Bulletin of the American Physical Society
72nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 64, Number 13
Saturday–Tuesday, November 23–26, 2019; Seattle, Washington
Session P24: Bubbly Flow II |
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Chair: Jack Keeler, University of Manchester Room: 606 |
Monday, November 25, 2019 5:16PM - 5:29PM |
P24.00001: Experimental and numerical study of bubble transport within multiphase cross flow over a cylinder. Eric Thacher, Simo Makiharju Vortex-induced vibration from cross flow over a cylinder is an important design consideration in numerous applications. For single phase flow, this phenomenon has been studied extensively; however, while past researchers have shown that increasing phase fraction decreases vibration amplitude while increasing shedding frequency, the mechanisms causing these changes are not fully understood. Studying individual bubble transport may provide insight, as indicated by Voutsinas et al. (2009) who demonstrated that the frequency shift depends on bubble size. In this work we begin by studying the flow of individual bubbles over a cylinder in cross-flow, to assess the time needed for bubbles to be captured in the shed vortices. Following the method of Oweis et al. (2005), the capture time is predicted using a point-particle tracking model, as a function of bubble size, release position, and flow rate. The numerical results are then verified experimentally using high speed camera visualization of a cylinder in cross flow within a vertical flow loop. The time-resolved transport of a single stream of monodisperse bubbles from a needle and co-flow apparatus is used to assess the impact of capture time on cylinder pressure fluctuations, before expanding the study to higher void fraction flows. -/abstract- Authors: Eric W. Thacher and Simo A. M\ [Preview Abstract] |
Monday, November 25, 2019 5:29PM - 5:42PM |
P24.00002: Modeling and validation of bubble-induced fluctuation in bubbly flows. Jubeom Lee, Hyungmin Park The nature of two-phase turbulence is of a great interest academically and practically. In general, it is decomposed into two contributions; one from the shear-induced turbulence in the absence of bubbles and the other from the bubble-induced turbulence (agitation or fluctuation). Regarding the latter, it is further broken down into non-turbulent (a potential drift) and turbulent (perturbations due to bubble wake) parts. In the present work, we are interested in this bubble-induced fluctuation and suggest a model, which is derived based on a mixing length model (analogous to the single-phase flow turbulence) but considering the effect of neighboring bubble and bubble-induced liquid flow. As a result, we suggest models for turbulent and streamwise normal stresses that includes the contributions from gradients of liquid velocity, void distribution, and bubble velocity. That is, we try to combine the non-turbulent and turbulent parts together. Finally, we validate our models with available data in the literature, including our own, obtained from various flow configurations such as a bubble-swarm, laminar pipe, and bubbly wake flows. We discuss the advantages and limitations of our models. [Preview Abstract] |
Monday, November 25, 2019 5:42PM - 5:55PM |
P24.00003: Some notes on eddy viscosity in wall-bounded turbulent bubbly flows Tian Ma, Yixiang Liao, Dirk Lucas, Andrew Bragg Recently, based on data from DNS, Ma et al. (Phys. Rev. Fluids 2, 034301, 2017) proposed a model for closing the bubble-induced turbulence (BIT) in a typical Euler-Euler two-equation model, which appears to yield improved performance for predicting $k$ and $\varepsilon$ over the previous models. The present study departures from this BIT model and purpose to use the same DNS data to investigate the behavior of the $C_\mu$ constant and standard eddy viscosity definition. It can be shown that $C_\mu$ constant computed using the DNS database has a very different behavior than that in single-phase flow. Checking closely, the deficiency originates from the description of the standard eddy viscosity that is intrinsic to this general hierarchy of Euler-Euler $k-\varepsilon$ type model, hence, cannot be overcome by a more complex correction function for $C_\mu$. Departing from this point, a modification to the definition of the eddy viscosity in bubbly flows is derived for the Euler-Euler two-equation models. We focus on the intermediate region -- a region extended from the core region, where bubble-induced production and dissipation are nearly in balance, and find that the modified model can lead to significantly improved predictions for the mean liquid, when compared with DNS data. [Preview Abstract] |
Monday, November 25, 2019 5:55PM - 6:08PM |
P24.00004: ABSTRACT WITHDRAWN |
Monday, November 25, 2019 6:08PM - 6:21PM |
P24.00005: Numerical two-phase flows study in channels with variable cross-section. Gustavo R. Anjos Bubbles and drops dynamics through capillaries of variable cross-section still remains of considerable importance in two-phase flows. The aim of this work is to investigate two-phase flows found in the cooling of new generation of computer processor units in the scope of the ThermaSMART - Marie-Curie/RISE Consortium. We seek to study numerically the effects of domain boundaries to the bubble dynamics, including change in the film thickness, bubble shape and vortex shedding in channels with variable cross-section using a moving mesh/boundary domain scheme, which dramatically shortens the domain length. Such a scheme moves the computational boundary nodes according to the bubble's center of mass relative to the variable cross-section of a given problem. The new methodology proposed to simulate two-phase flows in variable cross-sectioned channels shows good accuracy to describe interface forces and bubble dynamics in different complex geometries with moving boundaries. [Preview Abstract] |
Monday, November 25, 2019 6:21PM - 6:34PM |
P24.00006: Bubbly flow in upward 90-degree elbow pipe: bubble dispersion and liquid flow structure Hongseok Choi, Hyungmin Park In gas-liquid 2-phase flows, interfacial structure plays an important role in determining the transport characteristics, which changes with geometry and inlet condition. We experimentally investigate change in bubble dynamics and flow structures for gas-liquid bubbly flow in 90-degree bent square pipe, varying mean void fraction up to 3.0{\%}. Continuous phase flows are chosen as laminar (Re $=$ 550) and turbulent flows (Re $=$ 7,000). We acquire the liquid-phase velocity using two-phase PIV technique, while gas-phase velocity and size distribution are measured with high-speed shadowgraphy. In laminar flow, bubbles move much faster than the liquid phase, resulting in a backflow and large recirculation region at the inner wall of the pipe. The size of this region increases with mean void fraction, inducing strong turbulence at the boundary. For turbulent flow, flow structure doesn't show significant change with considered void fraction, but bubble trajectories move from the inner wall to the outer wall as mean void fraction increases. As a result, location of the maximum liquid-phase turbulence changes accordingly. Analysis of the interfacial force balance and mechanism for flow structure change will be discussed further. [Preview Abstract] |
Monday, November 25, 2019 6:34PM - 6:47PM |
P24.00007: Transient bubble dynamics in a constricted Hele-Shaw channel Antoine Gaillard, Jack Keeler, Gr\'egoire Le lay, Gr\'egoire Lemoult, Anne Juel, Alice Thompson, Andrew Hazel We explore the applicability of dynamical systems concepts recently used to study the transition to turbulence in shear flows to other subcritical transitions in fluid mechanics. We are ultimately interested in the subcritical instability of the linearly stable Saffman-Taylor finger in a Hele-Shaw channel, where finite perturbations can initiate complex dynamics for sufficiently large values of the driving parameter. Here, we concentrate on a geometrically-perturbed Hele-Shaw channel which supports multiple stable modes. We experimentally investigate and classify the different time-evolution scenarios of an air bubble of given volume when varying both the flow rate and the initial bubble shape. As the flow rate increases, the bubble exhibits increasingly complex behaviors, including oscillatory deformations and transient explorations of multiple-tipped unstable modes which often lead to bubble breakup, followed by multiple bubble interactions. Besides, we show that long and disordered transients can be observed at large flow rates depending on the level of noise in the system. [Preview Abstract] |
Monday, November 25, 2019 6:47PM - 7:00PM |
P24.00008: The transient journey and eventual fate of an air bubble travelling in a Hele-Shaw channel Jack Keeler, Alice Thompson, Andrew Hazel, Gregoire Lemoult, Gregoire La-Lay, Antoine Galliard, Anne Juel Displacement flow in a Hele-Shaw channel is a canonical problem in fluid mechanics and is an archetypal example of a pattern-forming system. If an air-bubble is placed at the opposite end then it will propagate along the channel and change shape as it does so. Recent experimental results have shown that with the introduction of a depth-perturbation to the bottom of the channel the system exhibits regions of bistability, so that starting from an initially centered bubble a wide range of time-dependent outcomes are possible. For small flow-rates the bubble will settle towards a stable asymmetric state but for larger flow-rates the bubble shape will become increasingly deformed and a large range of transient phenomena is observed, including tip-splitting and oscillatory behaviour. In this talk, we attempt to understand the transition to disorder in this system for a bubble in Hele-Shaw channel by finding invariant solutions, in the form of steady states and periodic orbits, of the governing equations. Inspired by recent developments in the transition to turbulence in shear flow, and using dynamical systems theory, we discuss the idea, that when the flow-rate is large enough the bubble will transiently explore the stable manifolds of weakly unstable edge states of the system. [Preview Abstract] |
Monday, November 25, 2019 7:00PM - 7:13PM |
P24.00009: Instabilities in four-layer gravity-driven Hele-Shaw flow Ahmed Al Brahim, Sigurdur Thoroddsen We study the dynamical rearrangement of gravitationally unstable multi-layer fluid in the narrow vertical gap inside a Hele-Shaw cell. Four layers of immiscible fluids are superposed inside the cell, which is subsequently turned over. The rearrangements are filmed with high-resolution and high-speed video. We vary the fluid properties and relative thicknesses of the layers. The layers in order of increasing density are air, oil, water/glycerin mixture and perfluorohexane. The concentration of the glycerin/water mixture is used to vary its viscosity. We classify various different dynamics of stirring and breakthrough of adjacent layers. We note a prominent phenomenon, where an air-pocket breaks through the high-viscosity layer to erupt into the lower-viscosity perfluorohexane layer above it. We were able to establish that the eruption velocity and the interface before eruption are highly influenced by the viscosity of the glycerin/water mixture. We also find that the thickness of the layers and the locations of the eruption have minor impact on the eruption speed and the interface beyond a specific limit. We investigate the center-of-mass trajectories for each layer and notice counter-flows, where the center of a layer can temporarily move against buoyancy. [Preview Abstract] |
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