直径方差和速度方差都增大,液滴直径和速度分布趋于分散.     

油水气三相流液体流量调节系统研究与设计

针对油水气三相流研究试验平台液体流量调节范围宽、范围度大的问题,提出采用3条主回路与3条旁通回流调节支路来覆盖所有的流量范围,设计了液相系统原理图及管路布置图,对3条旁通回流调节支路中调节阀的计算流量、计算压差及Kv值进行了计算,并对调节阀的开度及实际可调比进行了验算,结果表明选定的调节阀性能满足油水气三相流研究试验的要求。     

Formation of high-purity organic thin films by gas flow deposition and the effect of impurities on device characteristics

A gas flow deposition (GFD) system was developed to manufacture large-scale organic light-emitting diodes (OLEDs). A N,N′-di(1-naphthyl)-N,N′-diphenylbenzidine (α-NPD) thin film with a high purity of 99.97% was obtained using the GFD system. The film properties such as morphology, and electrical and optical characteristics were almost the same as those of films made by conventional vacuum thermal evaporation.     

Broadening of neutral analyte band in electroosmotic flow through slit channel with different zeta potentials of the walls

The study is concerned with addressing hydrodynamic dispersion of an electroneutral non-adsorbed solute being transported by electroosmotic flow through a slit channel formed by walls with different zeta potentials. The analysis is conducted in terms of the plate height which, using the Van Deemter equation, can be expressed through the cross-sectional mean flow velocity, the solute molecular diffusion coefficient and a length scale parameter having meaning of the minimum achievable plate height and depending on the velocity distribution within the channel cross-section. The minimum plate height is determined by substituting distribution of electroosmotic velocity into the preliminary derived integral expression that is valid for any given velocity distribution within a slit channel cross-section. The electroosmotic velocity distribution within the slit channel cross-section is obtained by solving one-dimensional version of the Stokes equation accounting for electric force exerted on the local equilibrium electric space charge. The major obtained result is an analytical expression which represents the minimum plate height normalized by half of channel width as a function of two dimensionless parameters, namely, half of channel width normalized by the Debye length, and the ratio of the wall zeta potentials. The obtained result reveals a substantial increase in the minimum plate height compared with the case of equal wall zeta potentials. Different limiting cases of the obtained relationships are analyzed and possible applications are discussed.     

Stability of the nonparallel developing flow in a channel

A theoretical investigation into the linear, spatial instability of the developing flow in a twodimensional channel, incorporating the effects of non-parallelism of the main flow, has been made at several axial locations. It is found that the critical Reynolds number, frequency and wavenumber decrease with increasing axial distance in the entry region and approach asymptotically the corresponding values for the fully developed flow. From all the growth rates considered herein the critical Reynolds number obtained from the growth rate of the stream function at the centre line of the channel is the minimum. In comparison to this value, the parallel flow theory overpredicts the critical Reynolds number by 22.8 percent at $\text{X}\text{?}\text{= 0.001}$, reducing to 8.3 percent at $\text{X}\text{?}\text{= 0.008}$. The neutral curves based on growth rates of different flow quantities tend to merge into the neutral curve for the parallel flow theory as $\text{X}\text{?}$ increases, thereby implying that the nonparallel effects vanish for large $\text{X}\text{?}$ as they should.     

Controlling electroosmotic flow by polymer coating: a dissipative particle dynamics study

We have performed dissipative particle dynamics (DPDs) simulations of electroosmotic flow (EOF) through a polymer-grafted nanopore. In this model, charged particles including salt ions and counterions are not included explicitly, and EOF is created using an effective boundary condition. The screening effect of polymer layer on EOF is investigated in detail under different solvent qualities and boundary electroosmotic velocities. Results show that the solvent quality has a significant effect on the conformational properties of polymer chains and the flow characteristics of the solvent. The polymer layer undergoes a collapsed transition when decreasing the solvent quality from good to poor. Under different solvent qualities, enhancing the EOF leads to a different variation tendency of the layer thickness. The solvent-induced permeability change is inconsistent with the steady velocity away from the surface. The minimum value of the solvent permeability occurs at an intermediate solvent quality. However, the layer thickness drops gradually to a smallest value (corresponding to the largest effective pore radius) in the poor solvent condition. It is also found that the polymer inclination and stretching length exhibit a complex behavior under the combined effect of solvent quality and electroosmosis-induced shear.     

Calculation of a parallel-plate waveguide with a chiral insert by the mixed finite element method

A parallel-plate waveguide with metallic boundaries, containing an insert of a chiral substance, is considered. The field distribution inside the insert is studied when the system is excited by the guide’s normal wave incident on the insert. The problem is considered in the full vector formulation. The waveguide is calculated by the mixed finite element method, which makes it possible to avoid spurious modes (so-called spirits).     

Experimental and numerical study of the recirculation flow inside a liquid meniscus focused by air

The internal motions inside a liquid meniscus in the so-called liquid cone-jet mode, which can occur upon stimulation by a coflowing gas sheath in flow focusing, are explored by both numerical simulation and experimental visualization. The results for low viscosity liquids show that, as in previous numerical simulations, a recirculating cell inside the meniscus appears when the injected liquid flow rate is reduced. Thus, as the flow rate is reduced not only the average residence time of particles in the meniscus becomes longer, but the appearance of a recirculation cell provides a natural platform for the efficient micro-mixing of different species before they are ejected through the issuing jet. The numerical results were confirmed with experimental visualization of the flow inside the meniscus using a dyed liquid. However, when the viscosity of the liquid is increased the recirculating cell disappears. In this case, viscous stresses organize the streamlines and direct the flow to the meniscus tip, which prevents the recirculating cell from being formed even for very small injected rate of liquid flow.     

The dynamic stability of the flow in a meander channel

The meander channel is one of the most common channel patterns in nature.The characteristics of the flow and sediment in a meander channel which have significant effect on the development of watercourse are important subjects in river dynamics.The transition of the flow patterns in a meander channel concerns with the development mode of the channel pattern and the river regime including the generation conditions of the three-dimensional coherent vortex and secondary flow,the hierarchical scale of coherent v...     

Free-surface flow simulations in the presence of inclined walls

Difficulties associated with free-surface finite element flow simulations are related to (a) nonlinear and advective nature of most hydrodynamic flows, (b) requirements for compatibility between velocity and pressure interpolation, (c) maintaining a valid computational mesh in the presence of moving boundaries, and (d) enforcement of the kinematic conditions at the free surface. Focusing on the last issue, we present an extension of the free-surface elevation equation to cases where the prescribed direction of the surface node motion is not uniformly vertical. The resulting hyperbolic generalized elevation equation is discretized using a Galerkin/least-squares formulation applied on the surface mesh. The elevation field so obtained is then used to impose displacement boundary conditions on the elastic mesh update scheme that governs the movement of interior mesh nodes. The proposed method is used to solve a two-dimensional problem of sloshing in a trapezoidal tank, and a three-dimensional application involving flow in a trapezoidal channel with bridge supports.     

Heat transfer effects on carbon nanotubes suspended nanofluid flow in a channel with non-parallel walls under the effect of velocity slip boundary condition: a numerical study

The present article is dedicated to analyze the flow and heat transfer of carbon nanotube (CNT)-based nanofluids under the effects of velocity slip in a channel with non-parallel walls. Water is taken as a base fluid, and two forms of CNTs are used to perform the analysis, namely the single- and multi-walled carbon nanotubes (SWCNTs and MWCNTs, respectively). Both the cases of narrowing and widening channel are discussed. The equations governing the flow are obtained by using an appropriate similarity transform. Numerical solution is obtained by using a well-known algorithm called Runge–Kutta–Fehlberg method. The influence of involved parameters on dimensionless velocity and temperature profiles is displayed graphically coupled with comprehensive discussions. Also, to verify the numerical results, a comparative analysis is carried out that ensures the authenticity of the results. Variation of skin friction coefficient and the rate of heat transfer at the walls are also performed. Some already existing solutions of the particular cases of the same problem are also verified as the special cases of the solutions obtained here.

     

一种新型明渠流量计设计

作为环境保护的一个重要方面,排放污水的准确计量一直存在着成本高、精度低的问题.自主研发、改进了一种板式流量智能传感器,介绍了其系统的原理,硬件设计及软件结构.通过实验室巴歇尔槽的流量检测实验测得的数据进行比较,得到改进的板式流量计的平均误差为1.76％,大大减小了改进前的误差,实现了高精度、低成本的要求,具有广阔的市场应用前景.     

Thermocapillary convection and interface deformation in a liquid film within a micro-slot with structured walls

Thermocapillary convection in a thin liquid film inside a micro-slot with structured walls kept at different temperatures is studied. The liquid film is wetting the substrate wall and is separated from the cover wall by a gas layer. If the slot walls are structured, the temperature at the liquid–gas interface is non-uniform. The temperature variation induces thermocapillary stresses which bring the liquid into motion and lead to the interface deformation. We investigate the film flow inside the micro-slot, the heat transfer, the liquid–gas interface deformations and the film stability in the framework of the long-wave theory. We show that the amplitude of the interface deformation increases with increasing of the wall structure period. We demonstrate that the structured walls lead to the heat transfer enhancement, which effect is for the studied range of parameters stronger if the cover wall is structured. We also show that the wall structure enhances the long-wave Marangoni instability. The destabilizing effect of the substrate structure is stronger than that of the cover wall structure. This work has been originally presented at the 3rd International Conference on Microchannels and Minichannels, 13–15 June 2005, Toronto, Canada.     

Modelling compressible gas flow near the nozzle of a gas atomiser using a new unified model

In this paper, a unified numerical model is used to simulate the compressible gas flow, during the process of atomisation of liquid, near the atomiser nozzle in gas-only case studies. The flow of the under-expanded gas jets is studied, by analyzing the pressure field, density field and the spatial distribution of Mach number of the gas. The simulated predictions of gas status at the nozzle exit, the radial profile of Pitot pressure, aspiration pressure, and the spatial distribution of the density gradient, are compared with relevant experimental results and an analytical correlation, in order to validate and verify the application of this unified model in the numerical simulation of the gas dynamic behavior during gas atomisation. The simulation results show that the compressible gas flow near the nozzle of a discrete jet atomiser is different from that in a typical annular slit atomiser. Unlike existing models, this new formulation has the potential to be used in future to simulate the simultaneous flow of compressible gas and a weakly compressible liquid metal stream.