Low-Speed DSMC Simulations of Hotwire Anemometers at High-Altitude Conditions

Numerical simulations of hotwire anemometers in low-speed, high-altitude conditions have been carried out using the direct simulation Monte Carlo (DSMC) method. Hotwire instruments are commonly used for in-situ turbulence measurements because of their ability to obtain high spatial and temporal resolution data. Fast time responses are achieved by the wires having small diameters (1–5 μm). Hotwire instruments are currently being used to make in-situ measurements of high-altitude turbulence (20–40 km). At these altitudes, hotwires experience Knudsen number values that lie in the transition-regime between slip-flow and free-molecular flow. This article expands the current knowledge of hotwire anemometers by investigating their behavior in the transition-regime. Challenges involved with simulating hotwires at high Knudsen number and low Reynolds number conditions are discussed. The ability of the DSMC method to simulate hotwires from the free-molecular to slip-flow regimes is demonstrated. Dependence of heat transfer on surface accommodation coefficient is explored and discussed. Simulation results of Nusselt number dependence on Reynolds number show good agreement with experimental data. Magnitude discrepancies are attributed to differences between simulation and experimental conditions, while discrepancies in trend are attributed to finite simulation domain size.

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Non-equilibrium effects on flow past a circular cylinder in the slip and early transition regime

Journal of Fluid Mechanics ◽

10.1017/jfm.2018.869 ◽

2018 ◽

Vol 860 ◽

pp. 654-681 ◽

Cited By ~ 8

Author(s):

Xiao-Jun Gu ◽

Robert W. Barber ◽

Benzi John ◽

David R. Emerson

Keyword(s):

Reynolds Number ◽

Circular Cylinder ◽

Kinetic Theory ◽

Knudsen Number ◽

Flow Separation ◽

Velocity Slip ◽

Transition Regime ◽

Flow Past ◽

Non Equilibrium ◽

Early Transition

This paper presents a comprehensive investigation into flow past a circular cylinder where compressibility and rarefaction effects play an important role. The study focuses on steady subsonic flow in the Reynolds-number range 0.1–45. Rarefaction, or non-equilibrium, effects in the slip and early transition regime are accounted for using the method of moments and results are compared to data from kinetic theory obtained from the direct simulation Monte Carlo method. Solutions obtained for incompressible continuum flow serve as a baseline to examine non-equilibrium effects on the flow features. For creeping flow, where the Reynolds number is less than unity, the drag coefficient predicted by the moment equations is in good agreement with kinetic theory for Knudsen numbers less than one. When flow separation occurs, we show that the effects of rarefaction and velocity slip delay flow separation and will reduce the size of the vortices downstream of the cylinder. When the Knudsen number is above 0.028, the vortex length shows an initial increase with the Reynolds number, as observed in the standard no-slip continuum regime. However, once the Reynolds number exceeds a critical value, the size of the downstream vortices decreases with increasing Reynolds number until they disappear. An existence criterion, which identifies the limits for the presence of the vortices, is proposed. The flow physics around the cylinder is further analysed in terms of velocity slip, pressure and skin friction coefficients, which highlights that viscous, rarefaction and compressibility effects all play a complex role. We also show that the local Knudsen number, which indicates the state of the gas around the cylinder, can differ significantly from its free-stream value and it is essential that computational studies of subsonic gas flows in the slip and early transition regime are able to account for these strong non-equilibrium effects.

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Thermal Non-Equilibrium Effects at the Stagnation Wall in the Unsteady Micro-Flow

10.1115/imece2001/htd-24147 ◽

2001 ◽

Author(s):

Jae Hyun Park ◽

Seung Wook Baek

Keyword(s):

Reynolds Number ◽

Knudsen Number ◽

Higher Order ◽

Heat Fluxes ◽

Sinusoidal Oscillation ◽

Dsmc Method ◽

One Dimensional ◽

Micro Flow ◽

Equilibrium Effect ◽

Non Equilibrium

Abstract In the present study, the non-equilibrium effects at the stagnation wall in one-dimensional unsteady micro-flow responding to the external sinusoidal oscillation are investigated. The fluctuating behavior of the rarefied medium is simulated by employing the unsteady DSMC method for various conditions with the acoustic Reynolds number and the Knudsen number. The heat fluxes calculated from Maxwell-Smoluchowski’s relation and its higher order modifications are compared with the direct DSMC result. Consequently, it is found that there exists the non-equilibrium effect at the stagnation wall especially when the number of reflected molecules from the diffuse wall was small.

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Effect of Dimensionless Number (KN and RE) on Performance Index of Concentric Circular Micro Channel

International Journal of Innovative Technology and Exploring Engineering - Special Issue ◽

10.35940/ijitee.j1012.08810s219 ◽

2020 ◽

Vol 8 (10S2) ◽

pp. 41-54

Keyword(s):

Reynolds Number ◽

Heat Exchanger ◽

Knudsen Number ◽

Slip Flow ◽

Dimensionless Number ◽

Micro Channel ◽

Cfd Analysis ◽

Counter Flow ◽

Overall Performance ◽

Transfer Properties

Overall performance of any type of heat channel is largely depends upon Knudsen number and Reynolds number [1]. In present work the outcome of dimensionless number on heat transfer properties of micro channel heat exchanger [2]are investigated. CFD analysis of counter flowmicro channel was performed for both slip and no slip flow using Fluent as CFD code. Based on results obtain, pressure drop increases with increment in Reynolds number and Knudsen number. The effectiveness reduces with increment in Reynolds number. The comparison of thermal and hydrodynamics performance between slip and no slip flow for heat exchanger is investigated. The effectiveness increases with decreasing values of Reynolds number for counter flow. For higher effectiveness of the micro channel heat exchanger, Reynolds number should be less.

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High altitude low Reynolds number effect on the matching performance of a turbofan engine

Proceedings of the Institution of Mechanical Engineers Part G Journal of Aerospace Engineering ◽

10.1177/0954410012437505 ◽

2012 ◽

Vol 227 (3) ◽

pp. 455-466 ◽

Cited By ~ 9

Author(s):

Hai-Long Tang ◽

Min Chen ◽

Dong-hai Jin ◽

Zheng-ping Zou

Keyword(s):

Reynolds Number ◽

High Altitude ◽

Low Reynolds Number ◽

Turbofan Engine ◽

Reynolds Number Effect ◽

Matching Performance ◽

Low Reynolds

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Study of Gas Flow in Micronozzles Using an Unstructured DSMC Method

ASME 2009 7th International Conference on Nanochannels, Microchannels and Minichannels ◽

10.1115/icnmm2009-82180 ◽

2009 ◽

Author(s):

Ehsan Roohi ◽

Masoud Darbandi ◽

Vahid Mirjalili

Keyword(s):

Boundary Layer ◽

Knudsen Number ◽

Gas Flow ◽

Subsonic Flow ◽

Flow Behavior ◽

Boundary Layer Separation ◽

Back Pressure ◽

Viscous Force ◽

Dsmc Method ◽

Wide Range

The current research uses an unstructured direct simulation Monte Carlo (DSMC) method to numerically investigate supersonic and subsonic flow behavior in micro convergent–divergent nozzle over a wide range of rarefied regimes. The current unstructured DSMC solver has been suitably modified via using uniform distribution of particles, employing proper subcell geometry, and benefiting from an advanced molecular tracking algorithm. Using this solver, we study the effects of back pressure, gas/surface interactions (diffuse/specular reflections), and Knudsen number, on the flow field in micronozzles. We show that high viscous force manifesting in boundary layers prevents supersonic flow formation in the divergent section of nozzles as soon as the Knudsen number increases above a moderate magnitude. In order to accurately simulate subsonic flow at the nozzle outlet, it is necessary to add a buffer zone to the end of nozzle. If we apply the back pressure at the outlet, boundary layer separation is observed and a region of backward flow appears inside the boundary layer while the core region of inviscid flow experiences multiple shock-expansion waves. We also show that the wall boundary layer prevents forming shocks in the divergent part. Alternatively, Mach cores appear at the nozzle center followed by bow shocks and an expansion region.

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Laminar Forced Convection in Annular Microchannels With Slip Flow Regime

ASME 2009 7th International Conference on Nanochannels, Microchannels and Minichannels ◽

10.1115/icnmm2009-82013 ◽

2009 ◽

Cited By ~ 1

Author(s):

Arman Sadeghi ◽

Abolhassan Asgarshamsi ◽

Mohammad Hassan Saidi

Keyword(s):

Nusselt Number ◽

Fluid Flow ◽

Aspect Ratio ◽

Boundary Conditions ◽

Knudsen Number ◽

Gas Flow ◽

Slip Flow ◽

Outer Wall ◽

Graphical Form ◽

Uniform Temperature

Fluid flow and heat transfer at microscale have attracted an important research interest in recent years due to the rapid development of microelectromechanical systems (MEMS). Fluid flow in microdevices has some characteristics which one of them is rarefaction effect related with gas flow. In this research, hydrodynamically and thermally fully developed laminar rarefied gas flow in annular microducts is studied using slip flow boundary conditions. Two different cases of the thermal boundary conditions are considered, namely: uniform temperature at the outer wall and adiabatic inner wall (Case A) and uniform temperature at the inner wall and adiabatic outer wall (Case B). Using the previously obtained velocity distribution, energy conservation equation subjected to relevant boundary conditions is numerically solved using fourth order Runge-Kutta method. The Nusselt number values are presented in graphical form as well as tabular form. It is realized that for the case A increasing aspect ratio results in increasing the Nusselt number, while the opposite is true for the case B. The effect of aspect ratio on Nusselt number is more notable at smaller values of Knudsen number, while its effect becomes slighter at large Knudsen numbers. Also increasing Knudsen number leads to smaller values of Nusselt number for the both cases.

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