scholarly journals Planetesimals in rarefied gas: wind erosion in slip flow

2020 ◽  
Vol 493 (4) ◽  
pp. 5456-5463 ◽  
Author(s):  
Tunahan Demirci ◽  
Niclas Schneider ◽  
Tobias Steinpilz ◽  
Tabea Bogdan ◽  
Jens Teiser ◽  
...  
Keyword(s):  
Shear Stress ◽  
Wind Erosion ◽  
Rarefied Gas ◽  
Ambient Pressure ◽  
Slip Flow ◽  
Critical Shear Stress ◽  
Mean Free Path ◽  
The Mean ◽  
Gas Molecules ◽  
First Time

ABSTRACT A planetesimal moves through the gas of its protoplanetary disc where it experiences a head wind. Though the ambient pressure is low, this wind can erode and ultimately destroy the planetesimal if the flow is strong enough. For the first time, we observe wind erosion in ground-based and microgravity experiments at pressures relevant in protoplanetary discs, i.e. down to $10^{-1}\, \rm mbar$. We find that the required shear stress for erosion depends on the Knudsen number related to the grains at the surface. The critical shear stress to initiate erosion increases as particles become comparable to or larger than the mean free path of the gas molecules. This makes pebble pile planetesimals more stable at lower pressure. However, it does not save them as the experiments also show that the critical shear stress to initiate erosion is very low for sub-millimetre-sized grains.

Establishment of the Wake Behind a Disk

1964 ◽  
Vol 86 (4) ◽  
pp. 869-880 ◽  
Author(s):  
Thomas Carmody
Keyword(s):  
Shear Stress ◽  
Ambient Pressure ◽  
Continuous Distribution ◽  
Flow Characteristics ◽  
Graphical Form ◽  
Stress Distributions ◽  
Mean Velocity ◽  
Pressure Distributions ◽  
Energy Relationships ◽  
The Mean

An air-tunnel study of the establishment of the wake behind a disk at a Reynolds number of approximately 7 × 104 was undertaken. On the basis of the measured data, such a wake is fully established, that is, similarity profiles of the flow characteristics are formed, within 15 diameters of the disk, and approximately 95 percent of the transfer of energy from the mean motion to the turbulence motion takes place within 3 diameters of the disk, in the region of the mean standing eddy. The measured mean ambient-pressure and mean total-pressure distributions, mean velocity distributions, turbulence-intensity and shear-stress distributions, and the mean streamline pattern are presented in graphical form, as are the quantitative balances of the integrated momentum and mean-energy relationships. A stream function consisting of a continuous distribution of doublets is introduced to extend the radial limit of understanding of the flow characteristics to a very large if not infinite radius. Considerable attention is given to the problem of obtaining and interpreting turbulence shear-stress data immediately downstream from the point of flow separation. The applicability of a local diffusion coefficient or virtual viscosity of the Boussinesq or Prandtl type for relating the turbulence shear stress to the radial gradient of mean axial velocity is discussed. The Bernoulli sum and the energy changes along individual streamlines investigated in an associated study are incorporated herein to obtain a quantitative estimate of the local errors involved in the turbulence-shear-stress measurements.

Effects of Wall Slip on Convective Heat Transfers of Giesekus Fluid in Microannulus

2020 ◽  
Vol 142 (8) ◽  
Author(s):  
Mehdi Moayed Mohseni ◽  
Gilles Tissot ◽  
Michael Badawi
Keyword(s):  
Heat Transfer ◽  
Shear Stress ◽  
Convective Heat ◽  
Slip Flow ◽  
Critical Shear Stress ◽  
Wall Slip ◽  
Outer Wall ◽  
Bulk Temperature ◽  
Brinkman Number ◽  
Thermal Boundary Conditions

Abstract Convective heat transfer and effect of nonlinear wall slip are studied analytically in concentric microannulus for viscoelastic fluids obeying the Giesekus constitutive equation. Laminar, thermally, and hydrodynamically fully developed flow is considered. A nonlinear Navier model with nonzero slip critical shear stress is employed for the slip equation at both walls. Critical shear stress will cause three slip flow regimes: no slip condition, slip only at the inner wall, and slip at both walls. Thermal boundary conditions are assumed to be peripherally and axially constant fluxes at the walls. Governing equations are solved to obtain temperature profiles and Nusselt number and effects of slip parameters, elasticity, and Brinkman number are discussed. Two regimes are compared when slip occurs at both walls or only at the inner wall. The results indicate that by increasing slip effect and elasticity, heat transfer between wall and fluid is enhanced, but it decreases by increasing Brinkman number. In the case where the heat flux is dominant in the outer wall, the inner wall Nusselt curve shows a singularity for a critical Brinkman number because at this Brinkman number the bulk temperature will be equal to the wall temperature.

Poiseuille Flow and Thermal Transpiration of a Rarefied Gas Between Parallel Plates: Effect of Nonuniform Surface Properties of the Plates in the Transverse Direction

2015 ◽  
Vol 137 (10) ◽  
Author(s):  
Toshiyuki Doi
Keyword(s):  
Distribution Function ◽  
Flow Rate ◽  
Poiseuille Flow ◽  
Transverse Direction ◽  
Rarefied Gas ◽  
Mean Free Path ◽  
Accommodation Coefficient ◽  
Parallel Plates ◽  
Nonuniform Surface ◽  
The Mean

Poiseuille flow and thermal transpiration of a rarefied gas between parallel plates with nonuniform surface properties in the transverse direction are studied based on kinetic theory. We considered a simplified model in which one wall is a diffuse reflection boundary and the other wall is a Maxwell-type boundary on which the accommodation coefficient varies periodically and smoothly in the transverse direction. The spatially two-dimensional (2D) problem in the cross section is studied numerically based on the linearized Bhatnagar–Gross–Krook–Welander (BGKW) model of the Boltzmann equation. The flow behavior, i.e., the macroscopic flow velocity and the mass flow rate of the gas as well as the velocity distribution function, is studied over a wide range of the mean free path of the gas and the parameters of the distribution of the accommodation coefficient. The mass flow rate of the gas is approximated by a simple formula consisting of the data of the spatially one-dimensional (1D) problems. When the mean free path is large, the distribution function assumes a wavy variation in the molecular velocity space due to the effect of a nonuniform surface property of the plate.

scholarly journals Maximum Possible Colling Rate in Ultrafast Chip Nanocalorimetry: Fundamental Limitations Due to Thermal Resistance at the Membrane/Gas Interface

2021 ◽  
Author(s):  
Alexander A. Minakov ◽  
Christoph Schick
Keyword(s):  
Thermal Resistance ◽  
Cooling Rate ◽  
Materials Science ◽  
Mean Free Path ◽  
Heat Conducting ◽  
Controlled Cooling ◽  
Fundamental Limitations ◽  
The Mean ◽  
Gas Molecules ◽  
Hot Zone

Ultrafast chip nanocalorimetry opens up remarkable possibilities in materials science by allowing samples to be cooled and heated at extremely high rates. Due to heat transfer limitations, controlled ultrafast cooling and heating can only be achieved for tiny samples in calorimeters with a micron-thick membrane. Even if ultrafast heating can be controlled under quasi-adiabatic conditions, ultrafast controlled cooling can be performed if the calorimetric cell is located in a heat-conducting gas. It was found that the maximum possible cooling rate increases as 1/r0 with decreasing radius r0 of the hot zone of the membrane. The possibility of increasing the maximum cooling rate with decreasing r0 was successfully implemented in many experiments. In this regard, it is interesting to answer the question: what is the maximum possible cooling rate in such experiments if r0 tends to zero? Indeed, on submicron scales, the mean free path of gas molecules lmfp becomes comparable to r0, and the temperature jump that exists at the membrane/gas interface becomes significant. Considering the limitation associated with thermal resistance at the membrane/gas interface and considering the transfer of heat through the membrane, we show that the controlled cooling rate can reach billions of K/s, up to 1010 K/s.

Applicability of the Wright Polynomial Correction for Time-Resolved Laser-Induced Incandescence in the Transition Regime

2012 ◽  
Author(s):  
K. J. Daun ◽  
S. C. Huberman
Keyword(s):  
Heat Conduction ◽  
Layer Thickness ◽  
Boundary Layer Thickness ◽  
Mean Free Path ◽  
Knudsen Layer ◽  
Transition Regime ◽  
Time Resolved ◽  
Laser Induced Incandescence ◽  
The Mean ◽  
Gas Molecules

Sizing aerosolized nanoparticles through time-resolved laser-induced incandescence (TiRe-LII) requires an accurate model of the heat conduction from the laser-energized particle to the surrounding gas. Under transition regime conditions this is often done using Fuchs’ boundary-sphere method, which requires the analyst to specify the thickness of a collisionless layer surrounding the particle, representing the Knudsen layer. Traditionally the boundary layer thickness is set to the mean free path of the gas at the boundary temperature, but recently some TiRe-LII practitioners have adopted a more complex treatment that accounts for particle curvature and directional distribution of gas molecules. This paper presents a critical reassessment of this approach; while this modification is more representative of the true Knudsen layer thickness, it does not improve the accuracy of heat conduction rates estimated using Fuchs’ boundary sphere methods under conditions prevailing in most TiRe-LII experiments.

scholarly journals The role of galaxies and AGN in reionizing the IGM – III. IGM–galaxy cross-correlations at z ∼ 6 from eight quasar fields with DEIMOS and MUSE

2020 ◽  
Vol 494 (2) ◽  
pp. 1560-1578
Author(s):  
Romain A Meyer ◽  
Koki Kakiichi ◽  
Sarah E I Bosman ◽  
Richard S Ellis ◽  
Nicolas Laporte ◽  
...  
Keyword(s):  
Correlation Function ◽  
Cross Correlation ◽  
Mean Free Path ◽  
Lyman Break Galaxies ◽  
The Galaxy ◽  
Cross Correlations ◽  
The Mean ◽  
Improved Model ◽  
First Time

ABSTRACT We present improved results of the measurement of the correlation between galaxies and the intergalactic medium transmission at the end of reionization. We have gathered a sample of 13 spectroscopically confirmed Lyman-break galaxies (LBGs) and 21 Lyman-α emitters (LAEs) at angular separations 20 arcsec ≲ θ ≲ 10 arcmin (∼0.1–4 pMpc at z ∼ 6) from the sightlines to eight background z ≳ 6 quasars. We report for the first time the detection of an excess of Lyman-α transmission spikes at ∼10–60 cMpc from LAEs (3.2σ) and LBGs (1.9σ). We interpret the data with an improved model of the galaxy–Lyman-α transmission and two-point cross-correlations, which includes the enhanced photoionization due to clustered faint sources, enhanced gas densities around the central bright objects and spatial variations of the mean free path. The observed LAE(LBG)–Lyman-α transmission spike two-point cross-correlation function (2PCCF) constrains the luminosity-averaged escape fraction of all galaxies contributing to reionization to $\langle f_{\rm esc} \rangle _{M_{\rm UV}\lt -12} = 0.14_{-0.05}^{+0.28}\, (0.23_{-0.12}^{+0.46})$. We investigate if the 2PCCF measurement can determine whether bright or faint galaxies are the dominant contributors to reionization. Our results show that a contribution from faint galaxies ($M_{\rm UV} \gt -20 \, (2\sigma)$) is necessary to reproduce the observed 2PCCF and that reionization might be driven by different sub-populations around LBGs and LAEs at z ∼ 6.

DSMC Simulation of Supersonic Gas Flow in Microchannel

2011 ◽  
Author(s):  
Mohamad M. Joneidipour ◽  
Reza Kamali
Keyword(s):  
Gas Flow ◽  
Characteristic Length ◽  
Flow Characteristics ◽  
Mean Free Path ◽  
Incoming Flow ◽  
Characteristic Length Scale ◽  
Flow Pressure ◽  
Supersonic Gas Flow ◽  
The Mean ◽  
Gas Molecules

The present study is concerned with the flow characteristics of a microchannel supersonic gas flow. The direct simulation Monte Carlo (DSMC) method is employed for predicting the density, velocity and temperature distributions. For gas flows in micro systems, the continuum hypothesis, which underpins the Navier-Stokes equations, may be inappropriate. This is because the mean free path of the gas molecules may be comparable to the characteristic length scale of the device. The Knudsen number, Kn, which is the ratio of the mean free path of the gas molecules to the characteristic length scale of the device, is a convenient measure of the degree of rarefaction of the flow. In this paper, the effect of Knudsen number on supersonic microchannel flow characteristics is studied by varying the incoming flow pressure or the microchannel height. In addition, the microchannel height and the incoming flow pressure are varied simultaneously to investigate their effects on the flow characteristics. Meanwhile, the results show that until the diffuse reflection model is used throughout the microchannel, the temperature and the Mach number in the microchannel entrance may not be equal to free-stream values and therefore a discontinuity appear in the flow field.

scholarly journals Experimental and theoretical studies of the behaviour of slow electrons in air. I

1949 ◽  
Vol 196 (1046) ◽  
pp. 402-426 ◽  
Keyword(s):  
Drift Velocity ◽  
Mean Free Path ◽  
Free Electrons ◽  
Direct Measurements ◽  
Electronic Motion ◽  
The Mean ◽  
Gas Molecules ◽  
Slow Electrons ◽  
Experimental And Theoretical Studies ◽  
Unit Pressure

This paper is an account of an experimental investigation of the motions of free electrons in air by the method developed by Townsend. An improved form of apparatus is described with the appropriate theory. The following parameters of the electronic motion were determined as functions of the ratio Z/p of the electric field strength Z to the gas pressure p : Townsend’s energy factor k r the drift velocity W , the mean free path at unit pressure L and the mean proportion n of its energy lost in collisions with gas molecules. The experimental data are given in the form of tables and curves. The drift velocity W is found by a new procedure based on the Hall effect and by comparing the velocities W so obtained with the direct measurements of W by Nielsen & Bradbury it is seen that the velocities of agitation are distributed approximately according to Druyvesteyn’s law when Z/p exceeds 0.5. Bailey’s factor G , which is of importance in ionospheric studies, is obtained from the experimental dependence of η on k r . Theoretical formulae are derived for k r and W in terms of L, G and Z/p . The theory of the new method for measuring W is given in an appendix.

scholarly journals Direct stochastic molecular modelling of transport processes in gases

2021 ◽  
Vol 2056 (1) ◽  
pp. 012003
Author(s):  
V Ya Rudyak ◽  
E V Lezhnev
Keyword(s):  
Thermal Conductivity ◽  
Diffuse Reflection ◽  
Thermal Conductivity Coefficient ◽  
Rarefied Gas ◽  
Transport Coefficients ◽  
High Accuracy ◽  
Transport Processes ◽  
Modeling Transport ◽  
Gas Molecules ◽  
First Time

Abstract The stochastic molecular modeling method (SMM) of transport processes in rarefied gases developed by the authors is systematically discussed in this paper. It is shown that, it is possible to simulate the transport coefficients of rarefied gas with high accuracy, using a relatively small number of molecules. The data of modeling the thermal conductivity coefficient are presented for the first time. The second part of the paper is devoted to the generalization of the SMM method for modeling transport processes in confined conditions. To describe the dynamics of molecules in this case, the splitting of their evolution by processes is used: first, the movement of molecules in the configuration space is simulated, and then their dynamics in the velocity space is imitated. Anisotropy of viscosity and thermal conductivity in nanochannels has been established. The interaction of gas molecules with walls is described by specular or specular-diffuse reflection laws. Gas viscosity can be either greater than in the bulk or less, depending on the law of gas interaction with the channel walls.

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