Bulletin of the American Physical Society
75th Annual Meeting of the Division of Fluid Dynamics
Volume 67, Number 19
Sunday–Tuesday, November 20–22, 2022; Indiana Convention Center, Indianapolis, Indiana.
Session U30: Drops: Instability and Break-up I |
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Chair: Stephane Zaleski, Sorbonne University Room: 238 |
Tuesday, November 22, 2022 8:00AM - 8:13AM |
U30.00001: Instability near the contact line of an evaporating volatile droplet Nayoung Kim, Pallav Kant, Devaraj van der Meer The evaporation of a droplet deposited on a solid surface represents an intriguing system that displays typical complexities present in many free surface and coating flows. For example, when a droplet of aqueous surfactant solution is deposited on a substrate covered with thin water film, it spreads by propagating fingers. A similar fingered spreading can also be observed during the evaporation of a binary droplet or a surfactant-laden droplet deposited on a dry substrate. In contrast, here, we establish that such patterned spreading behaviour is also exhibited by a volatile droplet of a pure substance when deposited on a dry surface. We show that the fingered spreading of an ethanol droplet deposited on a slightly heated surface can be triggered by controlling the relative humidity around the droplet. Notably, the thermal properties of the underlying substrate also play an important role in determining the extent of fingered spreading. Additionally, our investigation explores the sudden shrinkage of the evaporating droplet in its final moment that leaves myriad small droplets behind. |
Tuesday, November 22, 2022 8:13AM - 8:26AM |
U30.00002: Formalizing the effect of Initial Shape of Droplets on the Fragmentation Threshold Aditya Parik, Jeffrey N Fonnesbeck, Tadd Truscott, Som Dutta A Drop undergoes secondary breakup under impulsive acceleration when the initial flow Weber number is greater than its critical Weber number (Wecr). For a spherical droplet, its Wecr is a function of its density ratio (ρ), drop (Ohd) and ambient (Oho) Ohnesorge numbers. Though, in nature droplets are rarely spherical and show free surface oscillations in various modes, with different modes superimposed with the fundamental mode. These initial modes depending on their initial phase can interact with the external forcing either constructively or destructively leading to significant changes in the breakup and deformation process. To illustrate these effects, droplets with different oscillation modes and magnitudes are studied using direct numerical simulations and compared against analogous spherical droplets. Significant changes to both breakup morphology and Wecr are observed. |
Tuesday, November 22, 2022 8:26AM - 8:39AM |
U30.00003: An experimental investigation of droplet morphology and size distribution in swirl flow Kirti C Sahu, Someshwar S Ade, Pavan K Kirar, Lakshmana D Chandrala We investigate the morphology and size distribution of satellite droplets resulting from the interaction of a freely falling water droplet with a swirling airstream of different strengths by employing shadowgraphy and deep-learning-based digital in-line holography techniques. We found that the droplet exhibits vibrational, retracting bag and normal breakup phenomena for the no swirl, low and high swirl strengths for the same aerodynamic field. In the high swirl scenario, the disintegrations of the nodes, rim, and bag-film contribute to the number mean diameter, resulting in smaller satellite droplets. In contrast, in the low swirl case, the breakup of the rim and nodes only contributes to the size distribution, resulting in larger droplets. The temporal variation of the Sauter mean diameter reveals that for a given aerodynamic force, a high swirl strength produces more surface area and surface energy than a low swirl strength. The theoretical prediction of the number-mean probability density of tiny satellite droplets under swirl conditions agrees with experimental data. However, for the low swirl, the predictions differ from the experimental results, particularly due to the presence of large satellite droplets. Our results reveal that the volume-weighted droplet size distribution exhibits two (bi-modal) and three (multi-model) peaks for low and high swirl strengths, respectively. The analytical model that takes into account various mechanisms, such as the nodes, rim, and bag breakups, accurately predicts the shape and characteristic sizes of each mode for the case of high swirl strength. |
Tuesday, November 22, 2022 8:39AM - 8:52AM Not Participating |
U30.00004: Aerobreakup of liquid metal droplet Shubham Sharma, Navin K Chandra, Aloke Kumar, Saptarshi Basu The aerobreakup of liquid metal has application in metal powder production, thermal spray coatings, explosive detonations, metalized propellant combustion, and liquid metal cooling systems. Liquid metals provide several challenges which differ from other common Newtonian liquids (like water, ethanol, and liquid fuels). The applicability of the results obtained from the aerobreakup of common liquids to the atomization of liquid metals is a marginally explored area and is the focus of the present work. Conventional fluids and liquid metals significantly differ in density, surface tension, and viscosities. Further, liquid metal exhibits properties such as surface oxidation, which are not observed in conventional fluid experiments. Here we use a liquid metal alloy (Galinstan) as a viable material, and the aerobreakup results are compared with Newtonian fluids (water and glycerol-water solution). The surface tension of chosen material is 10 times that of water, while its viscosity is ~ 2.4 times that of water. A similar range of Weber number (We) values is tested for Galinstan and Newtonian counterparts, and the focus was kept on understanding the role of surface oxidation on the atomization characteristics. Retardation in the breakup characteristics is observed for liquid metal droplets where a lesser number of secondary droplets (in comparison to Newtonian fluids) with irregular shape are formed. However, for the considered values, a similar breakup mode (Shear-induced entrainment (SIE)) is observed for the Galinstan and Newtonian droplets. |
Tuesday, November 22, 2022 8:52AM - 9:05AM |
U30.00005: Aerobreakup of a polymeric droplet Navin K Chandra, Shubham Sharma, Saptarshi Basu, Aloke Kumar Aerobreakup of a liquid droplet is the process of breaking a droplet into smaller fragments by subjecting it to a high-speed stream of gas. It is encountered in various natural and industrial processes. Aerobreakup of polymeric droplets is of special interest because the properties of any liquid can be engineered by adding a small amount of polymer to control the process of aerobreakup. Polymeric liquids exhibit viscoelastic behavior, and the presence of elasticity in the liquid phase can significantly retard the breakup process. We present a study on the aerobreakup of a polymeric droplet to get a better understanding of the role of liquid elasticity in the breakup process. A wide range of Weber number (∼102–104) and Elasticity number (∼10-4–102) is investigated by subjecting polymeric droplets of different concentrations to the induced airflow behind a moving shockwave. Experiments revealed that elasticity plays an insignificant role in the initial stages of aerobreakup, which involves droplet deformation and the appearance of hydrodynamic instabilities (Kelvin-Helmholtz and Rayleigh-Taylor). The dominant role of elasticity appears in the final stages of aerobreakup in terms of the morphology of the fragmenting liquid mass. |
Tuesday, November 22, 2022 9:05AM - 9:18AM |
U30.00006: Aerodynamic breakup of drops at moderate Weber numbers Taofiq H Mahmood, Yue Ling Aerobreakup of drops is essential to many industry applications such as fuel injection. Due to?the highly complex nature and the wide range of spatial scales involved, both experimental and numerical approaches have their own limitations. In this study, we have investigated the deformation of a water droplet in a uniform air stream through direct numerical simulation. Particular attention is paid at the regime of moderate Weber numbers, for which the drop deforms to a forward bag before breakup eventually occurs.?The open-source?Basilisk solver has been used for simulations. The sharp interface is resolved using a?mass-momentum consistent VOF method on an adaptive octree mesh. When the drop is suddenly exposed to the gas stream,?the drop first deforms to a disk and then an inflating bag with a peripheral rim. The Rayleigh-Taylor instability (RTI) plays a critical role in the bag formation and growth. The competition between baroclinic torque and the capillary retraction from the periphery rim determine whether the bag will be atomized. Since RTI?is closely associated with the drag on the drop and the resulting acceleration of the interface, it is important to accurately resolve the flow around the drop, such as the turbulent wake, so that the unsteady drag and the drop deformation. As the bag undergoes inflation, the liquid sheet thickness decreases over time rapidly. The high-fidelity simulation results showed that rupture of the bag is initiated through hole nucleation which gradually expands and gets collected into a rim which surrounds the hole. The hole-rim is unstable due to another RTI when the hole on the curved sheet expands, forming fingers on the rim from which small drops are detached. |
Tuesday, November 22, 2022 9:18AM - 9:31AM |
U30.00007: Breakup and Evaporation in Shock Driven Multiphase Mixing Vasco O Duke, Jacob A McFarland The Shock Driven Multiphase Instability (SDMI) involves several phenomena that occur concurrently across overlapping length and time scales. At the larger scales, the problem involves turbulent mixing due to acceleration across pressure and density gradients, like the classic Richtmyer-Meshkov Instability; however, the inclusion of effects on larger droplet size results in longer equilibration times. The SDMI involves multiphase phenomena across many length scales, from the mesoscales (cloud-scale) to the microscale (droplet-scale). At the mesoscales, bulk droplet vaporization rates, the lag time of the liquid phase, pressure, and density gradient can be described. Contrary, isolating the problem at the microscale particle-scale mixing can be explained by the droplet breakup and evaporation phenomena at a high Weber number. The concurrent phenomena at these conditions are complex and poorly understood, warranting further research in numerous physical systems such as detonation-driven propulsion engines, liquid-vapor cloud explosions, and atmospheric hypersonic flight. |
Tuesday, November 22, 2022 9:31AM - 9:44AM |
U30.00008: MDPD Simulation of Liquid Thread Break-up and Formation of Droplets Luis H Carnevale, Panagiotis E Theodorakis, Piotr Deuar, Zhizhao Che The breakup of liquid threads is a fundamental process in nature and relevant for many industrial |
Tuesday, November 22, 2022 9:44AM - 9:57AM |
U30.00009: Numerical Investigation of Liquid Droplet Interactions with Cylindrical Bow Shocks Andrew M Hess, David A Kessler, Ryan F Johnson Precipitation and suspended droplets in the atmosphere present a significant erosion risk to vehicles traveling and high-supersonic and hypersonic speeds. We will present the results of simulations considering the interactions of droplets with blunt projectiles in a high-supersonic (Mach 2-4) flow environment. We use a volume of fluid method implemented in the compressibleInterFoam solver of OpenFOAM to simulate the interaction of a water droplet with the bow shock generated by a cylindrical blunt body. The droplet size, droplet position, blunt body diameter, and flow mach number are varied to determine the regimes where direct impact, partial impact, or no impact occurs as governed by the degree of droplet breakup driven by aerodynamic forces in the shock layer. |
Tuesday, November 22, 2022 9:57AM - 10:10AM |
U30.00010: Effect of evaporation on interfacial instabilities of shock-driven droplets Prashant Tarey, Praveen K Ramaprabhu, Jacob A McFarland Instability-driven breakup and droplet evaporation play an important role in liquid fuel droplet combustion. In this work, through detailed numerical simulations of a 2D axisymmetric JP-10 fuel droplet, the effect of evaporation rate on surface instabilities were investigated. The Weber number for all the simulations was set at We = 100, representing flow conditions relevant to fuel droplets in detonation engines. The simulations were performed using IMPACT1, a multiphase shock physics code that employs a 5th order WENO scheme to solve the Euler Equations with Adaptive Mesh Refinement. The interface between the liquid and gaseous phase was tracked using the Level Set method, and a Riemann Ghost Fluid Method integrated with a multi-medium Riemann solver used to couple the two phases. The cases simulated, involved a 10 μm droplet, processed by an impinging shockwave, for different Mach numbers of M=2.0 and M=5.0, with and without evaporation. It was observed that a combination of higher post shock gas density and velocity, obtained at higher Mach numbers, enhanced the growth of thin sheets over the droplet and subsequent droplet breakup. The accumulation of vapor layer near the droplet surface reduced the interfacial shear instabilities. |
Tuesday, November 22, 2022 10:10AM - 10:23AM |
U30.00011: Ferrofluid Drop To Spike Reversible Transition Due To An Approaching Magnet Sachin Kumar K Jain, Utsab Banerjee, Chiranjit Mandal, Ashis K Sen Ferrofluid sessile droplets when exposed to magnetic field may go through surface normal instability, spike-like structures. This phenomenon occurs when the magnetization (M) of the ferrofluid droplet exceeds a critical magnetization (Mc) depending upon the interfacial tension, gravitational potential and the magnetic field strength (B) and its gradient (∇B) experienced by the ferrofluid-air interface. We discovered and showed that depending upon the concentration of magnetic nanoparticles (MNPs) in the ferrofluid, there may exist another reversible transition from spikes back to droplet shape of ferrofluid. The existing theory based on total energy minimization was extended to include the non-uniformity of the magnetic field on the plane perpendicular to the direction of magnetization. The theoretically predicted gap (δ) between the magnet and droplets for the first (δc1) and second (δc2) transitions were in a good agreement with experimental findings. The experimental characteristic wavelength characterized by the area averaged separation distance (sexpt) between the spikes also closely matched with theoretical estimations (λmodel). The instability induced spike formation has many applications such as sub-microlitre sized droplet formation, as object transport vehicle, etc. |
Tuesday, November 22, 2022 10:23AM - 10:36AM |
U30.00012: Numerical Study of the Droplet Bag Breakup Behaviour Kaitao Tang, Thomas A Adcock, Wouter D Mostert We present novel numerical simulations investigating the secondary atomisation of water droplets with a focus on the bag breakup regime, which has important implications for many physical processes, especially for understanding the generation of ocean sprays under high-wind conditions. We solve the two-phase incompressible Navier-Stokes equation using the adaptive mesh refinement (AMR) technique, with interface reconstruction by a volume-of-fluid (VOF) method. We first show good agreement with theoretical predictions at early time, and clarify the viscous effect on drop deformation. Next, in numerical simulations of the bag breakup regime, a bag film is developed at late time and is susceptible to spurious mesh-induced breakup, which has prevented previous numerical studies from reaching grid convergence of fragment statistics. We therefore adapt a recently developed manifold death (MD) algorithm which artificially perforates thin films once they reach a prescribed critical thickness independent of the grid size, and we vary the parameters of the hole perforation mechanism to examine the relative effects of Ohnesorge and Weber numbers on the subsequent fragmentation behaviour of the bag. We show grid convergence of the fragment size distribution when utilising the MD algorithm, and identify the physical mechanisms involved in the fragmentation process. These results highlight the utility of this numerical method for multiphase atomisation problems, and pave the way for physics-based numerical investigations into spume generation at the air-sea interface. |
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