Molecular dynamics and continuum simulations of fluid flows with slip boundary conditions
Niavaranikheiri, Anoosheh
Microfluidics
Fluid dynamics
Mechanical engineering
Physics
Thesis Ph. D. Michigan State University. Mechanical Engineering 2011.
Microfluidics is a rapidly developing field with applicationsranging from molecular biology, environmental monitoring, andclinical diagnostics. Microfluidic systems are characterized bylarge surface-to-volume ratios, and, therefore, fluid flows aresignificantly influenced by boundary conditions. The fundamentalassumption in fluid mechanics is the no-slip boundary condition,which states that the tangential fluid velocity is equal to theadjacent wall speed. Although this assumption is successful indescribing fluid flows on macroscopic length scales, recentexperimental and numerical studies have shown that it breaks down atmicroscopic scales due to the possibility of slip of the fluidrelative to the wall. The effect of slip is more pronounced forhighly viscous liquids like polymer melts or in the region near themoving contact line due to the large gradient in shear stress at theliquid/solid interface. The measure of slip is the so-called sliplength, which is defined as a distance between the real interfaceand imaginary plane where the extrapolated velocity profilevanishes. The slip length value is sensitive to several keyparameters, such as surface energy, surface roughness, fluidstructure, and shear rate.In this dissertation, the slip phenomena in thin liquid filmsconfined by either flat or structured surfaces are investigated bymolecular dynamics (MD) and continuum simulations. It is found thatfor flows of both monatomic and polymeric fluids over smoothsurfaces, the slip length depends nonlinearly on shear rate atsufficiently high rates. The laminar flow away from a curvedboundary is usually described by means of the effective slip length,which is defined with respect to the mean roughness height. MDsimulations show that for corrugated surfaces with wavelength largerthan the size of polymer chains, the effective slip length decreasesmonotonically with increasing corrugation amplitude. A detailedcomparison between the solution of the Navier-Stokes equation withthe local rate-dependent slip condition and results of MDsimulations indicates that there is excellent agreement between thevelocity profiles and the effective slip lengths at low shear rateand small-scale surface roughness. It was found that the main causeof the slight discrepancy between MD and continuum results at highshear rates is the reduction of the local slip length in the higherpressure regions where the boundary slope becomes relatively largewith respect to the mainstream flow. It was further shown that forthe Stokes flow with the local no-slip boundary condition, theeffective slip length decreases with increasing corrugationamplitude and a flow circulation is developed in sufficiently deepgrooves. Analysis of a numerical solution of the Navier-Stokesequation with the local slip condition shows that the inertialeffects promote the asymmetric vortex flow formation and reduce theeffective slip length.
Includes bibliographical references (pages 146-155).
Priezjev, Nikolai V
Petty, Charles A
Jaberi, Farhad A
Koochesfahani, Manoochehr M
2011
text
Electronic dissertations
Academic theses
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1 online resource (xiv, 155 pages) : digital, PDF file, color illustrations
etd:923
isbn:9781124583563
isbn:1124583564
umi:3450056
local:Niavaranikheiri_grad.msu_0128D_10274
eng
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