Bulletin of the American Physical Society
71st Annual Meeting of the APS Division of Fluid Dynamics
Volume 63, Number 13
Sunday–Tuesday, November 18–20, 2018; Atlanta, Georgia
Session M08: Multiphase Flows: Turbulence II |
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Chair: Bamdad Lessani, South Dakota School of Mines & Technology Room: Georgia World Congress Center B213 |
Tuesday, November 20, 2018 8:00AM - 8:13AM |
M08.00001: Bubble breakup in turbulence Daniel Ruth, Stephane Perrard, Luc Deike We present the design and characterization of an opposing-pumps water turbulence tank used to study the dynamics and breakup of air bubbles in turbulence. The tank is inspired by previous larger-scale setups, but differs from these in that the generated turbulence must be strong enough to break millimeter-scale bubbles. Particle image velocimetry is employed to analyze the flow and optimize the trade-off between turbulence intensity maximization and mean flow minimization. Bubbles are injected at a controlled air flow rate through needles of various sizes and their dynamics and breakup in the turbulence tank are measured with high-speed imagery. We discuss the dynamical breakup of bubbles in turbulence, using the bubbles' three-dimensional trajectories, deformation, and breakup locations. We present data on the breakup threshold, time scale, and child bubble distribution for various injected bubble sizes. |
Tuesday, November 20, 2018 8:13AM - 8:26AM |
M08.00002: The effect of single bubble-cluster in upward bubbly flow on large-scale vortex structure Masahiro Eguchi, Shu Takagi At present, bubbly flow has been used for industrial applications such as reducing frictional resistance by introducing bubbles into the turbulent boundary layer at the bottom of the ship. To utilize air bubbles in high efficiency, we conduct experiments of upward bubbly channel flow in the vertical channel to investigate the interaction of the bubble-cluster formed in the wall shear layer and liquids. At present, it is confirmed that the large-scale vortex disappears under the bubble-cluster formation condition. In addition, we confirmed that vortex fraction begins to decrease around the time when the bubble-cluster passed the test section. Collating experiment with simulation results and analyzing three dimensional velocity field and bubble image, we estimated when and where bubble structure effect liquids. Because the velocity of bubble-cluster exists in wall-shear layer is slower than the advection speed, the bubble-cluster hardly affects large-scale vortex directly. Furthermore, by comparing the transition of the vortices upstream and downstream in the test section, transition and structure of the vortex throughout the whole channel were evaluated. As a results, we suggested the possibility that bubble-cluster inhibits the growth of vortex in the wall shear layer. |
Tuesday, November 20, 2018 8:26AM - 8:39AM |
M08.00003: Formation and dynamics of bubbles generated by turbulent breaking waves Wai Hong Ronald Chan, Michael Dodd, Perry L Johnson, Javier Urzay, Parviz Moin Turbulent breaking waves in oceans entrain air pockets that break up and coalesce to form a polydisperse cloud of bubbles. The size distribution of super-Hinze scale bubbles is compatible with one due to the quasi-steady fragmentation of large air pockets by turbulent eddies in an inertial subrange, but the distribution of sub-Hinze scale bubbles, for which capillary stresses dominate turbulent stresses, remains a subject of active research. In this study, an ensemble of simulations of statistically-unsteady breaking waves is used to investigate the temporal evolution of these distributions. This requires the proper identification of bubbles in each ensemble member at individual time instances. Physical bubbles are distinguished from numerical bubbles by a grouping algorithm that identifies resolved bubbles without clipping their mass. The resulting ensemble-averaged distributions of bubble sizes momentarily exhibit power-law scalings that compare well with experimental findings. The power laws suggest a quasi-steady transfer of bubbles in radius space. A collision detection algorithm is employed to identify impacts between surfaces and investigate rates of breakup and coalescence, which contribute to changes in the relative proportions of bubble sizes over time. |
Tuesday, November 20, 2018 8:39AM - 8:52AM |
M08.00004: Turbulence interactions with large bubbles Alessio Roccon, Giovanni Soligo, Alfredo Soldati The dynamics of large and deformable bubbles in a turbulent channel flow is inves- tigated adopting the Phase Field Method (PFM) to define the distribution of the two phases (carrier and drops) inside the domain. The phase field variable is a marker func- tion defining the local concentration of each of the two phases; it is uniform in the bulk of the phases and it undergoes a smooth transition across the interface. All fluid properties are modelled as proportional to the phase field. A Cahn–Hilliard (CH) equation describes the transport of the phase variable in the entire domain. This CH equation is coupled to the Navier–Stokes (NS) equations via an interfacial term (Korteweg stress tensor). This CH–NS coupled system is solved using a pseudospectral technique based on a Fourier representation of variables in the periodic directions (streamwise and spanwise) and a Chebyshev representation in the wall-normal direction. We performed direct numerical simulations of a turbulent channel flow at a shear Reynolds number of Reτ = 300, laden with large and deformable bubbles (d+=120 w.u.) at a fixed Weber number (We = 0.75). The effect of different density ratios between the bubbles and the carrier fluid on turbulence has been investigated. |
Tuesday, November 20, 2018 8:52AM - 9:05AM |
M08.00005: Surfactant-laden drops in wall-bounded turbulence Giovanni Soligo, Alessio Roccon, Alfredo Soldati The dynamics of large surfactant-laden drops in a turbulent channel flow is investigated using direct numerical simulations. Such a multiphase system is here described using a modified Phase Field Method (PFM), which accounts for both the interface and surfactant dynamics. The phase field variable is a marker function that defines the local concentration of each phase; it is uniform in the bulk of the phases and undergoes a smooth transition across the interface. The transport of both phase field and surfactant concentration is described by two Cahn–Hilliard (CH) equations. Their contribution is included in the Navier–Stokes (NS) equations via an interfacial term. This coupled NS–CH system describes the complex dynamics of surfactant-laden droplets in turbulence. A Fourier-Chebychev pseudo-spectral method is adopted to discretize the coupled system of equations in a closed channel geometry. The presence of surfactant at the interface strongly affects the interfacial dynamics: surfactant locally reduces surface tension (with respect to a clean interface) and can induce Marangoni stresses. We examine the effect of surfactant loading and strength on the dynamics and coalescence/breakup rates of a swarm of large drops in a turbulent channel flow. |
Tuesday, November 20, 2018 9:05AM - 9:18AM |
M08.00006: Mass conservation improved Phase Field Methods for turbulent multiphase flows simulations Gaëlle Leloup, Giovanni Soligo, Alessio Roccon, Alfredo Soldati Phase Field Method (PFM) has emerged as a powerful tool for the simulation of multiphase flows. The method has great potentials for further developments and applications: It has a sound physical basis and when associated with a highly refined grid, physics is accurately rendered. However, in many cases, especially when dealing with turbulent flows, the available computational resources do not allow for a complete resolution of the interfacial phenomena and some undesired effects such as shrinkage, coarsening and misrepresentation of surface tension forces and thermo-physical properties can affect the accuracy of the simulations. Two improved PFM formulations (profile-corrected and flux-corrected) have been specifically developed to overcome the previously mentioned drawbacks. The improved formulations are tested and compared against the classic one, particularly focusing on the drawbacks of the classic formulation. Different benchmarks have been tested, starting from laminar cases up to the more challenging simulation of a droplet-laden turbulent flow. Our aim is to benchmark the different phase field method formulations, with the final goal of laying down useful guidelines for the accurate simu- lation of multiphase turbulent flows with the phase field method. |
Tuesday, November 20, 2018 9:18AM - 9:31AM |
M08.00007: Control and ultrasonic actuation of a coaxial two-fluid atomizer. Peter Dearborn Huck, Nathanael Machicoane, Rodrigo Osuna Orozco, Adam Maxwell, Alberto Aliseda We experimentally investigate the atomization of a laminar fluid stream by a coaxial turbulent air jet with gas Reynolds numbers ranging from 104-105. Multi-physics control strategies are employed to drive droplet spatio-temporal and size distributions are towards a desired objective for feedback control of sprays. We present results from ultrasonic spray manipulation in which six transducers, emitting tone bursts at 2.1 MHz, are aligned axially and attached to the back of the liquid/gas nozzle. The nozzle shape acts as an acoustic coupler and amplifier: the displacements applied at the wide back end are amplified as they travel towards the nozzle throat where the liquid jet atomization is perturbed at ultrasonic frequencies. High-speed shadowgraphy is used to investigate the effect of acoustic forcing on the primary and secondary instabilities in the spray's near-field. Phase Doppler Particle Analysis (PDPA) is employed to measure the droplet size, location, and velocity distributions as the spray structure develops. |
Tuesday, November 20, 2018 9:31AM - 9:44AM |
M08.00008: Spray angle model using Gaussian CDRSV theory Ben Trettel The spray angle of a Newtonian liquid jet injected into still gas is modeled with conditional damped random surface velocity (CDRSV) theory. This theory of turbulent jet breakup hypothesizes that velocity fluctuations at the free surface are the cause of breakup. These fluctuations are treated as Gaussian random variables in this work. Conditional ensemble averages are used to analytically compute quantities of interest because not all fluctuations cause breakup. Differences between CDRSV theory and previous theories are emphasized. The model considers not only the liquid Weber number and density ratio, but also the liquid turbulence intensity and Reynolds stress. The model is compared against an experimental database using only data points with known turbulence intensities to avoid unnecessary uncertainty and to better generalize the model. |
Tuesday, November 20, 2018 9:44AM - 9:57AM |
M08.00009: Break-up instabilities and resulting droplet distributions in a gas-liquid coaxial atomizer combined with electro-spray Nathanael Machicoane, Rodrigo Osuna-Orozco, Peter Dearborn Huck, Alberto Aliseda We present an experimental study that explores the combined physics of gas-assisted atomization and electrosprays, based on a canonical coaxial gas-liquid atomizer. The laminar liquid stream is injected through a long metallic needle at the center of the turbulent gas jet, with gas-to-liquid momentum ratio from 1 to 80. The needle is at high-voltage and the gas nozzle exit is grounded, creating a strong electric field in which the dielectric liquid is charged up to 1-10 C/m3. The relative influence of the high-speed gas to the liquid electric charge on the primary instability and jet break-up is studied, using high speed visualizations in the near field and Phase Doppler Particle Analysis in the mid field. The quantitative visualization captures the fast dynamics of the interface destabilization and shows the changes in the liquid instabilities caused by the electric charge, which control the droplet sizes and their spatial distribution in the spray. We apply an additional electric field along the spray development region, characterizing the ability of an external radial forcing to modify the structure of the electrically-charged spray as it develops, enabling control of the droplet position, velocity and acceleration distributions through modulated charge and external field. |
Tuesday, November 20, 2018 9:57AM - 10:10AM |
M08.00010: Destabilization and breakup of a planar liquid stream assisted by a co-flowing turbulent gas stream Delin Jiang, Stephane Zaleski, Gretar Tryggvason, Yue Ling In a planar gas-liquid mixing layer the destabilization and breakup of the liquid stream is assisted by a co-flowing high speed turbulent gas stream. The velocity difference between the gas and the liquid streams triggers a shear instability at the interface, which develops into an interfacial wave moving downstream. In this study we focus on the effect of the inlet gas turbulence on the interfacial instability, spray formation, and two-phase turbulence statistics. Direct numerical simulations for different gas inlet turbulence intensity levels are performed. A digital filter approach is used to generate temporally and spatially correlated velocity fluctuations at the inlet. The gas-liquid interface is resolved by a volume-of-fluid method. The most unstable frequency is observed to increase significantly with the inlet gas turbulence intensity, which is in agreement with experimental observations. |
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