7 results
Role of viscosity in turbulent drop break-up
- Palas Kumar Farsoiya, Zehua Liu, Andreas Daiss, Rodney O. Fox, Luc Deike
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- Journal:
- Journal of Fluid Mechanics / Volume 972 / 10 October 2023
- Published online by Cambridge University Press:
- 27 September 2023, A11
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We investigate drop break-up morphology, occurrence, time and size distribution, through large ensembles of high-fidelity direct-numerical simulations of drops in homogeneous isotropic turbulence, spanning a wide range of parameters in terms of the Weber number $We$, viscosity ratio between the drop and the carrier flow $\mu _r=\mu _d/\mu _l$, where d is the drop diameter, and Reynolds ($Re$) number. For $\mu _r \leq 20$, we find a nearly constant critical $We$, while it increases with $\mu _r$ (and $Re$) when $\mu _r > 20$, and the transition can be described in terms of a drop Reynolds number. The break-up time is delayed when $\mu _r$ increases and is a function of distance to criticality. The first break-up child-size distributions for $\mu _r \leq 20$ transition from M to U shape when the distance to criticality is increased. At high $\mu _r$, the shape of the distribution is modified. The first break-up child-size distribution gives only limited information on the fragmentation dynamics, as the subsequent break-up sequence is controlled by the drop geometry and viscosity. At high $We$, a $d^{-3/2}$ size distribution is observed for $\mu _r \leq 20$, which can be explained by capillary-driven processes, while for $\mu _r > 20$, almost all drops formed by the fragmentation process are at the smallest scale, controlled by the diameter of the very extended filament, which exhibits a snake-like shape prior to break-up.
Direct numerical simulations of bubble-mediated gas transfer and dissolution in quiescent and turbulent flows
- Palas Kumar Farsoiya, Quentin Magdelaine, Arnaud Antkowiak, Stéphane Popinet, Luc Deike
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- Journal:
- Journal of Fluid Mechanics / Volume 954 / 10 January 2023
- Published online by Cambridge University Press:
- 06 January 2023, A29
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We perform direct numerical simulations of a gas bubble dissolving in a surrounding liquid. The bubble volume is reduced due to dissolution of the gas, with the numerical implementation of an immersed boundary method, coupling the gas diffusion and the Navier–Stokes equations. The methods are validated against planar and spherical geometries’ analytical moving boundary problems, including the classic Epstein–Plesset problem. Considering a bubble rising in a quiescent liquid, we show that the mass transfer coefficient $k_L$ can be described by the classic Levich formula $k_L = (2/\sqrt {{\rm \pi} })\sqrt {\mathscr {D}_l\,U(t)/d(t)}$, with $d(t)$ and $U(t)$ the time-varying bubble size and rise velocity, and $\mathscr {D}_l$ the gas diffusivity in the liquid. Next, we investigate the dissolution and gas transfer of a bubble in homogeneous and isotropic turbulence flow, extending Farsoiya et al. (J. Fluid Mech., vol. 920, 2021, A34). We show that with a bubble size initially within the turbulent inertial subrange, the mass transfer coefficient in turbulence $k_L$ is controlled by the smallest scales of the flow, the Kolmogorov $\eta$ and Batchelor $\eta _B$ microscales, and is independent of the bubble size. This leads to the non-dimensional transfer rate ${Sh}=k_L L^\star /\mathscr {D}_l$ scaling as ${Sh}/{Sc}^{1/2} \propto {Re}^{3/4}$, where ${Re}$ is the macroscale Reynolds number ${Re} = u_{rms}L^\star /\nu _l$, with $u_{rms}$ the velocity fluctuations, $L^*$ the integral length scale, $\nu _l$ the liquid viscosity, and ${Sc}=\nu _l/\mathscr {D}_l$ the Schmidt number. This scaling can be expressed in terms of the turbulence dissipation rate $\epsilon$ as ${k_L}\propto {Sc}^{-1/2} (\epsilon \nu _l)^{1/4}$, in agreement with the model proposed by Lamont & Scott (AIChE J., vol. 16, issue 4, 1970, pp. 513–519) and corresponding to the high $Re$ regime from Theofanous et al. (Intl J. Heat Mass Transfer, vol. 19, issue 6, 1976, pp. 613–624).
Bubble-mediated transfer of dilute gas in turbulence
- Palas Kumar Farsoiya, Stéphane Popinet, Luc Deike
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- Journal:
- Journal of Fluid Mechanics / Volume 920 / 10 August 2021
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- 14 June 2021, A34
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Bubble-mediated gas exchange in turbulent flow is critical in bubble column chemical reactors as well as for ocean–atmosphere gas exchange related to air entrained by breaking waves. Understanding the transfer rate from a single bubble in turbulence at large Péclet numbers (defined as the ratio between the rate of advection and diffusion of gas) is important as it can be used for improving models on a larger scale. We characterize the mass transfer of dilute gases from a single bubble in a homogeneous isotropic turbulent flow in the limit of negligible bubble volume variations. We show that the mass transfer occurs within a thin diffusive boundary layer at the bubble–liquid interface, whose thickness decreases with an increase in turbulent Péclet number, $\widetilde {{Pe}}$. We propose a suitable time scale $\theta$ for Higbie (Trans. AIChE, vol. 31, 1935, pp. 365–389) penetration theory, $\theta = d_0/\tilde {u}$, based on $d_0$ the bubble diameter and $\tilde {u}$ a characteristic turbulent velocity, here $\tilde {u}=\sqrt {3}\,u_{{rms}}$, where $u_{{rms}}$ is the large-scale turbulence fluctuations. This leads to a non-dimensional transfer rate ${Sh} = 2(3)^{1/4}\sqrt {\widetilde {{Pe}}/{\rm \pi} }$ from the bubble in the isotropic turbulent flow. The theoretical prediction is verified by direct numerical simulations of mass transfer of dilute gas from a bubble in homogeneous and isotropic turbulence, and very good agreement is observed as long as the thin boundary layer is properly resolved.
Jetting in finite-amplitude, free, capillary-gravity waves
- Saswata Basak, Palas Kumar Farsoiya, Ratul Dasgupta
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- Journal:
- Journal of Fluid Mechanics / Volume 909 / 25 February 2021
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- 17 December 2020, A3
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We present a theoretical and computational study of the mechanics of a jet formed from a large amplitude, axisymmetric, capillary-gravity wave on the surface of a liquid pool in a cylindrical container. Jetting can cause a pronounced overshoot of the interface at the axis of symmetry. A linear theory presented earlier in Farsoiya et al. (J. Fluid Mech., vol. 826, 2017, pp. 797–818) was shown to be incapable of describing this jet. To understand its mechanics, we present here the inviscid, weakly nonlinear solution to the initial value problem where the initial surface perturbation is a single Fourier–Bessel mode on quiescent fluid. The theory predicts that energy injected into a primary (Bessel) mode initially is transferred nonlinearly to a spectrum of modes. The extent of the theoretically predicted energy transfer is found to be very accurate for modes up to the second harmonic. We show using numerical simulations that the jet originates as a small dimple formed at the trough of the wave, analogous to similar observations in bubble cavity collapse (Duchemin et al., Phys. Fluids, vol. 14, 2002, pp. 3000–3008; Lai et al., Phys. Rev. Lett. vol. 121, 2018, 144501). The theory is able to describe the jet overshoot and the velocity and pressure fields in the liquid qualitatively, but does not capture the temporal evolution of the dimple or the thinning of the jet neck leading to pinchoff. Modal analysis shows that the latter phenomenon requires higher-order approximations, beyond the second order presented here. The nonlinear theory yields explicit analytical expressions without any fitting parameters which are systematically tested against numerical simulations of the incompressible Euler equation. The theory contains cylindrical analogues of the singularities corresponding to second harmonic resonance (Wilton, Lond. Edinb. Dubl. Phil. Mag. J. Sci., vol. 29, 1915, pp. 688–700). The connection of these to triadic resonant interactions among capillary-gravity waves in a cylindrical confined geometry is discussed.
Azimuthal capillary waves on a hollow filament – the discrete and the continuous spectrum
- Palas Kumar Farsoiya, Anubhab Roy, Ratul Dasgupta
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- Journal:
- Journal of Fluid Mechanics / Volume 883 / 25 January 2020
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- 25 November 2019, A21
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We study the temporal spectrum of linearised, azimuthal, interfacial perturbations imposed on a cylindrical gaseous filament surrounded by immiscible, viscous, quiescent fluid in radially unbounded geometry. Linear stability analysis shows that the base state is stable to azimuthal perturbations of standing wave form. Normal mode analysis leads to a viscous dispersion relation and shows that in addition to the discrete spectrum, the problem also admits a continuous spectrum. For a given azimuthal Fourier mode and Laplace number, the discrete spectrum yields two eigenfunctions which decay exponentially to zero at large radii and thus cannot represent far field perturbations. In addition to these discrete modes, we find an uncountably infinite set of eigenmodes which decay algebraically to zero. The completeness theorem for perturbation vorticity may be expressed as a sum over the discrete modes and an integral over the continuous ones. We validate our normal mode results by solving the linearised, initial value problem (IVP). The initial perturbation is taken to be an interfacial, azimuthal Fourier mode with zero perturbation vorticity. It is shown that the expression for the time dependent amplitude of a capillary standing wave (in the Laplace domain, $s$) has poles and branch points on the complex $s$ plane. We show that the residue at the poles yields the discrete spectrum, while the contribution from either side of the branch cut provides the continuous spectrum contribution. The particular initial condition treated here in the IVP, has projections on the discrete as well as the continuous spectrum eigenmodes and thus both sets are excited initially. Consequently the time evolution of the standing wave amplitude and the perturbation vorticity field have the form of a sum over discrete exponential contributions and an integral over a continuous range of exponential terms. The solution to the IVP leads to explicit analytical expressions for the standing wave amplitude and the vorticity field in the fluid outside the filament. Linearised analytical results are validated using direct numerical simulations (DNS) conducted using a code developed in-house for solving the incompressible, Navier–Stokes equations with an interface. For small perturbation amplitude, analytical predictions show excellent agreement with DNS. Our analysis complements and extends earlier results on the discrete and the continuous spectrum for interfacial viscous, capillary waves on unbounded domain.
Faraday waves on a cylindrical fluid filament – generalised equation and simulations
- Sagar Patankar, Palas Kumar Farsoiya, Ratul Dasgupta
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- Journal of Fluid Mechanics / Volume 857 / 25 December 2018
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- 19 October 2018, pp. 80-110
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We perform linear stability analysis of an interface separating two immiscible, inviscid, quiescent fluids subject to a time-periodic body force. In a generalised, orthogonal coordinate system, the time-dependent amplitude of interfacial perturbations, in the form of standing waves, is shown to be governed by a generalised Mathieu equation. For zero forcing, the Mathieu equation reduces to a (generalised) simple harmonic oscillator equation. The generalised Mathieu equation is shown to govern Faraday waves on four time-periodic base states. We use this equation to demonstrate that Faraday waves and instabilities can arise on an axially unbounded, cylindrical capillary fluid filament subject to radial, time-periodic body force. The stability chart for solutions to the Mathieu equation is obtained through numerical Floquet analysis. For small values of perturbation and forcing amplitude, results obtained from direct numerical simulations (DNS) of the incompressible Euler equation (with surface tension) show very good agreement with theoretical predictions. Linear theory predicts that unstable Rayleigh–Plateau modes can be stabilised through forcing. This prediction is borne out by DNS results at early times. Nonlinearity produces higher wavenumbers, some of which can be linearly unstable due to forcing and thus eventually destabilise the filament. We study axisymmetric as well as three-dimensional perturbations through DNS. For large forcing amplitude, localised sheet-like structures emanate from the filament, suffering subsequent fragmentation and breakup. Systematic parametric studies are conducted in a non-dimensional space of five parameters and comparison with linear theory is provided in each case. Our generalised analysis provides a framework for understanding free and (parametrically) forced capillary oscillations on quiescent base states of varying geometrical configurations.
Axisymmetric viscous interfacial oscillations – theory and simulations
- Palas Kumar Farsoiya, Y. S. Mayya, Ratul Dasgupta
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- Journal:
- Journal of Fluid Mechanics / Volume 826 / 10 September 2017
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- 15 August 2017, pp. 797-818
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We study axisymmetric, free oscillations driven by gravity and surface tension at the interface of two viscous, immiscible, radially unbounded fluids, analytically and numerically. The interface is perturbed as a zeroth-order Bessel function (in space) and its evolution is obtained as a function of time. In the linearised approximation, we solve the initial value problem (IVP) to obtain an analytic expression for the time evolution of wave amplitude. It is shown that a linearised Bessel mode temporally evolves in exactly the same manner as a Fourier mode in planar geometry. We obtain novel analytical expressions for the time varying vorticity and pressure fields in both fluids. For small initial amplitudes, our analytical results show excellent agreement with those obtained from solving the axisymmetric Navier–Stokes equations numerically. We also compare our results with the normal mode approximation and find the latter to be an accurate representation at very early and late times. The deviation between the normal mode approximation and the IVP solution is found to increase as a function of viscosity ratio. The vorticity field has a jump discontinuity at the interface and we find that this jump depends on the viscosity and the density ratio of the two fluids. Upon increasing the initial perturbation amplitude in the simulations, nonlinearity produces qualitatively new features not present in the analytical IVP solution. Notably, a jet is found to emerge at the axis of symmetry rising to a height greater than the initial perturbation amplitude. Increasing the perturbation amplitude further causes the jet to undergo end pinch off, giving birth to a daughter droplet. This can happen either for an advancing or a receding jet, depending on the viscosity ratio. A relation is found between the maximum jet height and the perturbation amplitude. Hankel transform of the interface demonstrates that at large perturbation amplitudes higher wavenumbers emerge, sharing some of the energy of the lowest mode. When these additional higher modes are present, the interface has pointed crests and rounded troughs.