Turbulent Bubbly Research Articles

Breaking bore roller characteristics: Turbulence statistics using optical techniques

Surface wave breaking induces strong turbulence in the two-phase flow region. Detailed turbulence statistics were experimentally obtained using non-intrusive optical techniques in a breaking bore roller, at relatively large scale, with a bore Froude number Fr1 = 2.15 and Reynolds number Re = 2.3 × 105. These novel velocity data were ensemble-averaged based upon an instantaneous dataset of 24,320 images. In terms of the velocity field, the breaking bore roller was classified into three regions: the impinging jet, developing shear layer and flow reversal region. The vertical profiles of the longitudinal velocity data exhibited some self-similarity. The Reynolds stress data showed an anisotropic turbulent flow immediately downstream of the roller toe, and tended towards isotropy away from the roller toe. The vorticity data suggested that the breaking at the roller toe was responsible for the generation of vortices. The turbulent structures in the shear layer presented significantly smaller length and time scales with higher dissipation rate than other regions. A discussion between present turbulence statistical data and bubble dynamics from literature was developed. The comparison between present and past studies suggested a similarity in two-phase physical processes in the breaking roller region between the tidal bore, hydraulic jump, swash zone bore and breaking wave.

난류 유동에서 다변수 인자에 대한 기계 학습 기법 비교 연구

It is important to predict spatially varying parameters to model turbulent flows. In this study, the spatially varying parameters are modeled via machine learning techniques using experiment-based turbulent bubble flow data and DNS-based turbulent Prandtl number data. The prediction and generalization errors of machine learning models are evaluated, and the different techniques are compared. Among the artificial neural network (ANN) techniques, the regular ANN using the full-batch training and the stochastic gradient descent (SGD) ANN based on mini-batch training are compared with the random forest (RF) method. The prediction and generalization errors show different characteristics according to the data resolution. For the coarsest bubbly flow data set, SGD ANN shows stable training and prediction, which leads to the smallest prediction and generalization errors. For the data sets with a finer resolution, the generalization error of SGD ANN is smallest, whereas the average prediction errors of regular ANN and RF method are smaller compared to SGD ANN. When evaluated using the trained models, all machine learning techniques show similar spatial distribution to the original data.

The orientational dynamics of deformable finite-sized bubbles in turbulence

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Periodic forcing of a large turbulent separation bubble

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Dynamics of bubbles under stochastic pressure forcing.

Several studies have investigated the dynamics of a single spherical bubble at rest under a nonstationary pressure forcing. However, attention has almost always been focused on periodic pressure oscillations, neglecting the case of stochastic forcing. This fact is quite surprising, as random pressure fluctuations are widespread in many applications involving bubbles (e.g., hydrodynamic cavitation in turbulent flows or bubble dynamics in acoustic cavitation), and noise, in general, is known to induce a variety of counterintuitive phenomena in nonlinear dynamical systems such as bubble oscillators. To shed light on this unexplored topic, here we study bubble dynamics as described by the Keller-Miksis equation, under a pressure forcing described by a Gaussian colored noise modeled as an Ornstein-Uhlenbeck process. Results indicate that, depending on noise intensity, bubbles display two peculiar behaviors: when intensity is low, the fluctuating pressure forcing mainly excites the free oscillations of the bubble, and the bubble's radius undergoes small amplitude oscillations with a rather regular periodicity. Differently, high noise intensity induces chaotic bubble dynamics, whereby nonlinear effects are exacerbated and the bubble behaves as an amplifier of the external random forcing.

Two-layer thermally driven turbulence: mechanisms for interface breakup

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Experimental investigation of the acceleration statistics and added-mass force of deformable bubbles in intense turbulence

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The turbulent bubble break-up cascade. Part 1. Theoretical developments

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Simultaneous measurements of deforming Hinze-scale bubbles with surrounding turbulence

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Gas-Liquid Flow and Interphase Mass Transfer in LL Microreactors

This work investigates the impact of fluid (CO2(g), water) flow rates, channel geometry, and the presence of a surfactant (ethanol) on the resulting gas–liquid flow regime (bubble, slug, annular), pressure drop, and interphase mass transfer coefficient (kla) in the FlowPlateTM LL (liquid-liquid) microreactor, which was originally designed for immiscible liquid systems. The flow regime map generated by the complex mixer geometry is compared to that obtained in straight channels of a similar characteristic length, while the pressure drop is fitted to the separated flows model of Lockhart–Martinelli, and the kla in the bubble flow regime is fitted to a power dissipation model based on isotropic turbulent bubble breakup. The LL-Rhombus configuration yielded higher kla values for an equivalent pressure drop when compared to the LL-Triangle geometry. The Lockhart–Martinelli model provided good pressure drop predictions for the entire range of experimental data (AARE < 8.1%), but the fitting parameters are dependent on the mixing unit geometry and fluid phase properties. The correlation of kla with the energy dissipation rate provided a good fit for the experimental data in the bubble flow regime (AARE < 13.9%). The presented experimental data and correlations further characterize LL microreactors, which are part of a toolbox for fine chemical synthesis involving immiscible fluids for applications involving reactive gas–liquid flows.

Experimental characterization of dynamic behavior of single bubble breakage in an agitated tank

Experimental tests for characterizing bubble breakage dynamics in different locations of a stirred tank with a 6-bladed impeller were carried out for a range of impeller Reynolds number (Re) using water as a continuous phase. The dynamic behavior of the bubble from the injection point until arriving at the impeller region was studied by using a high speed imaging method. The deformation behavior, local breakage probability, and the local number of daughter bubbles produced from the injection point until crossing the impeller region were quantified for the whole range of power input. The dominant breakage mechanisms at different locations in the stirred tank were identified and discussed. The results revealed that during the bubble’s motion in the turbulent field, it experiences different scales of deformation depending on the location of the bubble. Highly deformed shapes were observed in the injection and impeller regions due to the bubble’s interaction with turbulent eddies. Three distinct regions between the injection position and the impeller region were observed in which different breakage mechanisms were found to govern the breakage behavior including turbulent fluctuation, shear forces, and bubble collision with the blade. High breakage probability was observed in front of the blade’s tip, with about 90% occurring within a distance of 0.35Ri from the blade’s tip (Ri is the impeller radius). In the impeller region, around 20% of the breakages occurred above the impeller close to the blade’s top, while the breakage percentage below the impeller was very small (2% as a maximum). The breakage probability and the number of daughter bubbles increased with increasing Re with a dependence of 1.3.

Streamwise Aspect Ratio Effects on Turbulent Flow Separations Induced by Forward–Backward-Facing Steps

Abstract Separated and reattached turbulent flows induced by two-dimensional forward–backward-facing steps (FBFS) with different streamwise lengths submerged in a thick turbulent boundary layer (TBL) are investigated using time-resolved particle image velocimetry (TR-PIV). The aspect ratios (AR) of the step range from 1 to 8, and the Reynolds number based on the freestream velocity and step height is 13 200. The thickness of the incoming turbulent boundary layer is 6.5 times the step height. The effects of varying aspect ratio of the steps on the mean flow, principal stretching, Reynolds stresses, triple velocity correlation, two-point autocorrelation, and unsteadiness of turbulent separation bubbles are studied. The results indicate that the mean flow reattaches over the step for FBFS with aspect ratios of 2 and higher. Two local peaks of Reynolds stresses occur irrespective of AR, and for a sufficiently large AR, a third local peak of Reynolds stresses appear in the shear layer emanating from the trailing edge of the bluff bodies. The streamwise decay of Reynolds stresses is slower for smaller AR cases. Incoming coherent structure is strongly disturbed near an inclined edge where principal stretching switches orientation abruptly. The temporal variation of the first proper orthogonal decomposition (POD) mode and reverse flow area over the bluff bodies shows remarkable correlation, which signifies the flapping motion of separation bubble.

Simulation of turbulent bubbly pipe flow with high density ratio and high reynolds number by using the lattice boltzmann method and a multi-phase field model

Direct numerical simulation (DNS) has been widely employed to study the dynamics of turbulent bubbly flows; however, it is used in a limited setting, namely, turbulent bubbly channel flows with low density ratio and low Reynolds number. The present study aims to overcome these limitations and simulate turbulent bubbly pipe flow of a water-air system, a common experimental setting, with high density ratio and Reynolds number. Recently, we developed a cumulant lattice Boltzmann method for two-phase flows and simulated violent two-phase flows with high density ratio and high Reynolds number (Sitompul and Aoki, 2019). In this study, that method is extended by incorporating a multi-phase field model for simulating dispersed bubbles. The proposed method was evaluated by using several cases, namely, 3D bubble rising, turbulent single-phase channel and pipe flows with friction Reynolds number (Reτ≈180,550), turbulent bubbly channel flows with (Reτ≈180) and void fraction (α=1.5%,19.4%), and turbulent bubbly pipe flow with (Reτ=550,α=9.5%). The proposed method can stably simulate the cases, and the results obtained by the proposed method agree well with numerical and experimental results given in the references for given computational domain and grid sizes.

One-equation model to assess population balance kernels in turbulent bubbly flows

A simplified population balance model has been developed to predict the Sauter mean diameter, and to optimize any breakage and coalescence kernel. Firstly, the shortcut model is detailed, and the simplifying assumptions are argued. Then the model is applied in a comparison of 60 combinations of a selection of classical breakage, collision frequency and coalescence efficiency kernels. The models are fitted and then compared with an experimental dataset measured in two different technologies of interest for biotechnology: bubble columns (Gemello et al., 2018), and stirred tanks (Cappello et al., 2020). The best kernels are identified for each flow configuration separately, and some kernels are identified as giving acceptable predictions simultaneously of both flows (average error on bubble size less than 20%).

Turbulent bubbly channel flows: Effects of soluble surfactant and viscoelasticity

Interface-resolved direct numerical simulations are performed to examine the combined effects of soluble surfactant and viscoelasticity on the structure of a bubbly turbulent channel flow. The incompressible flow equations are solved fully coupled with the FENE-P viscoelastic model and the equations governing interfacial and bulk surfactant concentrations. The latter coupling is achieved through a non-linear equation of state which relates the surface tension to the surfactant concentration at the interface. The two-fluid Navier-Stokes equations are solved using a front-tracking method, augmented with a very efficient FFT-based pressure projection method that allows for massively parallel simulations of turbulent flows. It is found that, for the surfactant-free case, bubbles move toward the wall due to inertial lift force, resulting in formation of wall layers and a significant decrease in the flow rate. Conversely, a high-enough concentration of surfactant changes the direction of lateral migration of bubbles, i.e., the contaminated bubbles move toward the core region and spread out across the channel. When viscoelasticity is considered, viscoelastic stresses counteract the Marangoni stresses, promoting formation of bubbly wall-layers and consequently strong decrease in the flow rate. The formation of bubble wall-layers for combined case depends on the interplay of the inertial and elastic, and Marangoni forces.

Measurements of pressure and velocity fluctuations in a family of turbulent separation bubbles

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Radial spreading of turbulent bubble plumes.

Weak bubble plumes carry liquid from the environment upwards and release it at multiple intermediate levels in the form of radial intrusive currents. In this study, laboratory experiments are performed to explore the spreading of turbulent axisymmetric bubble plumes in a liquid with linear density stratification. The thickness, volumetric flowrate and spreading rates of multiple radial intrusions of plume fluid were measured by tracking the movement of dye injected at the source of bubbles. The experimental results are compared with scaling predictions. Our findings suggest that the presence of multiple intrusions reduces their spreading rate, compared to that of a single intrusion. This work is of relevance to the spreading of methane plumes issuing from the seabed in the Arctic Ocean, above methane-hydrate deposits. The slower, multiple spreading favours the presence of methane-rich seawater close to the plume, which may reduce the dissolution of methane in the bubbles, and thus promote the direct transport of methane to the atmosphere. This article is part of the theme issue 'Stokes at 200 (part 2)'.

Structure of a turbulent bubbly flow and heat transfer in a vertical tube

The results of numerical simulation of the flow structure and heat transfer in a vertical polydispersed bubbly flow are presented. The mathematical model is based on the Euler approach taking into account the effect of bubbles on the mean characteristics and turbulence of the carrier phase. The polydispersed distribution of bubbles size in a two-phase flow is modeled by the method of delta approximation considering the process of bubble break-up and coalescence. Carrier phase (fluid) turbulence is predicted using the Reynolds stress transport model. The simulation results showed good agreement with the experimental data presented in the literature. The measured and predicted thermal-hydraulic parameter distribution indicates that in a turbulent bubbly flow, the wall friction increase is greater than heat transfer enhancement.

Interface capturing simulations of bubble population effects in PWR subchannels

As the computational power of high-performance computing (HPC) facilities grows, so too does the feasibility of using first principle based simulation to study turbulent two-phase flows within complex pressurized water reactor (PWR) geometries. Direct numerical simulation (DNS), integrated with an interface capturing method, allows for the collection of high-fidelity numerical data using advanced analysis techniques. The presented research employs the massively parallel, finite-element based, unstructured mesh code, PHASTA, to simulate a set of two-phase bubbly flows through PWR subchannel geometries including auxiliary structures (spacer grids and mixing vanes). The main objective of the presented work is to analyze bubble dynamics and turbulence interactions at varying bubble concentrations to support the development of advanced two-phase flow closure models. Turbulent two-phase flows in PWR subchannels were simulated at hydraulic Reynolds numbers of 81,000 with bubble concentrations of 3%–15% by gas volume fraction (768–3928 resolved bubbles, respectively) and compared against a 1% void fraction case (262 bubbles) that had been previously simulated. The finite element mesh utilized for the study at higher bubble concentrations was composed of 1.55 billion elements, compared to the previous study which employed 1.11 billion elements, ensuring all turbulence scales and individual bubbles within the flow are fully resolved. For each case, the resolved initial bubble size was 0.65 mm in diameter (resolved with 25 grid points across the diameter). The simulations were analyzed to find flow features such as the mean velocity profile, bubble relative velocity and the effect of the bubbles on the turbulent conditions.

Tuft deflection velocimetry: a simple method to extract quantitative flow field information

A novel method capable of assessing flow fields in a quick and relatively simple manner is introduced. In an extension to the classical qualitative flow visualization by means of cotton or polymeric tufts, digital data processing is used to extract the orientation of these tufts. This information can be related to physical quantities, in particular to time- and space-dependent velocity signals. The capability of this method is demonstrated in two test scenarios. First, it is applied to gain information on the unsteady near-wall flow along a turbulent separation bubble. Second, the two-component velocity field in the wake of a generic car model is measured, allowing for a quantification of the recirculation zone dimensions. Validation measurements with conventional techniques, e.g., particle image velocimetry, unsteady pressure measurements and hot wire anemometry, are conducted throughout the study. These generally suggest that the novel approach provides a quick and reasonably good quantitative overview of the flow configurations. However, the measurement error may be substantial in flow regions of low velocity or dominated by high-frequency oscillations.Graphic abstract