Bulletin of the American Physical Society
76th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 19–21, 2023; Washington, DC
Session T19: Drops: Instability and Break-up I |
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Chair: Stephane Zaleski, Sorbonne Universite Room: 146B |
Monday, November 20, 2023 4:25PM - 4:38PM |
T19.00001: Large droplet breakup in free fall Tadd T Truscott, Aqeel Almanashid, Sandip L Dighe, Spencer S Truman, Dilip K Maity, Nilamani Sahoo, Aditya Parik, Som Dutta Large droplet break-up occurs in many industrial processes and biological systems but is rarely studied in experiments because of the multiple scales and complexity. Further, the phenomenon is made more complex by examining shear thinning fluids. Here, we seek to unravel this complex phenomenon by looking at very large droplet breakup of both Newtonian and non-Newtonian liquids with droplet sizes greater than 30mm. Our experimental setup consists of a free-falling high-speed camera-light system simultaneously with a proprietary large droplet maker. This allows us to analyze droplets in a gravitationally accelerating flow field. Image analysis techniques are used to analyze the breakup in this controlled environment. The breakup of large Newtonian droplets can vary from vibrational breakup at low speeds to bagging at critical speeds. However, large shear thinning droplets have different Ohnesorge numbers which result in sheared droplets and multimode breakup. We show the various breakup types and compare them to smaller droplet breakup studies. |
Monday, November 20, 2023 4:38PM - 4:51PM |
T19.00002: An experimental investigation of droplet morphology in swirl flow Pavan Kumar Kirar, Surendra K Soni, Pankaj S Kolhe, Kirti C Sahu The interaction of a droplet with a swirling airstream is investigated experimentally by shadowgraphy and particle image velocimetry techniques. In swirl flow, the droplet experiences oppose-flow, cross-flow, and co-flow conditions depending on its ejection location, the velocity of the airstream, and swirl strength, which results in distinct droplet morphologies as compared to the straight airflow situation. We observe a new breakup phenomenon, termed as `retracting bag breakup', as the droplet encounters a differential flow field created by the wake of the swirler's vanes and the central recirculation zone in swirl airflow. A regime map demarcating the various modes, such as no breakup, vibrational breakup, retracting bag breakup, and bag breakup modes, is presented for different sets of dimensionless parameters influencing the droplet morphology and its trajectory. In contrast to the straight flow, the swirl flow promotes the development of the Rayleigh-Taylor instability, enhancing the stretching factor in the droplet deformation process, resulting in a larger number of fingers on the droplet's surface. In order to gain physical insight, a modified theoretical analysis based on the Rayleigh-Taylor instability is proposed for the swirl flow. The experimental behaviour of droplet deformation phenomena in swirl flow conditions can be determined by modifying the stretching factor in the theoretical model. |
Monday, November 20, 2023 4:51PM - 5:04PM |
T19.00003: Size distribution of a drop undergoing breakup at moderate Weber numbers Kirti C Sahu, Someshwar S Ade, Pavan K Kirar, Lakshmana D Chandrala We investigated the morphology and size distribution of satellite droplets formed when a water droplet falls freely and interacts with a swirling airstream of varying strengths. We used shadowgraphy and deep-learning-based digital in-line holography techniques to analyse this phenomenon. Our findings indicate that the behavior of the droplet differs based on the strength of the swirling motion, leading to vibrational motion, retracting bag formation, and normal breakup phenomena. These effects occur within the same aerodynamic field, with no swirl, low swirl, and high swirl scenarios. In the high swirl scenario, the disintegration of nodes, rim, and bag film plays a significant role in determining the number mean diameter of the satellite droplets. This results in smaller droplets being produced. Conversely, in the low swirl case, only the breakup of the rim and nodes contributes to the size distribution, leading to the formation of larger droplets. We observed that the temporal variation of the Sauter mean diameter indicates that a high swirl strength generates more surface area and surface energy than a low swirl strength under a given aerodynamic force. Theoretical predictions of the number-mean probability density for tiny satellite droplets under swirl conditions align well with experimental data. However, these predictions differ from the experimental results in the case of low swirl, mainly due to the presence of large satellite droplets. Our research reveals that the volume-weighted droplet size distribution exhibits two (bi-modal) and three (multi-modal) peaks for low and high swirl strengths, respectively. To accurately predict the shape and characteristic sizes of each mode in the case of high swirl strength, we developed an analytical model that considers various mechanisms, including the breakup of nodes, rims, and bags. The analytical model accurately predicts the shape and characteristic sizes of each mode in the case of high swirl strength. Overall, our findings shed light on the intricate dynamics of droplet interaction with swirling airstreams, highlighting the influence of swirl strength on droplet morphology and size distribution. |
Monday, November 20, 2023 5:04PM - 5:17PM |
T19.00004: Droplets on angled fibers: A surprising change to the Rayleigh-Plateau instability Dilip K Maity, Sandip L Dighe, Tadd T Truscott The Rayleigh-Plateau (RP) instability on a thin viscous film flowing down on a fiber (aka droplets on a wire) is a complex phenomenon studied mostly on vertical wires. Here, we reexamine the RP instability by gradually varying the angle of the fiber and the position of the fiber within the nozzle. We observe that the different regime instabilities can stably occur by varying the angle when the flow rate is held constant. Further, for non-angled wires, the same can occur by altering the position of the fiber within the nozzle. The wavelength and velocity of the RP regime are highly sensitive to the angle and position of the fiber. Interestingly, the effect of the position of the fiber within the nozzle becomes negligible as the inclination angle is increased. One important finding is that the effective surface area of the film increases by up to 50% when the angle of the fiber is increased which could be useful in industrial applications where it would allow for more efficient heat or mass transfer. |
Monday, November 20, 2023 5:17PM - 5:30PM |
T19.00005: Escape from pinch-off during contraction of low-viscosity liquid sheets Ajay Harishankar Kumar, Xiao Liu, Hansol Wee, Osman A Basaran In spraying and polymer processing, fluid is ejected from a nozzle or a die as a liquid sheet. The cross-sections of sheets are rectangular in shape but with rounded ends which contract towards each other due to surface tension forces. If sufficiently thin, sheets can rupture due to van der Waals forces. However, it has been shown by Burton and Taborek (PoF, 2007) that a contracting inviscid liquid sheet or a 2D drop can break up even in the absence of vdW forces. Here, we demonstrate that in the presence of small yet finite viscosity, contracting liquid sheets escape from pinch-off when vdW forces are absent. We investigate the problem using 2D free-surface flow simulations and the 1D slender-sheet equations, both of which show an escape from pinch-off. However, the physics underlying escape differs in 1D and 2D: in the former it is due to viscous resistance, but in the latter its cause can be attributed to vorticity generated by the free surface. The latter mechanism can only be observed in 2D simulations as the 1D model is vorticity-free at leading-order. Moreover, these two distinct mechanisms also give rise to different scaling laws relating the minimum sheet thickness when escape occurs to the Ohnseorge number (the ratio viscous stress to inertial and capillary stresses). |
Monday, November 20, 2023 5:30PM - 5:43PM |
T19.00006: Aerobreakup of a liquid metal droplet Shubham Sharma, Navin K Chandra, Aloke Kumar, Saptarshi Basu The current investigation employs Galinstan as the test fluid to explore the gas induced breakup phenomenon of a liquid metal droplet under high Weber number conditions (We ~ 400 - 8000). Three distinct test environments are considered: oxidizing (Galinstan-air), inert (Galinstan-nitrogen), and conventional fluids (DI water-air). In industrial setups, liquid metal atomization usually takes place in inert atmospheres due to their propensity for oxidation. Surprisingly, no previous research has focused on the gas-induced secondary atomization of liquid metals under inert conditions. Typically, laboratory-scale models employ conventional fluids like DI water or liquid fuels to address the challenges related to molten metals. However, there is a need for studies investigating the transferability of results from conventional fluids to liquid metal atomization. |
Monday, November 20, 2023 5:43PM - 5:56PM |
T19.00007: Fragmentation from inertial detachment of a sessile droplet: implications for pathogen transport Naijian N Shen, Y. Kulkarni, T. Jamin, S. Popinet, S. Zaleski, L. Bourouiba Various modes of fluid fragmentation inherent to foliar disease transmission have been linked to the average-wetting dominating most crop leaves (Gilet and Bourouiba 2015). One of such modes is the inertial detachment: Upon impact from rain, irrigation, or dew drops, the motion of a compliant leaf locally transfers its impulse to the sessile contaminated drop residing on it. The resulting fragmentation of the sessile contaminated drop is particularly interesting for the application domain for its ability to produce highly contaminated ejected droplets not undergoing any dilution and typically producing a primary tip drop. Inertial detachment is also interesting as a fundamental fragmentation process on its own merit, in which it is the asymmetric stretching under impulsive axial forcing that shapes the fragmentation of the initially sessile drop. Although the related filament end-pinching phenomenon is well studied, the wetting at the liquid-solid interface significantly complicates the dynamics. Nevertheless, in a controlled analog experimental system to that of leaves, we find that the radius, $R_{tip}$, of the tip drop ejected for $Bo>1$ become insensitive to the Bond number value itself. Here, the Bond number quantifies the inertial effects via the relative axial impulsive acceleration compared to capillary effects. This insensitivity to the Bond number is also recovered for the time of tip drop breakup, $t_{tip}$. In a combined experimental and theoretical study we investigate these results, to decipher what sets the primary drop radius and its sensitivity to the surface-wetting and foot dynamics on the substrate. Asymptotic theory in the large $Bo$ limit for which the thin-film/slender-jet approximation holds and a reduced physical model enable further insights, including prediction of the properties of the ejected primary tip drop, consistent with experiments. Combined with numerical simulations which are developed and validated against the experiments and theory, the results also enable to shed light on how physical properties of pathogens (e.g., their wetting) within the sessile contaminated drop affect their distribution in the primary tip drop and secondary drops. |
Monday, November 20, 2023 5:56PM - 6:09PM |
T19.00008: RNN modeling for time-varying aerodynamic deformation and drag for a droplet at moderate Weber numbers Taofiq Hasan H Mahmood, Amanullah Kabir Tonmoy, Chad Sevart, Yi Wang, Yue Ling Aerodynamic deformation and breakup of droplets in gas flows are important in various natural and industrial applications. When an initially stationary droplet is subjected to a gas stream, the competition between destabilizing gas dynamic pressure and resisting surface tension is characterized by the Weber number (We). The present study is focused on the range of We lower than the critical threshold; as a result, the droplet does not break but rather undergoes deformation in an oscillatory manner. The droplet deformation influences the drag coefficient, which in turn plays a significant role in determining the trajectory of the droplet. In this study, 2D axisymmetric interface-resolved simulations were performed to resolve the droplet deformation and dynamics, using a mass-momentum consistent VOF method. The instantaneous drop shape is then decomposed into spherical harmonic modes. The time evolutions of the first ten spherical harmonic modes, the droplet centroid velocity, and the physical parameters are used to train the Non-linear Auto-Regressive with eXogenous input Neural Network (NARXNN) model. A NARXXNN model is first built to capture the time evolution of the drop deformation, and then the predicted mode coefficients and the drop velocity are used to train the second model for the drag. An excellent agreement between the model predictions and the simulation results is achieved. |
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