11 results found
Batchvarov A, Kahouadji L, Constante-Amores CR, et al., 2021, Three-dimensional dynamics of falling films in the presence of insoluble surfactants, Journal of Fluid Mechanics, Vol: 906, Pages: A16-1-A16-13, ISSN: 0022-1120
We study the effect of insoluble surfactants on the wave dynamics of vertically falling liquid films. We use three-dimensional numerical simulations and employ a hybrid interface-tracking/level-set method, taking into account Marangoni stresses induced by gradients of interfacial surfactant concentration. Our numerical predictions for the evolution of the surfactant-free, three-dimensional wave topology are validated against the experimental work of Park & Nosoko (AIChE J., vol. 49, 2003, pp. 2715–2727). The addition of surfactants is found to influence significantly the development of horseshoe-shaped waves. At low Marangoni numbers, we show that the wave fronts exhibit spanwise oscillations before eventually acquiring a quasi-two-dimensional shape. In addition, the presence of Marangoni stresses is found to suppress the peaks of the travelling waves and preceding capillary wave structures. At high Marangoni numbers, a near-complete rigidification of the interface is observed.
Constante-Amores CR, Kahouadji L, Batchvarov A, et al., 2020, Rico and the jets: Direct numerical simulations of turbulent liquid jets, Physical Review Fluids, Vol: 5, Pages: 110501-1-110501-4, ISSN: 2469-990X
This paper is associated with a poster winner of a 2019 American Physical Society's Division of Fluid Dynamics (DFD) Milton van Dyke Award for work presented at the DFD Gallery of Fluid Motion. The original poster is available online at the Gallery of Fluid Motion, https://doi.org/10.1103/APS.DFD.2019.GFM.P0020.
Batchvarov A, Kahouadji L, Magnini M, et al., 2020, Effect of surfactant on elongated bubbles in capillary tubes at high Reynolds number, Physical Review Fluids, Vol: 5, Pages: 093605 – 1-093605 – 21, ISSN: 2469-990X
The effect of surfactants on the tail and film dynamics of elongated gas bubbles propagating through circular capillary tubes is investigated by means of an extensive three-dimensional numerical study using a hybrid front-tracking/level-set method. The focus is on the visco-inertial regime, which occurs when the Reynolds number of the flow is much larger than unity. Under these conditions, “clean” bubbles exhibit interface undulations in the proximity of the tail, with an amplitude that increases with the Reynolds number. We perform a systematic analysis of the impact of a wide range of surfactant properties, including elasticity, bulk surfactant concentration, solubility, and diffusivity, on the bubble and flow dynamics in the presence of inertial effects. The results show that the introduction of surfactants is effective in suppressing the tail undulations as they tend to accumulate near the bubble tail. Here large Marangoni stresses are generated, which lead to a local “rigidification” of the bubble. This effect becomes more pronounced for larger surfactant elasticities and adsorption depths. At reduced surfactant solubility, a thicker rigid film region forms at the bubble rear, where a Couette film flow is established, while undulations still appear at the trailing edge of the downstream “clean” film region. In such conditions, the bubble length becomes an influential parameter, with short bubbles becoming completely rigid.
Constante-Amores CR, Kahouadji L, Batchvarov A, et al., 2020, Dynamics of retracting surfactant-laden ligaments at intermediate Ohnesorge number, Physical Review Fluids, Vol: 5, Pages: 084007 – 1-084007 – 24, ISSN: 2469-990X
The dynamics of ligaments retracting under the action of surface tension occurs in a multitude of natural and industrial applications; these include inkjet printing and atomization. We perform direct, fully three-dimensional, two-phase numerical simulations of the retracting process over a range of system parameters that account for surfactant solubility, sorption kinetics, and Marangoni stresses. Our results indicate that the presence of surfactant inhibits the “end-pinching” mechanism and promotes neck reopening through Marangoni-flow; this is induced by the formation of surfactant concentration gradients that drive flow-reversal toward the neck. The vortical structures associated with this flow are also analyzed in detail. We also show that these Marangoni stresses lead to interfacial rigidification, observed through a reduction of the retraction velocity and ligament kinetic energy.
Russell AW, Kahouadji L, Mirpuri K, et al., 2019, Mixing viscoplastic fluids in stirred vessels over multiple scales: An experimental and CFD approach, Chemical Engineering Science, Vol: 208, ISSN: 1873-4405
Dye visualisation techniques and CFD are used to characterise the flow of viscoplastic CarbopolTM solutions in stirred vessel systems over multiple scales. Centrally-mounted, geometrically-similar Rushton turbine (RT) impellers are used to agitate various Carbopol 980 (C980) fluids. The dimensionless cavern diameters, Dc/D, are scaled against a combination of dimensionless parameters: Rem-0.3Rey0.6n-0.1ks-1, where Rem, Rey, n and ks are the modified power-law Reynolds number, yield stress Reynolds number, flow behaviour index and impeller geometry constant, respectively. Excellent collapse of the data is demonstrated for the fluids and flows investigated. Additional data are collected using a pitched-blade turbine (PBT) with cavern size similarity being shown between the RT and PBT datasets. These results are important in the context of scale-up/scale-down mixing processes in stirred vessels containing complex fluids and can be used to show that flow similarity can be achieved in these systems if the processes are scaled appropriately.
Kahouadji L, Nowak E, Kovalchuk N, et al., 2018, Simulation of immiscible liquid-liquid flows in complex microchannel geometries using a front-tracking scheme, MICROFLUIDICS AND NANOFLUIDICS, Vol: 22, ISSN: 1613-4982
The three-dimensional two-phase flow dynamics inside a microfluidic device of complex geometry is simulated using a parallel, hybrid front-tracking/level-set solver. The numerical framework employed circumvents numerous meshing issues normally associated with constructing complex geometries within typical computational fluid dynamics packages. The device considered in the present work is constructed via a module that defines solid objects by means of a static distance function. The construction combines primitive objects, such as a cylinder, a plane, and a torus, for instance, using simple geometrical operations. The numerical solutions predicted encompass dripping and jetting, and transitions in flow patterns are observed featuring the formation of drops, ‘pancakes’, plugs, and jets, over a wide range of flow rate ratios. We demonstrate the fact that vortex formation accompanies the development of certain flow patterns, and elucidate its role in their underlying mechanisms. Experimental visualisation with a high-speed imaging are also carried out. The numerical predictions are in excellent agreement with the experimental data.
Seungwon S, Chergui J, Juric D, et al., 2018, A hybrid interface tracking – level set technique for multiphase flow with soluble surfactant, Journal of Computational Physics, Vol: 359, ISSN: 0021-9991
A formulation for soluble surfactant transport in multiphase flows recently presented by Muradoglu & Tryggvason (JCP 274 (2014) 737–757) is adapted to the context of the Level Contour Reconstruction Method, LCRM, (Shin et al. IJNMF 60 (2009) 753–778) which is a hybrid method that combines the advantages of the Front-tracking and Level Set methods. Particularly close attention is paid to the formulation and numerical implementation of the surface gradients of surfactant concentration and surface tension. Various benchmark tests are performed to demonstrate the accuracy of different elements of the algorithm. To verify surfactant mass conservation, values for surfactant diffusion along the interface are compared with the exact solution for the problem of uniform expansion of a sphere. The numerical implementation of the discontinuous boundary condition for the source term in the bulk concentration is compared with the approximate solution. Surface tension forces are tested for Marangoni drop translation. Our numerical results for drop deformation in simple shear are compared with experiments and results from previous simulations. All benchmarking tests compare well with existing data thus providing confidence that the adapted LCRM formulation for surfactant advection and diffusion is accurate and effective in three-dimensional multiphase flows with a structured mesh. We also demonstrate that this approach applies easily to massively parallel simulations.
Kahouadji L, Périnet N, Tuckerman LS, et al., 2015, Numerical simulation of super-square patterns in Faraday waves, Journal of Fluid Mechanics, ISSN: 1469-7645
We report the first simulations of the Faraday instability using the full three-dimensional Navier–Stokes equations in domains much larger than the characteristic wavelength of the pattern. We use a massively parallel code based on a hybrid front-tracking/level-set algorithm for Lagrangian tracking of arbitrarily deformable phase interfaces. Simulations performed in square and cylindrical domains yield complex patterns. In particular, a superlattice-like pattern similar to those of Douady & Fauve (Europhys. Lett., vol. 6, 1988, pp. 221–226) and Douady (J. Fluid Mech., vol. 221, 1990, pp. 383–409) is observed. The pattern consists of the superposition of two square superlattices. We conjecture that such patterns are widespread if the square container is large compared with the critical wavelength. In the cylinder, pentagonal cells near the outer wall allow a square-wave pattern to be accommodated in the centre.
Kahouadji L, Witkowski LM, 2014, Free surface due to a flow driven by a rotating disk inside a vertical cylindrical tank: Axisymmetric configuration, Physics of Fluids, Vol: 26, Pages: 072105-072105, ISSN: 1070-6631
Kahouadji L, Houchens BC, Witkowski LM, 2011, Thermocapillary instabilities in a laterally heated liquid bridge with end wall rotation, Physics of Fluids, Vol: 23, Pages: 104104-104104, ISSN: 1070-6631
Kahouadji L, Witkowski LM, Le Quéré P, 2010, Seuils de stabilité pour un écoulement à surface libre engendré dans une cavité cylindrique tournante à petit rapport de forme, Mécanique & Industries, Vol: 11, Pages: 339-344, ISSN: 1296-2139
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