Nonlinear interaction and coalescence features of oscillating bubble pairs are investigated experimentally and numerically. The spark technique is used to generate in-phase bubble pairs with similar size and the simulation is performed with the compressible volume of fluid (VOF) solver in OpenFOAM. The initial conditions for the simulation are determined from the reference case, where the interbubble distance is sufficiently large and the spherical shape is maintained at the moment of maximum volume. Although the microscopic details of the coalescing behaviors are not focused, the compressible VOF solver reproduces the important features of the experiment and shows good grid convergence. We systematically investigate the effects of the dimensionless interbubble distance γ (scaled by the maximum bubble radius) and define three different coalescing patterns, namely, coalescence due to the expansion in the first cycle for γ < 1.1 (Pattern I), bubble breaking up and collapsing together with coalescence at the initial rebounding stage for 1.1 < γ < 2.0 (Pattern II), and coalescence of the rebounding toroidal bubbles for 2.0 < γ < 3.65 (Pattern III). For Pattern I, prominent gas flow and velocity fluctuation can be observed in the coalescing region, which may induce the annular protrusion in the middle of the coalesced bubble. For Patterns II and III, migration of the bubbles toward each other during the collapsing and rebounding stages greatly facilitates the bubble coalescence.
The dynamics of a bubble bypassing or passing between spherical obstacles, which is associated with many industrial applications, is investigated numerically. A gas–liquid–solid interaction model is established by combining the lattice Boltzmann method and the immersed boundary method. The deformation and the surface velocity of the bubble, as well as the streamlines of the flow field, are studied as the bubble bypasses a single spherical obstacle or passes between a pair of such obstacles. It is found that for the case of a single sphere, the rise velocity reaches a minimum value at the moment at which an annular bubble forms and the whole sphere is enveloped by the bubble. The initial distance between the bubble and the sphere, as well as the ratio of their sizes, has distinct influences on bubble shape and rise velocity. For a pair of spherical obstacles, the rise velocity of the bubble reaches a minimum value twice as the bubble rises between the obstacles. The distance between the two obstacles has a stronger influence on bubble motion than does their size, although when the two obstacles are of different sizes, the bubble will deviate toward the smaller one.
Flow separation control has a wide application prospect in drag reduction for industry. This paper numerically studies the effect of microstructures on flow separation and drag reduction. Simple morphological microstructures, derived from the tilted shark scales, are attached to the wing at an angle of attack. The spacing and height of microstructures are made dimensionless by using the microstructure width and half of the wing width, respectively, that is, d̃m=dm/dAB and h̃m=hm/(H/2). The angle of attack is set to 10°. It is found that microstructures can reduce the motion amplitude of shed vortices, thereby suppressing flow separation and reducing drag. Both the planar and curved microstructures have excellent drag reduction performance. The microstructure spacing d̃m and tilt angle θ should not be too large or too small; otherwise, it will weaken the drag reduction ability. Cases d̃m=1.51, θ=20°, and θ=30° exhibit excellent drag reduction performance. The microstructure has the characteristic for being small, yet it needs to reach a certain height h̃m to effectively reduce drag. The case h̃m=0.667 is the most superior choice. Based on the proposed microstructure shape and spacing, the drag reduction performance of microstructures can reach more than 28%. Meanwhile, the drag reduction performance of microstructures increases with the improvement of the attachment proportion pm, and case pm≥50% is suggested for significant drag reduction performance. Finally, we discuss the drag reduction performance of microstructures on the wing at different angles of attack and find that microstructures can achieve good drag reduction, provided that the pressure drag caused by the flow separation is a significant proportion of the total drag and the flow separation occurs within the controllable range of microstructures.
Numerical dissipation is ubiquitous in multiphase flow simulation. This paper introduces a phase interface compression term into the recently developed multiphase lattice Boltzmann flux solver and achieves an excellent interface maintenance. Here, the phase interface compression term only works in the interface region and is solved as the flux in finite volume discretization. At each cell interface, the interfacial compression velocity [Formula: see text] is determined by local reconstruction velocities of the multiphase lattice Boltzmann flux solver, which maintains the consistency of the flux evaluation. Meanwhile, the interfacial order parameter C in the phase interface compression term is obtained by the second order upwind scheme according to the interface normal direction. Numerical validation of the present model has been made by simulating the Zalesak problem, the single vortex problem, Rayleigh–Taylor instability, and bubble rising and coalescence. The obtained results indicate the validity and reliability of the present model.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.