Browsing by Subject "Spatio-temporal instability"

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Open Access
Spatio-temporal evolution of evaporating liquid films sheared by a gas
(2019-11) Mohamed, Omair A. A.
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.
Open Access
Spatiotemporal instability of femtosecond pulses in graded-index multimode fibers
(Institute of Electrical and Electronics Engineers Inc., 2017) Teǧin, U.; Ortaç, B.
We study the spatiotemporal instability generated by a universal unstable attractor in normal dispersion graded-index multimode fiber for femtosecond pulses for the first time. Experimentally observed spatiotemporal instability sidebands are 91-THz detuned from the pump wavelength of 800 nm. Detailed analysis carried out numerically by employing coupled-mode pulse propagation model. Numerically obtained results are well-aligned with experimental observations. Spatial evolution of the total field and spatiotemporal instability sidebands is calculated numerically, and for the input pulses of 200-fs duration, formation and evolution of spatiotemporal instability are shown in both spatial and temporal domains. Our results present the unique features of spatiotemporal instability, such as remarkable frequency shift with inherited beam shape of instability sidebands.