On the role of the Knudsen layer in rapid granular flows

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...

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Bibliographic Details
Published in:Journal of fluid mechanics 2007-08, Vol.585, p.73-92
Main Authors: GALVIN, J. E., HRENYA, C. M., WILDMAN, R. D.
Format: Article
Language:English
Subjects:
Cross-disciplinary physics: materials science
rheology
ENGINEERING
Exact sciences and technology
Fluid mechanics
Fluids
GRANULAR MATERIALS
Granular solids
HEAT FLUX
KNUDSEN FLOW
Material form
MEAN FREE PATH
Mechanics
MOLECULAR DYNAMICS METHOD
Physics
Rheology
SOLIDS FLOW
THICKNESS
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Summary: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.
ISSN:0022-1120
1469-7645
DOI:10.1017/S0022112007006489