Spatio-temporal evolution of evaporating liquid films sheared by a gas
Author
Mohamed, Omair A. A.
Advisor
Biancofiore, Luca
Date
2019-11Publisher
Bilkent University
Language
English
Type
ThesisItem Usage Stats
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Abstract
The main purpose of this work is the investigation of the spatio-temporal characteristics
of an evaporating liquid film under the in
uence of inertia, hydrostatic
pressure, thermocapillary effects, vapor recoil, and shear stress imparted by a
gas. The effects of the shearing gas are included via the introduction of a constant
shear agent quantity modeling the effect of a constant shear stress applied
along the liquid interface. Subsequently, long wave theory is used to derive an
interface evolution equation accounting for all the previous effects which then is
used to analyze the linear stability characteristics of the film for different parameter
combinations. First a temporal analysis is performed to determine the
stable/unstable parameter sets, followed by spatio-temporal analysis to differentiate
the absolute/convective stability domains. It is demonstrated that the shear
agent acts as a modifier to the base
ow's existing inertia and therefore doesn't
change perturbation growth rates in a stationary base
ow, however it does have a
strong effect on the phase speed. Therefore it can cause convective/absolute transitions
of thermal instabilities. As for its effect on inertial instabilities, namely
the H-mode, positive values of the shear agent promote its growth, while negative
ones suppress it, to the point of completely eliminating it for sufficiently negative
values. As for the effects of evaporation it is found that the reduction in film
height due to evaporation suppresses the advection of perturbations through the
film and therefore promotes absolute instabilities.
In order to investigate the non-linear evolution of the interface, the evolution
equation is solved numerically. Initially, the interface evolution is simulated for
infinitesimal perturbations over a periodic domain for the purposes of validation
by comparison to the linear temporal stability results, and also to existing literature.
Once the numerical procedure is validated, the non-linear evolution of
the interface is studied. Finally, the shear gas's effect on film rupture location
nd time are studied where it is found that the shear agent can strongly affect
rupture location and time, but doesn't change the self-similar rupture mechanics
as the minimum film height approaches zero.