Bulletin of the American Physical Society
66th Annual Meeting of the APS Division of Fluid Dynamics
Volume 58, Number 18
Sunday–Tuesday, November 24–26, 2013; Pittsburgh, Pennsylvania
Session L9: Instability: Interfacial and Thin-Film V |
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Chair: Dan Lathrop, University of Maryland Room: 333 |
Monday, November 25, 2013 3:35PM - 3:48PM |
L9.00001: Acoustic Coupling to Kelvin-Helmholtz Instability at a Discontinuity Layer of Zero and Finite Thickness and Viscosity Orlando Ugarte, V'yacheslav Akkerman The analytical formulation of Funada and Joseph [J. Fluid.Mech. 445 (2001) 263] on the Kelvin-Helmholtz (KH) instability developing at a surface separating two fluids is extended to the event of imposed acoustics field by means of the Bychkov method [Phys. Fluids 11 (1999) 3168]. Specifically, acoustical modification, mitigation and stabilization of the KH instability as well as the excitation of the parametric instability by sound waves are considered. The limits for stable/unstable regimes as a function of hydrodynamic and acoustic parameters are determined considering a linear dispersion relation for the perturbed interface. Two interacting modes are of particular interest: resonant and parametric, characterized by their frequency in relation to the disturbance oscillation. We start with an infinitely thin approach of the discontinuity surface, which is subsequently extended to a finite thickness layer (i.e. continuous velocity and density gradients are considered). A parametric study of the influence of surface tension and viscosity to the KH-acoustic coupling and stability limits is also performed. [Preview Abstract] |
Monday, November 25, 2013 3:48PM - 4:01PM |
L9.00002: Modeling air-driven flow of a viscous film coating the interior of a rigid, vertical tube Reed Ogrosky, Roberto Camassa, Greg Forest, Jeffrey Olander The upwards, air-driven flow of a viscous fluid film coating the interior of a rigid, vertical tube is studied theoretically and numerically. The free surface of the film develops instabilities due to the interplay between interfacial stress from the airflow and surface tension from azimuthal curvature. Simple closure models for turbulent airflow coupled to long-wave asymptotic models for the liquid film have been shown to reproduce qualitatively the dynamics of the instabilities past initial transients observed in experiments. However, quantitative agreement requires improving the turbulent airflow modeling beyond leading order theories of free surface stress. An attempt in this direction is described here; the resulting model is compared with others in the literature and with experiments, for the case where the free surface is replaced by a rigid, wavy wall. This comparison is made for both wavy pipe and wavy channel flows, and the mean stress is seen to be out of phase with the wavy wall itself by a phase shift dependent on both the Reynolds number and the amplitude of the wall modulations. The free surface model is then studied through linear stability analysis and numerical solutions, both of which show improved agreement with experiments. [Preview Abstract] |
Monday, November 25, 2013 4:01PM - 4:14PM |
L9.00003: Studying gas-sheared liquid film in horizontal rectangular duct with laser-induced fluorescence technique Andrey Cherdantsev, David Hann, Barry Azzopardi High-speed LIF-technique is applied to study gas-sheared liquid film in horizontal rectangular duct with 161 mm width. Instantaneous distributions of film thickness resolved in both longitudinal and transverse coordinates were obtained with a frequency of 10 kHz and spatial resolution from 0.125 mm to 0.04 mm. Processes of generation of fast and slow ripples by disturbance waves are the same as described in literature for downwards annular pipe flow. Disturbance waves are often localized by transverse coordinate and may have curved or slanted fronts. Fast ripples, covering disturbance waves, are typically horseshoe-shaped and placed in staggered order. Their characteristic transverse size is of order 1 cm and it decreases with gas velocity. Entrainment of liquid from film surface can also be visualized. Mechanisms of ripple disruption, known as ``bag break-up'' and ``ligament break-up,'' were observed. Both mechanisms may occur on the same disturbance waves. Various scenarios of droplet deposition on the liquid film are observed, including the impact, slow sinking and bouncing, characterized by different outcome of secondary droplets or entrapped bubbles. Number and size of bubbles increase greatly inside the disturbance waves. Both quantities increase with gas and liquid flow rates. [Preview Abstract] |
Monday, November 25, 2013 4:14PM - 4:27PM |
L9.00004: Wavy liquid films in interaction with a strongly confined laminar gas flow: Modeling and direct numerical simulations Georg F. Dietze, Christian Ruyer-Quil Different technological settings concern the flow of a wavy liquid film in contact with a strongly confined gas flow. Micro-gaps for instance, which are employed for the cooling of electronic equipment, involve a pressure-driven evaporating liquid film flowing co-currently to its own vapor. In packed columns used for distillation, falling liquid films sheared by a counter-current gas flow occur within narrow channels. Surface waves on the liquid-gas interface of these flows play an important role as they intensify scalar transfer and may cause flooding of the channel. However, their accurate prediction by full numerical simulation is associated with a substantial computational cost. We evaluate an alternative approach based on a low-dimensional integral boundary layer formulation applied to both fluid layers. The resulting model captures the long-wave (Yih and Kapitza) instabilities of the flow accurately and allows calculations on long domains at low computational cost. These evince a number of intricate wave-induced flow structures within the film and gas as well as a possible route to the flooding of narrow channels under counter-current gas flow conditions. Comparisons with direct numerical simulations using the VOF-CSF approach as well as experiments are convincing. [Preview Abstract] |
Monday, November 25, 2013 4:27PM - 4:40PM |
L9.00005: Liquid falling films: linear stability and direct numerical simulation Patrick Schmidt, Lennon O'Naraigh, Prashant Valluri, Mathieu Lucquiaud Interfacial instability of falling liquid films in counter-current contact with a turbulent gas phase is investigated by means of an Orr-Sommerfeld analysis. This study is complemented by a full energy budget analysis, identifying the key mechanisms of the instability. This gives first insight into the dynamic behaviour of the two-phase system, which is relevant for a wide range of technical applications, such as absorption and distillation. The linear stability analysis is also used to identify the operating limits of a counter-current operation i.e. the so-called loading and flooding limits. In addition, the results of this analysis are benchmark for direct numerical simulations using the newly launched Two-Phase Level Set (http://sourceforge.net/projects/tpls/) solver. High resolution DNS is used to obtain detailed knowledge of important mechanisms at play, especially with regard to interfacial instability and transient system behaviour, which can help to design more efficient mass transfer equipment such as structured packings. [Preview Abstract] |
Monday, November 25, 2013 4:40PM - 4:53PM |
L9.00006: An experimental investigation of fingering instability and growth dynamics in inclined counter-current gas-liquid channel flow Jordan Purvis, Ravi Mistri, Christos Markides, Omar Matar The results of an experimental study involving low Reynolds number, counter-current flows of glycerol and air on an inclined glass substrate inside a rectangular channel are presented. The interface forms a thickened front immediately upstream of a thin, precursor layer region. This front is vulnerable to spanwise perturbations which, under certain conditions, grow to acquire the shape of ``fingers.'' Decreasing the inclination angle has a stabilizing effect on the front: complete stability is achieved below a critical angle whose value depends on the remaining system parameters. Regions of transient finger formation are also observed. It is also found that increasing the ratio of the precursor to the inlet film thickness, and increasing the liquid and air flow-rates also exerts a stabilizing effect on the interface. Analyses of the initial finger growth-rate corroborate the findings of previous theoretical work, showing this growth-rate to be independent of inclination angle and liquid film Reynolds number, and weakly-dependent on the air flow-rate for low inclination angles. [Preview Abstract] |
Monday, November 25, 2013 4:53PM - 5:06PM |
L9.00007: Absolute and convective instabilities in turbulent gas-laminar liquid film flows Rajagopal Vellingiri, Dmitri Tseluiko, Serafim Kalliadasis Gas-liquid flows are important from a fundamental fluid mechanics point of view, but are also central in a variety of engineering applications, such as distillation, absorption and cooling of electronic devices. Our prototypical system for such flows consists of a thin laminar liquid film flowing down an inclined plate in the presence of a countercurrent turbulent gas. The liquid flow is influenced by the gas through the tangential and normal stresses acting at the interface. We develop low-dimensional models for the liquid-flow problem: a long-wave model and a weighted integral-boundary layer (WIBL) model. These models, along with the Orr-Sommerfeld problem derived from the full Navier-Stokes equations and associated boundary conditions are used to explore the linear stability of the liquid-gas system. For a given liquid flow rate, we show that the wave velocity decreases with increasing gas shear before changing direction at the ``flooding point.'' The appearance of this point is linked to the onset of absolute instability, where a localized disturbance gets amplified and contaminates the whole domain. This is also marked by the collision of two spatial branches at a saddle point. We supplement our stability analysis with time-dependent computations of the WIBL model. [Preview Abstract] |
Monday, November 25, 2013 5:06PM - 5:19PM |
L9.00008: Linear stability analysis of thin films in wall bounded shear flow Ahmed Kaffel, Amir Riaz In this study we examine the stability of core annular flow of two fluids with large density and viscosity ratios to investigate the physical mechanisms associated to thin liquid films flow in microgap channels. Emphasis will be placed on predicting and controlling the growth of interfacial instabilities which can lead to the rupture of the thin liquid films encountered in annular flows. A multi-domain Chebyshev collocation spectral method along with QZ eigenvalue solver are used to solve the Orr-Sommerfeld stability equations in both layers. The algorithm is computationally efficient and accurate in reproducing the whole spectrum of the eigenvalues and associated eigenfunctions. The derivation of the asymptotics of these modes shows that the numerical eigenvalues are in agreement with the analytic formula obtained previously by Yih (1967), Orszag (1971), Higgins et al (1988), Dongarra (1996) and Sahu et al (2007). The numerical simulations and experiments are carried out to quantify unstable wave patterns with respect to the underlying fluid dynamic mechanism for various flows rates. We consider the case of isothermal, non-adiabatic, parallel flow of liquid and vapor phases. A parametric study is analyzed and the numerical stability results are presented and will be used later as a tool to validate the direct numerical solver and to identify the physical mechanisms in two-phase liquid vapor flows. [Preview Abstract] |
Monday, November 25, 2013 5:19PM - 5:32PM |
L9.00009: Electrostatic control of flows of moderate Reynolds number Demetrios Papageorgiou, Alex Wray, Omar Matar Film flow down an inclined plane is a widely investigated problem because it serves as an important prototypical situation for analysis. Under the assumption that the characteristic wavelength of coherent structures is long relative to the thickness of the film, this system can be modelled to second order by the boundary-layer equations. However, the perturbative approach, which enslaves the system to the dynamics of the interface, typically results in equations (e.g. the Benney equation) that exhibit finite-time blow-up. It has been shown that a weighted-residual approach gives rise to simple equations which exhibit very good agreement with both direct numerical simulations and experiments in both the drag-gravity and drag-inertia regimes. We extend this system by allowing the plane to serve as an electrode, and incorporating a second parallel plane positioned above the fluid. The variation in the resultant electric fields in each region induces a Maxwell stress at the interface. We validate our model in one dimension via comparisons of linear theory, and by direct numerical simulations of both transient solutions and traveling waves. We then extend this to the two dimensional case to exhibit the degree of control afforded by the electric field. [Preview Abstract] |
Monday, November 25, 2013 5:32PM - 5:45PM |
L9.00010: FRAP in thin film flows Jason Wexler, Ian Jacobi, Howard Stone A new technique is proposed for measuring the velocity field within thin liquid films, which combines Fluorescence Recovery After Photobleaching (FRAP) measurements with two-dimensional Taylor dispersion analysis. FRAP is a technique used largely by biologists to measure the diffusion coefficient of compounds in living cells. A small spot of fluorescent dye is bleached and then monitored for subsequent fluorescence recovery. The rate of recovery can be related to the coefficient of molecular diffusion. In our experiments we apply FRAP to a flowing liquid film, where advection, in addition to molecular diffusion, contributes to the evolution of the bleached spot. By employing simple optical measurements of the rate of advection and diffusion, combined with an analysis of dispersion, we can uniquely determine the velocity profile within a thin film. As a proof of concept we apply this technique to shear-driven flow over a liquid film within a micro-patterned surface. [Preview Abstract] |
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