Session X35: Multiphase Flows: General III

Development of dual-plane observation techniques of shock wave interaction with solid objects

A better understanding of the effects of shock wave interaction with materials and objects commonly found in natural and urban environments is highly desirable, to facilitate both prevention and mitigation of blast damage. This study focuses on the effects of the shock wave on solid objects made from different materials and placed in a variety of geometric formations. These formations were impacted with a planar shock wave at Mach numbers ranging from 1.2 to 2.0. High-speed visualization of the flow carrying debris takes place in two perpendicular planes simultaneously, allowing tracking of individual debris pieces. Pressure readings both upstream and downstream of the initial object location were recorded with four transducers. We compare our laboratory observations with historical results from large-scale explosions, such as declassified nuclear tests, to develop a scaling framework applicable to both.   

Analysis of the equilibrium Eulerian approximation of particle velocity in a spherical blast

The explosive dispersal of particles is an important multiphase physics of considerable interest in many applications. Calculating the particle velocity within these blasts can be costly, however, since it must be calculated separate from the gas velocity, and the two phases can only be assumed to have the same velocity when particles are sufficiently small. However, by using the equilibrium Eulerian approximation, particle velocity can be approximated as a function of gas velocity, acceleration, and the particle time scale [1]. The appropriateness of the equilibrium Eulerian approximation depends on the Stokes number of the particles, which in the present flows varies over both space and time. Several simulations were performed using the in-house code HyBurn, where an Euler-Euler approach was used to solve the gas and particle phase governing equations [2]. The effect of four different parameters were tested: the particle size, initial particulate volume fraction, initial higher pressure, and initial temperature of the high-pressure region. These parameters were studied to examine in which regime the dusty gas and equilibrium Eulerian assumptions are appropriate as these factors would play a significant part in the calculation of Stokes Number.   

Exploring Aerosol-Gas Dynamics in Fluidic Oscillator Ventilation

The reality of infectious disease and the threat of biological and chemical weapons—all of which can cause respiratory depression or failure—have become more apparent in the wake of COVID-19. Battlefield resuscitation and pre-hospital care utilize a bag valve mask (BVM) to provide positive pressure ventilation for patients with inadequate breaths, though this tool is not ideal due to its inconsistency and heavy resource load. A fluidic logic oscillating ventilator (FLOV) is a pocket-sized device that achieves the same goal while self-sustaining within safe breathing parameters. The source of energy is a constant motive flow passing through a constricting nozzle and further into a region of expansion. "Searching vortices" form due to instabilities in this expansion field and they "pick" a direction for aiming the bulk flow. This results in a natural oscillation between the patient and exhaust. This technology holds great potential in pre-hospital care if droplets are introduced to the flow, as aerosol inhalation therapy can be used to administer many life-saving medications. Investigations were conducted to learn the gas' effects on droplets (feed-forward) and the droplets' effects on the carrier gas (feedback) in compressible transitional flows. While feed-forward is expected and will affect the exit aerosol distribution, it is the feedback effect that might result in unintended negative consequences in FLOV performance. Variables such as droplet size, flow rate, and numerical parameters were considered. It was found that there was a threshold of droplet size below which the droplets did not modify the vortices, but the feedback threshold was affected by droplet concentration. We aim to produce a "safety" map to help users define a safe operating range for administering treatment via the FLOV.   

Characterization of the swirling bubbly flow and its application to oil transportationCharacterization of the swirling bubbly flow and its application to oil transportation

We experimentally characterized the swirling bubbly flow, induced by the swirling blade, in a circular pipe with an inner diameter (D) of 70 mm. At the superficial water velocities of 0.1-2.0 m/s, we injected air (up to 20 LPM) through a sparger to generate millimeter-scale bubbles before the blade. The experiment was carried out in both vertical and horizontal pipes to examine the evolution and decay of bubble swirls under various conditions. Additionally, we tested different blade design by adjusting the rotating angle and number. In a horizontal pipe, the two-dimensional void fraction increased as the flow develops away from the blade, indicating a decrease in swirling intensity. However, after the distance of 4D, void fraction decreased because of the dominant buoyancy effect over the swirling. In a vertical pipe, void fraction notably decreased when the superficial velocity of the water exceeds a critical value (~ 0.6 m/s), which depends on the blade design and air superficial velocity. Based on the further investigation of the swirling morphology and strength, we finally introduced the oil in the pipe and examined its transportation performance with the swirling bubbly flow.   

Computation of the flow due to entrainment for a sphere settling in stratified fluids

Understanding particle sedimentation in stratified fluids is important in many environmental applications. In particular, we study the cases with long residence times at the density transition layer, similar to the ones of marine snow aggregates in the ocean. A sphere's motion exhibits complex mechanics when falling through a stable stratification of miscible, viscous fluids, due to the deformation of the entrained boundary layer. Following a first principles model, we develop a numerical scheme to compute the stratification-induced flow. Properties of the Oseen tensor and its approximations near and far from the sphere are used to improve computation of the flow. We provide a convergence analysis and comparison of the model with experimental data.   

Role of bubble deformation in interfacial instability across a two-layer miscible liquid

We conducted experiments to investigate interfacial phenomena when soft/rigid bodies cross a two-layer miscible stratified liquid, focusing on the role of bubble deformation. We used water and water-glycerol mixtures at the upper (μ_u = 1 mPa·s, ρ_u = 998 kg/m³) and lower (μ_l = 91 - 872 mPa·s, ρ_l = 1021 - 1257 kg/m³) liquids, respectively, in which the cap bubble (R_cap = 30 - 35 mm) and a rigid hollow sphere rise due to buoyancy at the speed of 0.3 - 0.4 and 0.2 - 0.9 m/s, respectively. According to the viscosity ratio (μ_l/μ_u = 97.3 - 933), we analyzed how the interfacial phenomena on the surface of soft and rigid bodies grow. When μ_l/μ_u = 97.3, bulges were generated on the bubble surface and the Kelvin-Helmholtz (KH) instability was triggered due to bubble deformation. At a higher viscosity ratio (436 and 933), however, the bulges could not grow into the roll-up waves. Without the surface deformation (rigid body), no bulges were generated in all viscosity ratios. Instead, the roll-up waves directly occurred at the surface of the rigid sphere when the rising velocity is as fast as 0.7 m/s, of which the wavelength becomes longer with increasing the viscosity ratio. We will explain more detail on this difference.   

Modulation of the path instability of buoyancy-driven swimming sheet.

Recently, the buoyancy-driven, low-energy swimming sheet robot (artificial bubble) has been reported, and here we focus on its path instability analogous to that of rising bubbles, to improve its controllability. For such a sheet-shaped body, the stability of the motion is affected by not only the geometry but the formation of an unstable wake flow. We systematically varied key parameters governing this path instability, such as moment of inertia, Reynolds number, and thickness-to-width ratio. Using shadowgraphy and particle image velocimetry, we measured the three-dimensional trajectory, velocity, and wake flow of the buoyant sheets, and tried to establish the geometric conditions for stable swimming at the Reynolds numbers up to 3,000. We also analyze the hydrodynamic forces acting on the body in the generalized Kirchhoff equations to understand the mechanism of identified conditions.   

Turbulence effect on the bubble-particle collision efficiency

Understanding bubble-particle collisions is a crucial aspect of the flotation process. Here, we employed numerical methods to investigate the impact of turbulence on collision efficiency between a fully contaminated bubble and small inertial particles in a moderately turbulent flow. The bubble is represented by a rigid sphere moving at a constant velocity through homogeneous and isotropic turbulence at a Taylor Reynolds number of 32 and a bubble Reynolds number of 120, using the immersed boundary method to fully resolve the coupling between the bubble and the fluid. The small particles were modeled as point particles, and the particle Stokes number ranges from 0.01 to 5.2. We observe that collisions in the turbulent flow occurred for particles coming from a significantly wider region ahead of the bubble. This led to a remarkable enhancement in the collision efficiency of up to 100% compared to the results in the quiescent flow. Additionally, we found that the critical collision angle increased for all considered particle Stokes numbers due to turbulent fluctuation. To elucidate these findings, we proposed a statistical model based on turbulent sweeping which involves the results obtained in a quiescent flow. The model well predicts the collision probability in the incoming flow region. It turns out that the collision is enhanced by the turbulent fluctuation which temporarily enhances the velocity of fluid sweeping through the bubble, as well as the temporary bubble Reynolds number.   

Regime transitions in drag reduction by air lubrication

Friction drag accounts on average for approximately 70% of the overall resistance of a ship, and thus a large part of a typical ship’s propulsive power is required to overcome it. To reduce this drag air layer lubrication techniques have been proposed and investigated over the past years. These techniques could lead to fuel cost savings and a lower environmental impact. While drag reduction measurements using a variety of injectors and flow conditions have been performed by different research groups to date, no reliable scaling laws are available. Depending on the liquid freestream velocity, the air injection rate, and the injection method, different air phase regimes are commonly observed: a bubbly regime, a transitional regime and an air layer regime. The onset of the air layer regime is particularly important since this is the desired regime in terms of drag reduction. The present study aims to investigate regime transitions via comparing experimental data from various experimental facilities resulting in a wide range of Reynolds and Froude numbers. The imaging procedure to acquire and quantify these data is described and preliminary conclusions are drawn on the important parameters taking into account the real-life application of this technology.   

Morphology of surfactant-free and surfactant-laden droplets in homogeneous isotropic turbulence

We perform direct numerical simulations of surfactant-free and surfactant-laden droplets in statistically-stationary homogeneous isotropic turbulence at moderate Taylor Reynolds number, Re_λ≈180. The volume of fluid method is used to track the interface, while the soluble surfactant is transported using an advection-diffusion equation. Surfactants reduce surface tension at the interface according to their local concentration; surface tension gradients along the interface, generated by a inhomogeneous surfactant concentration, induce Marangoni stresses acting tangential to the interface. In our simulation setup, we find the average surface tension reduction to be the dominant effect, allowing us to extend the Kolmogorov-Hinze (KH) theory to surfactant-laden droplets: we compute the KH scale by using a lower, averaged value of surface tension. We characterize the morphology of the droplets, and find two different regimes for the interfacial area of each droplet. The interfacial area of droplets smaller than the KH scale is proportional to d², while that of larger droplets is proportional to d³, with d being the characteristic size of the droplet. The two different regimes originate from the shape of the droplets: droplets smaller than the KH scale take regular, spheroid-like shapes, whereas droplets larger than the KH scale take long, coiled, filamentous shapes.   

Modelling of interfacial flows with surfactants: micelle formation and interfacial viscosity effects

Surfactant-laden multiphase flow is crucial in all kinds of applications because of the presence of surfactants which can be either as desirable additives or undesirable contaminants. Many types of physical behaviours can occur such as the reduction of the local value of surface tension, the generation of Marangoni stress, the endowment of the interface with shear and dilatational viscosities, micelle formation, the interaction with solid substrates and modification of contact line motion. Most of the numerical frameworks that are able to deal with surfactant-laden systems are based on many assumptions and consider, or prioritise, certain limiting situations, e.g., insoluble surfactant, concentrations below the critical micelle concentration, and the absence of interfacial viscous effects. There is therefore a need for a generalised model that can encompass all of the aforementioned effects. Here, we present the progress we have made in the development of such a model and provide several flow examples motivated by industrial and environmental applications.   

Evaporating Water Droplets on Hydrophobic Surfaces: Influence of Insoluble Surfactants

When a water droplet is deposited on a hydrophobic, thermally conductive substrate, the evaporation rate is maximum at the apex, resulting in lower temperatures at the top. Consequently, density and surface tension gradients emerge within the droplet and at its surface, giving rise to flows from the apex towards the contact line and vice versa, respectively. In small droplets, capillary forces (thermal Marangoni effects) are expected to dominate over buoyancy forces ("Rayleigh effects''). However, contrary to theoretical predictions, in recent experiments dominance of the circulation from the apex to the contact line was observed, indicating prevailing Rayleigh convection. Furthermore, the same experiments showed an unexpected asymmetric flow that persisted for several minutes. We hypothesize that a small amount of insoluble surfactants, which may arise from dust particles or experimental imperfections, reduces the capillary effects, thereby promoting the dominance of Rayleigh convection. Our Finite Element numerical simulations demonstrate that, under the experiment's conditions, a mere 0.5% reduction in the initial surface tension caused by surfactants leads to a reversal in the flow direction compared to the theoretical prediction without surfactants. Additionally, we investigate the stability of the solutions obtained under azimuthal perturbation at a specific mode, revealing that the presence of surfactants also affects the axisymmetry of the flow.