Session P28: Multiphase Flows: Turbulence

Bubble-mediated gas transfer of dilute gas in homogeneous and isotropic turbulent flow

Gas exchange from bubbles in turbulent flow is critical in bubble column chemical reactors as well as for the ocean-atmosphere interface where the air is 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) regimes is important as it can be used for improving the models on a larger scale. We study 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, Pe. We present a model for exchange velocity that is verified by direct numerical simulations of mass transfer of dilute gas having up to Schmidt number, Sc = 100 from a bubble in homogeneous and isotropic turbulence in region 50 < Pe < 10⁴. The direct numerical simulations are performed with an efficient wavelet-based adaptive mesh refinement resolving the thin boundary layers for high Schmidt numbers along with Kolmogorov and Batchelor scales (Farsoiya et. al. 2021).   

Fragmentation in turbulence by small eddies

From air-sea gas exchange, oil pollution, to bioreactors, the ubiquitous fragmentation of bubbles/drops in turbulence has been modeled by relying on the classical Kolmogorov-Hinze paradigm since the 1950s. This framework assumes that bubbles/drops are broken solely by eddies of the same size, even though turbulence is well known for its wide spectrum of eddy scales. Here, by designing a new experiment that can physically disentangle eddies of various sizes, we report an experimental work to challenge this assumption and show that bubbles are preferentially broken by sub-bubble scale eddies. Our work also highlights that fragmentation cannot be quantified solely by the stress criterion; The competition between different time scales is equally important. Instead of being elongated slowly and persistently by large-scale flows, bubbles are fragmented in turbulence by a burst of intense local deformation caused by small eddies that could release their energy within a short time.    

Origin of the sub-Hinze scale bubble production in turbulence

In turbulent flow, bubble breaking generates a broad distribution of sizes $d$, which scales as $d^{-10/3}$ and holds down to the Hinze scale $d_h$, the size separation between stable ($dd_h$). The sub-Hinze bubble distribution remains poorly understood, while it plays important roles in many practical situations, such as low solubility gas fluxes at the ocean-atmosphere interface. Combining experimental and numerical approaches, we report a power law scaling $d^{-3/2}$ for the small bubble size distribution, for sufficiently large separation of scales between the injection size and the Hinze scale. From an analysis of individual events, we show that break-up events generating small bubbles are driven by capillary effects, and that their break-up time scales as $d^{3/2}$, which physically explains the sub-Hinze scaling observed.   

Experimental observations of sub-Hinze bubble production in turbulence with surfactant

Knowledge of the spectrum of sizes of bubbles existing in turbulent dispersions of gases in liquids is essential to understanding many natural and industrial processes. However, the distribution of bubbles that are smaller than the Hinze scale d_H (at which turbulent stresses to bubbles are balanced by surface tension) is not well understood. We approach the problem with two experiments involving air bubbles injected into forced water turbulence: in the first, the bubble size distribution is described statistically using a range of parent bubble sizes and concentrations of surfactant; in the second, break-up events are resolved dynamically and in three dimensions using multiple high-speed cameras. Consistent with recent numerical observations, the sizes of the sub-Hinze bubbles produced are set by the capillary pinching dynamics instigated by turbulent deformations (of scales larger than d_H) to the parent bubble. Increased concentration of surfactant promotes greater production of sub-Hinze scale bubbles, pointing to distinct mechanisms for sub- and super-Hinze bubble production and an important role of surface tension in the former.   

Controlling turbulent bubbly flow through a jet array

Bubble deformation strongly affects its lateral migration in a shear flow, including both the mean or turbulent shear. But the classical experiments studying this problem in a pipe flow cannot disentangle these two effects. To control the profile of the mean flow and turbulence in a water tunnel, we modified our vertical tunnel, V-ONSET, to move away from the canonical homogeneous and isotropic conditions. The tunnel is equipped with a unique jet array with 88 independently controlled nozzles, each of which is connected through a dedicated adjustable valve and solenoid valve to control the flow rate and its on-off state. With this large degree of controllability, we aim to generate a well-prescribed level of shear in the test section. The turbulent shear flow is then inspected in detail by using our in-house 3D Lagrangian particle tracking system, and the results will also be presented in this talk.   

Dynamics of turbulence kinetic energy in droplet-laden homogeneous shear turbulence

Our objective is to study the effects of droplets on the turbulence kinetic energy (TKE) in homogeneous shear turbulence via direct numerical simulation (DNS). The main difference with the study of droplet-laden decaying isotropic turbulence by Dodd & Ferrante (J. Fluid Mech., 2016) is the presence of the mean shear in the flow which results in the production term for the TKE equation. We performed DNS of 1260 non-evaporating droplets of diameter approximately equal to twice the Taylor lengthscale of turbulence at the time the droplets are released, and with 5% droplet volume-fraction in homogeneous shear turbulence at an initial Re_λ,0 = 40 using a mesh of 1200 × 600 × 600 grid points. For the cases studied, we varied the initial We_rms from 0.02 to 0.5 while keeping the density and viscosity ratios constant, ρ_d/ρ_c = 10 and μ_d/μ_c = 10. The results show that increasing We_rms causes a decrease in TKE production, and that for We_rms = 0.02 and We_rms = 0.5, the TKE production is larger and smaller, respectively, than that of the droplet-free case. The underlying physical mechanisms for such dynamics will be discussed.   

How do deformable bubbles modulate turbulence?

In this talk, we will present an experimental study that was designed to answer a specific question: How do finite-sized deformable bubbles modulate their surrounding turbulence? In contrast to previous works on turbulence generated by a swarm of rising bubbles in an otherwise quiescent medium, our study was conducted in an environment with intense background turbulence, which can constantly exchange energy with bubbles through interface deformation. In particular, we utilize a unique experimental setup, named V-ONSET, within which fluid flows, bubble shape, and bubble dynamics are simultaneously measured. We will show how turbulence is modulated in the vicinity of bubbles that undergo strong deformation. The results will pave the foundation for a new regime in turbulent bubbly flows where the pre-existing background turbulence is intense and there is strong two-phase couplings through deformation.   

Comparison of Reynolds shear stress methods for RANS turbulence modelling of a cloud cavitation in a venturi

Cavitation is defined as the process when the local pressure of the fluid drops below the vapor pressure at a roughly constant temperature resulting in the formation of vapor cavities or bubbles. It leads to several performance degrading effects like noise, vibrations and there exists a need to investigate it experimentally & numerically to study the mechanisms behind it. On the numerical side, the flow is resolved using a cavitation model coupled with a turbulence model. For coupling of a cavitation model with a URANS turbulence model for a cloud cavitating flow (cavitating flow where main cloud cavity of vapor detaches from the main wall periodically and results in large scale clouds of bubbles), the Reynolds shear stress is expected to be dependent on the turbulent eddy viscosity since the velocity fluctuations are averaged out but it is shown in this study that both formulas yield different results. A 2D model of a venturi-type section is selected as the numerical model to study the flow using the multiphase solver, interPhaseChangeFoam available in OpenFOAM. Here,the Merkle cavitation model is used along with the k-omega Shear Stress Transport (SST) turbulence model.  The Reynolds stress data is plotted globally and at local probe stations using both methods- the time-averaged product of eddy viscosity and the velocity gradients and the time-averaged product of the subtraction of time-averaged velocity from the instantaneous velocity in each direction.The results of both methods are discussed and compared with experimental data obtained using X-ray high speed Particle-Image Velocimetry (PIV) experiments. On a local comparison, it is observed that both methods yield different results: the Reynolds shear stresses calculated using the eddy viscosity method shows a near linear profile at local stations regardless of the distance from the throat while the stress calculated using the time-averaged fluctuations agrees with experimental data close to the throat.   

Simulations of finite-size evaporating droplets in weakly-compressible homogeneous shear turbulence

We perform interface-resolved simulations of finite-size evaporating droplets in weakly-compressible homogeneous shear turbulence (HST). The study is conducted by varying three dimensionless physical parameters: the initial gas temperature over the critical temperature, the initial droplet diameter over the Kolmogorov scale and the surface tension, i.e. the shear-based Weber number. We first discuss the impact on the evaporation rate of the three thermodynamic models employed to evaluate the gas thermophysical properties: a constant property model and two variable-properties approaches where either the gas density or all the gas properties are allowed to vary. Taking this last approach as reference, the model assuming constant gas properties and evaluated with the "1/3" rule, is shown to predict the evaporation rate better than the model where the only variable property is the gas density. Next, we show that the ratio between the actual evaporation rate in turbulence and the one computed in stagnant condition is always much higher than one for weakly deformable droplets. Finally, we examine the overall evaporation rate and the local interfacial mass-flux, showing a positive correlation between evaporation rate and interfacial curvature.   

Numerical study of emulsions in homogeneous and isotropic turbulence

We present an analysis of the effects of volume fraction, viscosity ratio and surface tension on the turbulent flow of emulsions in a tri-periodic box where Homogeneous and Isotropic Turbulence is forced at large scale. We will present integral quantities defining the flow as well as spectral scale-by-scale analysis of the energy transport through scales. Our findings reveal a significant modification of the classical energy transport occurring in single phase flows. More specifically, liquid-liquid interface offers an alternative path for energy transport towards small scales, where surface tension forces act alongside non-linear transport. We observed that this novel mechanism for energy transport scales proportionally with the total interfacial area. Furthermore, energy transfer via surface tension forces can be related to the coalescence and breakup mechanisms and, to this extent, droplet size distribution is analyzed, where we report both -10/3 and -3/2 power-law scalings for all cases. Finally, strong modulation of small scale dynamics is observed and its behavior with property variations is described.   

Catastrophic Phase Inversion in High Reynolds Number Turbulent Taylor–Couette Flow

The dynamics of metastable oil-water emulsions in highly turbulent (10^11 <= Ta <= 10^13) Taylor-Couette flow, far from equilibrium, is investigated. By varying the oil-in-water void fraction, catastrophic phase inversion between oil-in-water and water-in-oil emulsions can be triggered, changing the morphology, including droplet sizes, and rheological properties of the mixture, dramatically. The manifestation of these different states is exemplified by combining global torque measurements and local in situ laser induced fluorescence microscopy imaging. Despite the turbulent state of the flow and the dynamic equilibrium of the oil-water mixture, the global torque response of the system is found to be as if the fluid were Newtonian, and the effective viscosity of the mixture was found to be several times bigger or smaller than either of its constituents.   

Estimating time and length scales of indirect transmission of respiratory diseases

The recent pandemic has highlighted several open questions across many disciplines. One of the main epidemiological uncertainty relates to the mechanisms of pathogen transmission between individuals. In particular, a dichotomy exists between direct (short-range) transmission and indirect (long-range) transmission.  Direct transmission is associated with larger droplets falling immediately in the proximity of the infected person and spreading the virus with deposition on other individuals or objects. On the contrary, indirect transmission is related to smaller droplets that evaporate faster and can remain suspended in the air for longer times as aerosol nuclei reaching longer distances from the emission point. Here, we will use Direct Numerical Simulations (DNS) to assess the indirect transmission in a poorly ventilated room to quantify the relevant resident times and distances of respiratory droplets. We will show that droplets with an initial diameter of less than 30 microns can remain suspended for more than an hour, reaching distances even of the order of 10 meters. Assuming typical values of viral loads, we can explain the occurrence of superspreading events and evaluate the efficiency of non-pharmaceutical interventions such as social distances and masks.