scholarly journals Modelling corners flow in rectangular microchannel

2021 ◽  
Vol 2119 (1) ◽  
pp. 012114
Author(s):  
D S Gluzdov ◽  
E Ya Gatapova
Keyword(s):  
Shear Stress ◽  
Fluid Flow ◽  
Boundary Conditions ◽  
Fluid Velocity ◽  
Rectangular Microchannel ◽  
Cross Sectional ◽  
Rectangular Microchannels ◽  
Lab On Chip ◽  
Slip Boundary ◽  
Wall Boundary Conditions

Abstract Rectangular microchannels are most common configuration in microfluidics. They can be used in many industries, for example in lab-on-chip devices. Despite standard fluid dynamics, microfluidics has a significant impact of wall boundary conditions on fluid flow. And in microfluidics, we cannot simply set no-slip boundary conditions if our goal is accurate modeling results. In rectangular microchannels, there is another important moment in modeling that is not present in circular pipes. The velocity profile of the fluid depends on the shear stress at the edges and the velocities at the walls of the microchannel change at different points of the cross-sectional wall of the microchannel. The fluid velocity is lower at the corners of a rectangular microchannel. In this paper, a solution is proposed to find a more accurate way to model the fluid flow in a rectangular microchannel by knowing the friction factor without shear stress distribution.

MEMS-Scale Turbomachinery Based Vacuum Roughing Pump

2014 ◽  
Vol 136 (10) ◽  
Author(s):  
Anthony J. Gannon ◽  
Garth V. Hobson ◽  
Michael J. Shea ◽  
Christopher S. Clay ◽  
Knox T. Millsaps
Keyword(s):  
Boundary Condition ◽  
Shear Stress ◽  
Wall Shear Stress ◽  
Boundary Conditions ◽  
Knudsen Number ◽  
Slip Flow ◽  
Slip Boundary Condition ◽  
3D Simulations ◽  
Slip Boundary ◽  
Wall Shear

This study forms part of a program to develop a micro-electro-mechanical systems (MEMS) scale turbomachinery based vacuum pump and investigates the roughing portion of such a system. Such a machine would have many radial stages with the exhaust stages operating near atmospheric conditions while the inlet stages operate at near vacuum conditions. In low vacuum such as those to the inlet of a roughing pump, the flow can still be treated as a continuum; however, the no-slip boundary condition is not accurate. The Knudsen number becomes a dominant nondimensional parameter in these machines due to their small size and low pressures. As the Knudsen number increases, slip-flow becomes present at the walls. The study begins with a basic overview on implementing the slip wall boundary condition in a commercial code by specifying the wall shear stress based on the mean-free-path of the gas molecules. This is validated against an available micro-Poiseuille classical solution at Knudsen numbers between 0.001 and 0.1 with reasonable agreement found. The method of specifying the wall shear stress is then applied to a generic MEMS scale roughing pump stage that consists of two stators and a rotor operating at a nominal absolute pressure of 500 Pa. The zero flow case was simulated in all cases as the pump down time for these machines is small due to the small volume being evacuated. Initial transient two-dimensional (2D) simulations are used to evaluate three boundary conditions, classical no-slip, specified-shear, and slip-flow. It is found that the stage pressure rise increased as the flow began to slip at the walls. In addition, it was found that at lower pressures the pure slip boundary condition resulted in very similar predictions to the specified-shear simulations. As the specified-shear simulations are computationally expensive it is reasonable to use slip-flow boundary conditions. This approach was used to perform three-dimensional (3D) simulations of the stage. Again the stage pressure increased when slip-flow was present compared with the classical no-slip boundaries. A characteristic of MEMS scale turbomachinery are the large relative tip gaps requiring 3D simulations. A tip gap sensitivity study was performed and it was found that when no-slip boundaries were present the pressure ratio increased significantly with decreasing tip gap. When slip-flow boundaries were present, this relationship was far weaker.

Impact of gold nanoparticles along with Maxwell velocity and Smoluchowski temperature slip boundary conditions on fluid flow: Sutterby model

2021 ◽  
Author(s):  
Tanveer Sajid ◽  
Wasim Jamshed ◽  
Faisal Shahzad ◽  
Esra Karatas Akgül ◽  
Kottakkaran Sooppy Nisar ◽  
...  
Keyword(s):  
Gold Nanoparticles ◽  
Fluid Flow ◽  
Boundary Conditions ◽  
Slip Boundary Conditions ◽  
Slip Boundary

scholarly journals Slip Velocity Distribution in a Parallel Plate Channel on Asymmetric Thermal Boundary Condition

2016 ◽  
Vol 35 ◽  
pp. 113-126
Author(s):  
Md Tajul Islam
Keyword(s):  
Boundary Conditions ◽  
Velocity Distribution ◽  
Parallel Plate ◽  
Control Volume ◽  
Navier Stokes ◽  
Working Fluid ◽  
Slip Boundary Conditions ◽  
Cross Sectional ◽  
Slip Boundary ◽  
Knudsen Numbers

Steady, laminar and fully developed flows in parallel plate microchannel with asymmetric thermal wall conditions are solved by control volume technique. In order to examine the influence of Reynolds number and Knudsen number on the velocity distributions, a series of simulations are performed for different Reynolds and Knudsen numbers. Nitrogen gas is used as working fluid and we assume the fluid as continuum but employ the slip boundary conditions on the walls. The Navier-Stokes and energy equations are solved simultaneously. The results are found in good agreement with those predicted by analytical solutions in 2D continuous flow model employing first order slip boundary conditions. It is concluded that the rarefaction flattens the velocity distribution. If the product of Reynolds numbers and Knudsen numbers is fixed, the cross sectional average velocity is fixed for incompressible flow.GANIT J. Bangladesh Math. Soc.Vol. 35 (2015) 113-126

Kinematic description of fluids in motion and approximations

2018 ◽  
Author(s):  
Marcel Escudier
Keyword(s):  
Fluid Velocity ◽  
Slip Boundary Condition ◽  
Cross Sectional Area ◽  
Constant Density ◽  
Sectional Area ◽  
Cross Sectional ◽  
Slip Boundary ◽  
Flow Problems ◽  
Mass Flowrate ◽  
Flow Through

In this chapter some of the terminology and simplifications which enable us to begin to describe and analyse practical fluid-flow problems are introduced. The terms ‘fluid particle’ and ‘streamline’ are defined. The principle of conservation of mass applied to steady one-dimensional flow through a streamtube of varying cross-sectional area resulted in the continuity equation. This important equation relates mass flowrate ṁ, volumetric flowrate Q̇, average fluid velocity V̄, fluid density ρ‎, and cross-sectional area A: m = ρ‎ Q̇ = ρ‎AV̅ = constant. For a constant-density fluid this result shows that fluid velocity increases if the cross-sectional area decreases, and vice versa. The no-slip boundary condition, a consequence of which is the boundary layer, is introduced.

Accuracy and grid convergence of wall shear stress measured by lattice Boltzmann method

2014 ◽  
Vol 25 (11) ◽  
pp. 1450057 ◽  
Author(s):  
Xiuying Kang ◽  
Zhiya Dun
Keyword(s):  
Shear Stress ◽  
Relaxation Time ◽  
Wall Shear Stress ◽  
Boundary Conditions ◽  
Lattice Boltzmann ◽  
Velocity Shear ◽  
Wall Boundary ◽  
Wall Boundary Conditions ◽  
Grid Convergence ◽  
Wall Shear

Based on a two-dimensional Poiseuille and Wormersley flow, accuracy and grid convergence of velocity, shear stress and wall shear stress (WSS) measurements were investigated using the single-relaxation-time (SRT) and multiple-relaxation-time (MRT) lattice Boltzmann models under various open and wall boundary conditions. The results showed that grid convergence of shear stress and WSS are not consistent with that of velocity when flow channels are not aligned to the grids, and strongly rely on the used wall boundary conditions. And the MRT model is slightly superior to the SRT when simulating the complicated border flow. Moreover the WSS should be approximately calculated on the fluid nodes closest to walls under the curved boundary (CB) condition but not for the bounce-back (BB) boundary scheme. As applications, distributions of WSS in a wavy-walled channel and distensible carotid artery were simulated which would much more depend on local roughness of vessel intima than channel diameters.

Fluid-Induced Forces in Pump Liquid Seals With Large Aspect Ratio

2011 ◽  
Author(s):  
Alexandrina Untaroiu ◽  
Vahe Hayrapetian ◽  
Costin D. Untaroiu ◽  
Paul E. Allaire ◽  
Houston G. Wood ◽  
...  
Keyword(s):  
Shear Stress ◽  
Fluid Flow ◽  
Aspect Ratio ◽  
Stokes Equations ◽  
Complex Geometry ◽  
Dynamic Properties ◽  
Fluid Velocity ◽  
Bulk Flow ◽  
Large Aspect Ratio ◽  
Aspect Ratios

The instability due to fluid flow in seals is a known phenomenon that can occur in pumps and compressors as well as in steam turbines. Traditional annular seal models are based on bulk flow theory. While these methods are computationally efficient and can predict dynamic properties fairly well for short seals, they lack accuracy in cases of seals with complex geometry or with large aspect ratios (above 1.0). Unlike the bulk flow models, computational fluid dynamics (CFD) makes no simplifying assumption on the seal geometry, shear stress at the wall, relationship between wall shear stress and mean fluid velocity, or characterization of interfaces between control volumes through empirical friction factors. This paper presents a method to calculate the linearized rotor-dynamic coefficients for a liquid seal with large aspect ratio (balance drum) subjected to incompressible turbulent flow by means of a three dimensional CFD analysis to calculate the fluid-induced forces acting on the rotor. The Reynolds-averaged Navier-Stokes equations for fluid flow are solved by dividing the volume of fluid into a discrete number of points at which unknown variables are computed. As a result, all the details of the flow field, including the fluid forces with potential destabilizing effects, are calculated. A 2nd order curve fit is then used to express the fluid-induced forces in terms of equivalent linearized stiffness, damping, and fluid inertia coefficients.

Sign in / Sign up

Export Citation Format

Share Document