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Knudsen number
About: Knudsen number is a research topic. Over the lifetime, 5052 publications have been published within this topic receiving 104278 citations.
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TL;DR: In this paper, the authors used the molecular dynamics method to simulate shear driven argon gas flows in the early transition and free molecular flow regimes to investigate surface effects as a function of the surface-gas potential strength ratio (ewf/eff).
Abstract: Molecular dynamics (MD) method is used to simulate shear driven argon gas flows in the early transition and free molecular flow regimes to investigate surface effects as a function of the surface–gas potential strength ratio (ewf/eff). Results show a bulk flow region and a near wall region that extends three molecular diameters away from the surfaces. Within the near wall region the velocity, density, and shear stress distributions exhibit deviations from the kinetic theory predictions. Increased ewf/eff results in increased gas density, leading toward monolayer adsorption on surfaces. The near wall velocity profile shows reduced gas slip, and eventually velocity stick with increased ewf/eff. Using MD predicted shear stress values and kinetic theory, tangential momentum accommodation coefficients (TMAC) are calculated as a function of ewf/eff, and TMAC values are shown to be independent of the Knudsen number. Presence of this near wall region breaks down the dynamic similarity between rarefied and nanoscale gas flows.
40 citations
TL;DR: Caravella et al. as discussed by the authors evaluated the influence of strongly adsorbed species on the permeation of weakly adorbed ones and found that the blocking effect causes a higher selectivity at lower temperatures, at which adsorption is relatively stronger.
40 citations
TL;DR: In this article, a numerical solution of the Boltzmann equation with the Bhatnagar-Gross-Krook model is obtained for Poiseuille flow and thermal creep of a rarefied gas between two parallel plates.
Abstract: A numerical solution of the Boltzmann equation with the Bhatnagar‐Gross‐Krook model is obtained for Poiseuille flow and thermal creep of a rarefied gas between two parallel plates. The numerical results of Poiseuille flow are in fair agreement wth the experimental data of Dong and predict the Knudsen minimum in volume flow rate. The present results of the thermal creep approach a thermal transpiration formula analyzed on the basis of the elementary kinetic theory of gases as the density of the gas decreases and agree well with Maxwell's equation continuum theory in the range of low Knudsen number. The effect of the thermal creep on the velocity profile of Poiseuille flow is very large, especially at the Knudsen layer near the plate.
40 citations
TL;DR: In this article, a three-dimensional lattice Boltzmann method based simulation of a microduct has been undertaken to understand the different physical phenomena occurring at these small scales and to investigate when the flow can be treated as two-dimensional.
Abstract: Three-dimensional lattice Boltzmann method based simulations of a microduct have been undertaken in this paper. The objectives are to understand the different physical phenomena occurring at these small scales and to investigate when the flow can be treated as two dimensional. Toward this end, the Knudsen number and aspect ratio (depth to width ratio) are varied for a fixed pressure ratio. The pressure in the microduct is nonlinear with the nonlinearity in pressure reducing with an increase in the Knudsen number. The pressure behaves somewhat similar to two-dimensional microchannels, even when the aspect ratio is unity. The slip velocity at the impenetrable wall has two components: along and perpendicular to the primary flow direction. Our results show that the streamwise velocity near the centerline is relatively invariant along the depth for an aspect ratio of more than three, suggesting that the microduct can be modeled as a two-dimensional microchannel. On the other hand, the velocity component along the depth is never identically zero, implying that the flow is not truly two dimensional, although for practical purposes a two-dimensional treatment might suffice. A curious change in the vector direction in a plane normal to the flow direction is observed around an aspect ratio of four. These three-dimensional results are significant because they will help in theoretical development and flow modeling at microscales.
40 citations
TL;DR: In this paper, a combination of molecular dynamics simulations, theoretical predictions and previous experiments are used in a two-part study to determine the role of the Knudsen layer in rapid granular flows.
Abstract: A combination of molecular dynamics simulations, theoretical predictions and previous experiments are used in a two-part study to determine the role of the Knudsen layer in rapid granular flows. First, a robust criterion for the identification of the thickness of the Knudsen layer is established: a rapid deterioration in Navier–Stokes order prediction of the heat flux is found to occur in the Knudsen layer. For (experimental) systems in which heat flux measurements are not easily obtained, a rule-of-thumb for estimating the Knudsen layer thickness follows, namely that such effects are evident within 2.5 (local) mean free paths of a given boundary. Secondly, comparisons of simulation and experimental data with Navier–Stokes order theory are used to provide a measure as to when Knudsen-layer effects become non-negligible. Specifically, predictions that do not account for the presence of a Knudsen layer appear reliable for Knudsen layers collectively composing up to 20% of the domain, whereas deterioration of such predictions becomes apparent when the domain is fully comprised of the Knudsen layer.
40 citations