Turbulence and its organization, long conceptualized in terms of "coherent structures,"has resisted clear description. A significant limitation has been the lack of tools to identify instantaneous, spatially finite structures, while unraveling their superposition. We present a framework of generalized correlations, which can be used to readily define a variety of correlation measures, aimed at identifying field patterns. Coupled with Helmholtz-decomposition, this provides a paradigm to identify and disentangle structures. We demonstrate the correlations using vortex-based canonical flows and then apply them to incompressible, homogeneous, isotropic turbulence. We find that high turbulence kinetic energy (Ek) regions form compact velocity-jets that are spatially exclusive from high enstrophy (ω 2) regions that form vorticity-jets surrounded by swirling velocity. The correlation fields reveal that the energetic structures in turbulence, being invariably jets, are distinct from those in vortex-based canonical flows, where they can be jet-like as well as swirling. A full Biot-Savart decomposition of the velocity field shows that the velocity-jets are neither self-induced, nor induced by the interaction of swirling, strong vorticity regions, and are almost entirely induced, non-locally, by the permeating intermediate range (rms level) vorticity. Velocity-swirls, instead, are a superposition of self-induced and background-induced velocity. Interestingly, it is the mild intermediate vorticity that dominantly induces the velocity-field everywhere. This suggests that turbulence organization could result from non-local and non-linear field interactions, leading to an emergent description unlike the notion of a strict structural hierarchy. Our correlation-decomposition framework lends itself readily to the study of generic vector and scalar fields associated with diverse phenomena.

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We perform direct numerical simulations (DNS) of emulsions in homogeneous isotropic turbulence using a pseudopotential lattice-Boltzmann (PP-LB) method. Improving on previous literature by minimizing droplet dissolution and spurious currents, we show that the PP-LB technique is capable of long stable simulations in certain parameter regions. Varying the dispersed-phase volume fraction , we demonstrate that droplet breakup extracts kinetic energy from the larger scales while injecting energy into the smaller scales, increasingly with higher , with approximately the Hinze scale (Hinze, AIChE J., vol. 1 (3), 1955, pp. 289-295) separating the two effects. A generalization of the Hinze scale is proposed, which applies both to dense and dilute suspensions, including cases where there is a deviation from the inertial range scaling and where coalescence becomes dominant. This is done using the Weber number spectrum , constructed from the multiphase kinetic energy spectrum , which indicates the critical droplet scale at which . This scale roughly separates coalescence and breakup dynamics as it closely corresponds to the transition of the droplet size distribution into a scaling (Garrett et al., J. Phys. Oceanogr., vol. 30 (9), 2000, pp. 2163-2171; Deane & Stokes, Nature, vol. 418 (6900), 2002, p. 839). We show the need to maintain a separation of the turbulence forcing scale and domain size to prevent the formation of large connected regions of the dispersed phase. For the first time, we show that turbulent emulsions evolve into a quasi-equilibrium cycle of alternating coalescence and breakup dominated processes. Studying the system in its state-space comprising kinetic energy , enstrophy and the droplet number density , we find that their dynamics resemble limit cycles with a time delay. Extreme values in the evolution of are manifested in the evolution of and with a delay of and respectively (with the large eddy timescale). Lastly, we also show that flow topology of turbulence in an emulsion is significantly more different from single-phase turbulence than previously thought. In particular, vortex compression and axial straining mechanisms increase in the droplet phase.

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The mixing of two immiscible fluids, often under turbulent conditions, can lead to the formation of an emulsion, where droplets of one fluid are embedded in another fluid. The occurrence of emulsions is commonplace across industries, ranging from the oil industry to food processing and biotechnology. Why emulsions serve diverse applications, in grossly simple terms, is due to their structural organization, as the two fluids in an emulsion form exhibit very different physical properties than they do when separated. The stability of the emulsion structure, hence, is key for its utility. The presence of impurities, or surfactants, in the constituent fluids, greatly enhances emulsion stability, by preventing the coalescence of droplets (which would lead to phase segregation). Emulsion research, over the past century, has developed into a thriving field, driven by the force of detailed experimentation that has significantly informed modeling, control and design of processes dealing with emulsification. Despite being predictable to a degree, the true nature of droplet dynamics at the heart of emulsification remains unknown. It is experimentally exceedingly difficult to illumine the evolution of interfaces undergoing coalescence and breakup, while simultaneously reporting the three-dimensional, turbulent flow features. It is slowly becoming feasible, however, to tackle these problems by using numerical simulations. Such simulations, too, involve a level of modeling complexity and pose heavy computational demands, and have hence remained an exception. It is only now becoming feasible to simulate such complex flows, allowing us to augment experiments with numerical insights. In this thesis, we attempt to unravel emulsification (to a small extent) by using simulations resolving both flow and interfaces, while considering fluids with impurities.@en

We present a pseudopotential lattice Boltzmann method to simulate liquid–liquid emulsions with a slightly soluble surfactant. The model is investigated in 2-D, over a wide parameter space for a single, stationary, immiscible droplet, and surface tension reduction by up to 15% is described in terms of a surfactant strength Λ (which roughly follows a Langmuir isotherm). The basic surfactant model is shown to be insufficient for arresting phase segregation—which is then achieved by changing the liquid–liquid interaction strength locally as a function of the surfactant density. 3-D spinodal decomposition (phase separation) is simulated, where the surfactant is seen to adapt rapidly to the evolving interfaces. Finally, for pendent droplet formation in an immiscible liquid, the addition of surfactant is shown to alter the droplet-size distribution and dynamics of newly formed droplets.

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While various multiphase flow simulation techniques have found acceptance as predictive tools for processes involving immiscible fluids, none of them can be considered universally applicable. Focusing on accurate simulation of liquid-liquid emulsions at the scale of droplets, we present a comparative assessment of the single-component multiphase pseudopotential lattice Boltzmann method (PP-LB, classical and modified) and the Volume of Fluid method (VOF, classical and modified), highlighting particular strengths and weaknesses of these techniques. We show that a modified LB model produces spurious velocities 1–3 orders of magnitude lower than all VOF models tested, and find that LB is roughly 10 times faster in computation time, while VOF is more versatile. Simulating falling liquid droplets, a realistic problem, we find that despite identical setups, results can vary with the technique in certain flow regimes. At lower Reynolds numbers, all methods agree reasonably well with experimental values. At higher Reynolds numbers, all methods underpredict the droplet Reynolds number, while being in good agreement with each other. Particular issues regarding LB simulations at low density ratio are emphasized. Finally, we conclude with the applicability of VOF vis-à-vis PP-LB for a general range of multiphase flow problems relevant to myriad applications.

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The predictability horizon of convective boundary layers is investigated in this study. Large-eddy simulation (LES) and direct numerical simulation (DNS) techniques are employed to probe the evolution of perturbations in identical twin simulations of a growing dry convective boundary layer. Error growth typical of chaotic systems is observed, marked by two phases. The first comprises an exponential error growth as , with δ0 as the initial error, δ(t) as the error at time t, and Λ as the Lyapunov exponent. This phase is independent of the perturbation wavenumber, and the perturbation energy grows following a self-similar spectral shape dominated by higher wavenumbers. The nondimensional error growth rate in this phase shows a strong dependence on the Reynolds number (Re). The second phase involves saturation of the error. Here, the error growth follows Lorenz dynamics with a slower saturation of successively larger scales. An analysis of the spectral decorrelation times reveals two regimes: an Re-independent regime for scales larger than the boundary layer height and an Re-dependent regime for scales smaller than , which are found to decorrelate substantially faster for increasing Reynolds numbers@en

This investigation highlights rationale of solvent bonding and adhesive bonding for fabrication of a transparent polymer such as polycarbonate with a high-throughput process. Studies under ultra violet spectra and visible spectra reveal that in comparison with adhesive bonding of a polymer, solvent diffusion bonding is more transparent. Polycarbonate is hydrophilic in nature resulting in a low contact angle of water as well as the presence of polar functional groups on the polymer surface. It is observed that a lap shear tensile strength of a solvent bonding polymer is significantly higher than that of an acrylic adhesive bonded polycarbonate, and fabrication of polycarbonate by solvent bonding merely takes few seconds. Solvent bonding of a polymer results in a cohesive failure from polymer as analyzed under the scanning electron microscopy, this is why solvent bonding shows a significantly higher bond strength.@en

Conference papers
Mukherjee, S., Zarghami, A., Haringa, C., Kenjeres, S., van den Akker, H.E.A., A comparative assessment of Lattice Boltzmann and Volume of Fluid (VOF) approaches for generic multiphase problems, ICMF 2016 – 9th conference on multiphase flows, may 22-77 2016, Firenze, Italy@en