Bulletin of the American Physical Society
69th Annual Meeting of the APS Division of Fluid Dynamics
Volume 61, Number 20
Sunday–Tuesday, November 20–22, 2016; Portland, Oregon
Session E38: Flow Instability: Thermal |
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Chair: Lou Kondic, New Jersey Institute of Technology Room: Portland Ballroom 255 |
Sunday, November 20, 2016 5:37PM - 5:50PM |
E38.00001: Nucleation type instabilities in partially wetting nanoscale nematic liquid films Michael Lam, Linda Cummings, Lou Kondic Nucleation type instabilities are studied in nematic liquid crystal (NLC) films with thicknesses less than a micrometer. Within the framework of the long wave approximation, a 4th order nonlinear partial differential equation is proposed for the free surface height. Unlike simple fluids, NLC molecules have a dipole moment which induces an elastic response due to deformation in the bulk of the fluid. The model includes the balance between the bulk elasticity energy and the anchoring (boundary) energy at the substrate and free surface, and van der Waals' intermolecular forces, by means of a structural disjoining pressure. In this presentation, we focus on two-dimensional flow and present simulation results for a flat film with a localized perturbation. We are interested in the morphology of the dewetted film as a function of the initial film thickness. We will show that there exists a range of film thicknesses within the linearly unstable flat film regime where stability analysis does not explain the morphology of the dewetted film. Marginal stability criterion (MSC) is used to derive an analytical expression for the velocity at which a perturbation propagates into the unstable flat film. Finally, we discuss the degree to which MSC can be used to explain the observed morphology. [Preview Abstract] |
Sunday, November 20, 2016 5:50PM - 6:03PM |
E38.00002: High-frequency vibration of heated liquid layer covered by insoluble surfactant Alexander Mikishev, Alexander Nepomnyashchy We study the influence of high-frequency vertical vibration on thin liquid layer with insoluble surfactant adsorbed on the free surface. The layer is subjected to a vertical temperature gradient (the layer is heated either from below or from above). We perform the linear analysis of Marangoni instability. The system is characterized by monotonic and oscillatory modes. The characteristic frequency of long surfactant-induced Marangoni waves is O($\varepsilon^{\mathrm{2}})$, where $\varepsilon $ is the scale of wavenumber, hence the frequency of external vibrations of order one can be considered as a high frequency. The threshold of the onset of Marangoni convection is shifted by the vibration. Applying a long-wave approach we obtain a system of weakly nonlinear equations describing dynamics near the threshold. The standard Floquet method helps to investigate the excitation of short scale Marangoni waves. [Preview Abstract] |
Sunday, November 20, 2016 6:03PM - 6:16PM |
E38.00003: Instabilities of manometric fluid films on a thermally conductive substrate Lou Kondic, Nanyi Dong We consider thin fluid films placed on thermally conductive substrates and exposed to time-dependent spatially uniform heat source. The evolution of the films is considered within the long-wave framework in the regime such that both fluid/substrate interaction, modeled via disjoining pressure, and Marangoni forces, are relevant. The main finding is that when self-consistent computation of the temperature field is carried out, a complex interplay of different instability mechanisms results. This includes either monotonous or oscillatory dynamics of the free surface. In particular, we find that the oscillatory behavior is absent if the film temperature is assumed to be slaved to the current value of the film thickness. The results are discussed within the context of liquid metal films, but are of relevance to dynamics of any thin film involving variable temperature of the free surface, such that the temperature and the film interface itself evolve on comparable time scales. [Preview Abstract] |
Sunday, November 20, 2016 6:16PM - 6:29PM |
E38.00004: Evaporative suppression of Rayleigh-Taylor instability in pure and binary mixtures Ranga Narayanan, Dipin Pillai The classic configuration of an interface between a liquid lying above its vapor is well-known to be unstable due to the Rayleigh-Taylor instability. We study this heavy-over-light configuration in the presence of evaporation. For this, a model configuration of a liquid lying above its vapor confined between two flat plates is chosen. The system is heated from the vapor side by maintaining the temperature of the plate in contact with the vapor higher than that in contact with the liquid. A weighted residual-integral boundary layer model is developed for this system. We show that strong evaporation can linearly stabilize Rayleigh-Taylor instability. Interestingly, when evaporation is weak and the system is linearly unstable, it can still evolve nonlinearly to a steady interface configuration that sustains a stable layer of vapor. In the presence of weak evaporation, the interface reaches near the bottom plate. Under such conditions, the system exhibits features of a pure Rayleigh-Taylor. Inertia is shown to slow down the rate at which the interface reaches the steady state. It also results in non-monotonic evolution of the interface. The study is extended to the case of a binary mixture with solutal Marangoni taken into account. [Preview Abstract] |
Sunday, November 20, 2016 6:29PM - 6:42PM |
E38.00005: Nonlinear dynamics and three-dimensional stable patterns in Rayleigh-Taylor unstable condensing liquid layers: improved one- and 1.5-sided models Tao WEI, Fei DUAN Three-dimensional patterns of condensing layers suspended on cooled substrates are investigated with an introduced vapor boundary layer (VBL), to which the changes in gas composition and temperature are confined. The interfacial vapor transport equation incorporates convective and diffusive fluxes, coupled with a long-wave evolution equation for interface location. Our work extends that of Kanatani [1] on sessile evaporating films to Rayleigh-Taylor unstable condensing layers with nonlinear theory and involves effects of vapor recoil, gravity combined with buoyancy in condensate and heat flux in VBL. The general framework is termed as 1.5-sided model, which can reduce to a one-sided model. An extended 1.5-sided basic state is proposed, whose stability is investigated with linear stability analysis and nonlinear simulation. With one-sided model, finite-time rupture is found, suggesting that destabilizations of vapor recoil and negative gravity combined with buoyancy prevail over stabilizations of mass gain, thermocapillarity and surface tension. This is in sharp contrast to 1.5-sided model, in which such a layer can be stabilized by convection and diffusion of vapor on interface, and rupture is suppressed as stable pattern even with induced destabilization of thermocapillarity. [Preview Abstract] |
Sunday, November 20, 2016 6:42PM - 6:55PM |
E38.00006: Oscillatory instability of a self-rewetting film driven by thermal modulation William Batson, Yehuda Agnon, Alex Oron Here we consider the self-rewetting fluids (SRWFs) that exhibit a well-defined minimum surface tension with respect to temperature, in contrast to those where surface tension decreases linearly. Utilization of SRWFs has grown significantly in the past decade, due to observations that heat transfer is enhanced in applications such as film boiling and pulsating heat pipes. With similar applications in mind, we investigate the dynamics of a thin SRWF film which is subjected to a temperature modulation in the bounding gas. A model is developed within the framework of the long-wave approximation, and a time-averaged thermocapillary driving force for destabilization is uncovered for SRWFs that results from the nonlinear surface tension. Linear analysis of the nonlinear PDE for the film thickness is used to determine the critical conditions at which this driving force destabilizes the film, and, numerical integration of this evolution equation reveals that linearly unstable perturbations saturate to regular periodic solutions (when the modulational frequency is set properly). Properties of these flows such as bifurcation and long-domain flows, where multiple unstable linear modes interact, will also be discussed. [Preview Abstract] |
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