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Moise, Pradeep, i Joseph Mathew. "Bubble and conical forms of vortex breakdown in swirling jets". Journal of Fluid Mechanics 873 (24.06.2019): 322–57. http://dx.doi.org/10.1017/jfm.2019.401.
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Experimental investigations of laminar swirling jets had revealed a new form of vortex breakdown, named conical vortex breakdown, in addition to the commonly observed bubble form. The present study explores these breakdown states that develop for the Maxworthy profile (a model of swirling jets) at inflow, from streamwise-invariant initial conditions, with direct numerical simulations. For a constant Reynolds number based on jet radius and a centreline velocity of 200, various flow states were observed as the inflow profile’s swirl parameter $S$ (scaled centreline radial derivative of azimuthal velocity) was varied up to 2. At low swirl ($S=1$) a helical mode of azimuthal wavenumber $m=-2$ (co-winding, counter-rotating mode) was observed. A ‘swelling’ appeared at $S=1.38$, and a steady bubble breakdown at $S=1.4$. On further increase to $S=1.5$, a helical, self-excited global mode ($m=+1$, counter-winding and co-rotating) was observed, originating in the bubble’s wake but with little effect on the bubble itself – a bubble vortex breakdown with a spiral tail. Local and global stability analyses revealed this to arise from a linear instability mechanism, distinct from that for the spiral breakdown which has been studied using Grabowski profile (a model of wing-tip vortices). At still higher swirl ($S=1.55$), a pulsating type of bubble breakdown occurred, followed by conical breakdown at 1.6. The latter consists of a large toroidal vortex confined by a radially expanding conical sheet, and a weaker vortex core downstream. For the highest swirls, the sheet was no longer conical, but curved away from the axis as a wide-open breakdown. The applicability of two classical inviscid theories for vortex breakdown – transition to a conjugate state, and the dominance of negative azimuthal vorticity – was assessed for the conical form. As required by the former, the flow transitioned from a supercritical to subcritical state in the vicinity of the stagnation point. The deviations from the predictions of the latter model were considerable.
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SOTIROPOULOS, FOTIS, DONALD R. WEBSTER i TAHIRIH C. LACKEY. "Experiments on Lagrangian transport in steady vortex-breakdown bubbles in a confined swirling flow". Journal of Fluid Mechanics 466 (10.09.2002): 215–48. http://dx.doi.org/10.1017/s0022112002001271.
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In a recent study, Sotiropoulos et al. (2001) studied numerically the chaotic particle paths in the interior of stationary vortex-breakdown bubbles that form in a closed cylindrical container with a rotating lid. Here we report the first experimental verification of these numerical findings along with new insights into the dynamics of vortex-breakdown bubbles. We visualize the Lagrangian transport within the bubbles using planar laser-induced fluorescence (LIF) and show that even though the flow fields are steady – from the Eulerian standpoint – the spatial distribution of the dye tracer varies continuously, and in a seemingly random manner, over very long observation intervals. This finding is consistent with the arbitrarily long šil'nikov transients of upstream-originating orbits documented numerically by Sotiropoulos et al. (2001). Sequences of instantaneous LIF images also show that the steady bubbles exchange fluid with the outer flow via random bursting events during which blobs of dye exit the bubble through the spiral-in saddle. We construct experimental Poincaré maps by time-averaging a sufficiently long sequence of instantaneous LIF images. Ergodic theory concepts (Mezić & Sotiropoulos 2002) can be used to formally show that the level sets of the resulting time-averaged light intensity field reveal the invariant sets (unmixed islands) of the flow. The experimental Poincaré maps are in good agreement with the numerical computations. We apply this method to visualize the dynamics in the interior of the vortex-breakdown bubble that forms in the wake of the first bubble for governing parameters in the steady, two-bubble regime. In striking contrast with the asymmetric image obtained for the first bubble, the time-averaged light intensity field for the second bubble is remarkably axisymmetric. Numerical computations confirm this finding and further reveal that the apparent axisymmetry of this bubble is due to the fact that orbits in its interior exhibit quasi-periodic dynamics. We argue that this stark contrast in dynamics should be attributed to differences in the swirl-to-axial velocity ratio in the vicinity of each bubble. By studying the bifurcations of a simple dynamical system, with manifold topology resembling that of a vortex-breakdown bubble, we show that sufficiently high swirl intensities can stabilize the chaotic orbits, leading to quasi-periodic dynamics.
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Jasikova, Darina, Petr Schovanec, Michal Kotek i Vaclav Kopecky. "Comparison of cavitation bubbles evolution in viscous media". EPJ Web of Conferences 180 (2018): 02038. http://dx.doi.org/10.1051/epjconf/201818002038.
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There have been tried many types of liquids with different ranges of viscosity values that have been tested to form a single cavitation bubble. The purpose of these experiments was to observe the behaviour of cavitation bubbles in media with different ranges of absorbance. The most of the method was based on spark to induced superheat limit of liquid. Here we used arrangement of the laser-induced breakdown (LIB) method. There were described the set cavitation setting that affects the size bubble in media with different absorbance. We visualized the cavitation bubble with a 60 kHz high speed camera. We used here shadowgraphy setup for the bubble visualization. There were observed time development and bubble extinction in various media, where the size of the bubble in the silicone oil was extremely small, due to the absorbance size of silicon oil.
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Schovanec, Petr, Darina Jasikova, Michal Kotek, Karel Havlicek, Magda Nechanicka, Jakub Eichler, Jiri Cech i Petra Subrtova. "Sterilization of Biofilm in Foam Using a Single Cavitation Bubble". MATEC Web of Conferences 328 (2020): 05003. http://dx.doi.org/10.1051/matecconf/202032805003.
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This article presents the sterilization of bacteria using cavitation bubbles. Cavitation generated by ultrasound creates a cavitation cloud. Therefore is more advantageous to generate the cavitation bubbles by laser-induced breakdown, because it is possible to generate individual bubbles for the purpose of study single impact and physical mechanism of acting. The cavitation bubble is generated by a Nd: YAG 532nm laser beam, a short 10ns pulse. Here, we used optics to focus the laser beam and a high-speed camera to visualize characteristics the bubble. We used the method of long-distance microscopy and shadowgraph lightening for the visualization. We used the particle image velocimetry (PIV) method to determine the interaction of the bubble with the surrounding liquid and solid surface. The main goal of the research is to use cavitation to sterilize bacteria and biofilm in impact of single bubble collapse on living cells.
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Амромин, Э. Л. "О происхождении цепочек каверн во вращающемся потоке между цилиндрами". Журнал технической физики 91, nr 11 (2021): 1645. http://dx.doi.org/10.21883/jtf.2021.11.51523.119-21.
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Cavitation between rotating and immobile cylinders appears in the form of a regular chain of bubbles. The bubble sizes are practically equal, as well as the distances between the bubbles and their azimuthal locations. Though such a form of cavitation has been observed in numerous experiments (in particular, in the experiments with bearings), its nature was not clarified. The presented analysis shows that breakdown of the flow axial symmetry due to displacement of the axis of one of cylinders leads to the regular wave-similar three-dimensional flow perturbations. Their “wavelength” is predetermined by the minimal gap between cylinders. Though the flow between cylinders is not curl-free, these perturbations can be determined with the use of a velocity potential.
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Amromin E.L. "On the origin of chains of cavities in the rotating flow between cylinders". Technical Physics 67, nr 14 (2022): 2184. http://dx.doi.org/10.21883/tp.2022.14.55216.119-21.
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Cavitation between rotating and immobile cylinders appears in the form of a regular chain of bubbles. The bubble sizes are practically equal, as well as the distances between the bubbles and their azimuthal locations. Though such a form of cavitation has been observed in numerous experiments (in particular, in the experiments with bearings), its nature was not clarified. The presented analysis shows that breakdown of the flow axial symmetry due to displacement of the axis of one of cylinders leads to the regular wave-similar three-dimensional flow perturbations. Their "wavelength" is predetermined by the minimal gap between cylinders. Though the flow between cylinders is not curl-free, these perturbations can be determined with the use of a velocity potential. Keywords: Cavitation, circular cylinders, misaligned cylinders, non-viscous flow.
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BILLANT, PAUL, JEAN-MARC CHOMAZ i PATRICK HUERRE. "Experimental study of vortex breakdown in swirling jets". Journal of Fluid Mechanics 376 (10.12.1998): 183–219. http://dx.doi.org/10.1017/s0022112098002870.
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The goal of this study is to characterize the various breakdown states taking place in a swirling water jet as the swirl ratio S and Reynolds number Re are varied. A pressure-driven water jet discharges into a large tank, swirl being imparted by means of a motor which sets into rotation a honeycomb within a settling chamber. The experiments are conducted for two distinct jet diameters by varying the swirl ratio S while maintaining the Reynolds number Re fixed in the range 300<Re<1200. Breakdown is observed to occur when S reaches a well defined threshold Sc≈1.3–1.4 which is independent of Re and nozzle diameter used. This critical value is found to be in good agreement with a simple criterion derived in the same spirit as the first stage of Escudier & Keller's (1983) theory. Four distinct forms of vortex breakdown are identified: the well documented bubble state, a new cone configuration in which the vortex takes the form of an open conical sheet, and two associated asymmetric bubble and asymmetric cone states, which are only observed at large Reynolds numbers. The two latter configurations differ from the former by the precession of the stagnation point around the jet axis in a co-rotating direction with respect to the upstream vortex flow. The two flow configurations, bubble or cone, are observed to coexist above the threshold Sc at the same values of the Reynolds number Re and swirl parameter S. The selection of breakdown state is extremely sensitive to small temperature inhomogeneities present in the apparatus. When S reaches Sc, breakdown gradually sets in, a stagnation point appearing in the downstream turbulent region of the flow and slowly moving upstream until it reaches an equilibrium location. In an intermediate range of Reynolds numbers, the breakdown threshold displays hysteresis lying in the ability of the breakdown state to remain stable for S<Sc once it has taken place. Below the onset of breakdown, i.e. when 0<S<Sc, the swirling jet is highly asymmetric and takes the shape of a steady helix. By contrast above breakdown onset, cross-section visualizations indicate that the cone and the bubble are axisymmetric. The cone is observed to undergo slow oscillations induced by secondary recirculating motions that are independent of confinement effects.
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Gould, John. "Build me up to break me down: frothed spawn in the sandpaper frog, Lechriodus fletcheri, is formed by female parents and later broken down by their offspring". Australian Journal of Zoology 67, nr 3 (2019): 153. http://dx.doi.org/10.1071/zo20038.
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Several genera of anuran amphibians deposit their eggs within mucous secretions that have been aerated by the parents to produce a foam or bubble spawn body. This is a dynamic medium for embryo development given that it gradually breaks down over time, and one that has been hypothesised to serve a variety of purposes including protecting embryos from external stresses, such as suboptimal temperatures, desiccation and predation. In this study, I provide additional details of bubble spawn production in the sandpaper frog, Lechriodus fletcheri. Field and laboratory observations showed that females aerate spawn while in inguinal amplexus, using flanged fingers to transport air bubbles into the mucous. While the frothed spawn is initially resistant to breakdown, it gradually loses bubbles and flattens out into a film. This temporal shift in structure is likely to be adaptive, as the resultant increase in surface area allows embryos to come in direct contact with the open water, which may accommodate their increased oxygen demands or ease extrication from the mass. I provide evidence that this process is controlled by the residing embryos, given that spawn in the absence of embryos does not break down, highlighting the ability of offspring to modify their immediate environment even before hatching occurs to ensure conditions remain suitable for their changing needs.
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Falbo, Paolo, i Rosanna Grassi. "Market Dynamics When Agents Anticipate Correlation Breakdown". Discrete Dynamics in Nature and Society 2011 (2011): 1–33. http://dx.doi.org/10.1155/2011/959847.
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The aim of this paper is to analyse the effect introduced in the dynamics of a financial market when agents anticipate the occurrence of a correlation breakdown. What emerges is that correlation breakdowns can act both as a consequence and as a triggering factor in the emergence of financial crises rational bubbles. We propose a market with two kinds of agents: speculators and rational investors. Rational agents use excess demand information to estimate the variance-covariance structure of assets returns, and their investment decisions are represented as a Markowitz optimal portfolio allocation. Speculators are uninformed agents and form their expectations by imitative behavior, depending on market excess demand. Several market equilibria result, depending on the prevalence of one of the two types of agents. Differing from previous results in the literature on the interaction between market dynamics and speculative behavior, rational agents can generate financial crises, even without the speculator contribution.
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Rajamanickam, Kuppuraj, i Saptarshi Basu. "Insights into the dynamics of conical breakdown modes in coaxial swirling flow field". Journal of Fluid Mechanics 853 (22.08.2018): 72–110. http://dx.doi.org/10.1017/jfm.2018.549.
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The main idea of this paper is to understand the fundamental vortex breakdown mechanisms in the coaxial swirling flow field. In particular, the interaction dynamics of the flow field is meticulously addressed with the help of high fidelity laser diagnostic tools. Time-resolved particle image velocimetry (PIV) (${\sim}1500~\text{frames}~\text{s}^{-1}$) is employed in $y{-}r$ and multiple $r{-}\unicode[STIX]{x1D703}$ planes to precisely delineate the flow dynamics. Experiments are carried out for three sets of co-annular flow Reynolds number $Re_{a}=4896$, 10 545, 17 546. Furthermore, for each $Re_{a}$ condition, the swirl number ‘$S_{G}$’ is varied independently from $0\leqslant S_{G}\leqslant 3$. The global evolution of flow field across various swirl numbers is presented using the time-averaged PIV data. Three distinct forms of vortex breakdown namely, pre-vortex breakdown (PVB), central toroidal recirculation zone (CTRZ; axisymmetric toroidal bubble type breakdown) and sudden conical breakdown are witnessed. Among these, the conical form of vortex breakdown is less explored in the literature. In this paper, much attention is therefore focused on exploring the governing mechanism of conical breakdown. It is should be interesting to note that, unlike other vortex breakdown modes, conical breakdown persists only for a very short band of $S_{G}$. For any small increase/decrease in $S_{G}$ beyond a certain threshold, the flow spontaneously reverts back to the CTRZ state. Energy ranked and frequency-resolved/ranked robust structure identification methods – proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD) respectively – are implemented over instantaneous time-resolved PIV data sets to extract the dynamics of the coherent structures associated with each vortex breakdown mode. The dominant structures obtained from POD analysis suggest the dominance of the Kelvin–Helmholtz (KH) instability (axial $+$ azimuthal; accounts for ${\sim}80\,\%$ of total turbulent kinetic energy, TKE) for both PVB and CTRZ while the remaining energy is contributed by shedding modes. On the other hand, shedding modes contribute the majority of the TKE in conical breakdown. The frequency signatures quantified from POD temporal modes and DMD analysis reveal the occurrence of multiple dominant frequencies in the range of ${\sim}10{-}400~\text{Hz}$ with conical breakdown. This phenomenon may be a manifestation of high energy contribution by shedding eddies in the shear layer. Contrarily, with PVB and CTRZ, the dominant frequencies are observed in the range of ${\sim}20{-}40~\text{Hz}$ only. We have provided a detailed exposition of the mechanism through which conical breakdown occurs. In addition, the current work explores the hysteresis (path dependence) phenomena of conical breakdown as functions of the Reynolds and Rossby numbers. It has been observed that the conical mode is not reversible and highly dependent on the initial conditions.
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SOTIROPOULOS, FOTIS, i YIANNIS VENTIKOS. "The three-dimensional structure of confined swirling flows with vortex breakdown". Journal of Fluid Mechanics 426 (10.01.2001): 155–75. http://dx.doi.org/10.1017/s0022112000002342.
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In a recent experimental study, Spohn, Mory & Hopfinger (1998) investigated in detail the flow in a closed cylindrical container with a rotating bottom for Reynolds numbers in the steady and unsteady regimes. Their visualization photographs revealed that the stationary vortex breakdown bubbles, which form along the container axis within a range of governing parameters, are open, with inflow and outflow, and asymmetric at their downstream end. For Reynolds numbers within the unsteady regime, visualizations of the limiting streamlines on the cylindrical wall showed that the Stewartson layer separates asymmetrically along stationary spiral convergence lines that form below the top cover. We study numerically the container flow, by solving the unsteady, three-dimensional Navier–Stokes equations, in order to clarify the origin and elucidate the underlying physics of these complex, three-dimensional flow features. The stationary vortex breakdown bubbles we simulate exhibit all the asymmetries observed in the laboratory. By analysing the Lagrangian characteristics of the calculated flow fields, we explain the origin of these asymmetries, clarify the experimentally documented filling and emptying mechanisms, and show that the flow in the interior of stationary vortex breakdown bubbles exhibits chaotic particle paths. We also show that the spiral separation lines observed by Spohn et al. (1998) inside the Stewartson layer at high Reynolds numbers are due to the growth of pairs of counter-rotating, spiral vortices and the interaction of these vortices with the stationary-cover boundary layer.
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SERRE, E., i P. BONTOUX. "Vortex breakdown in a three-dimensional swirling flow". Journal of Fluid Mechanics 459 (25.05.2002): 347–70. http://dx.doi.org/10.1017/s0022112002007875.
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Time-dependent swirling flows inside an enclosed cylindrical rotor–stator cavity with aspect ratio H/R = 4, larger than the ones usually considered in the literature, are studied. Within a certain range of governing parameters, vortex breakdown phenomena can arise along the axis. Very recent papers exhibiting some particular three-dimensional effects have stimulated new interest in this topic. The study is carried out by a numerical resolution of the three-dimensional Navier–Stokes equations, based on high-order spectral approximations in order to ensure very high accuracy of the solutions.The first transition to an oscillatory regime occurs through an axisymmetric bifurcation (a supercritical Hopf bifurcation) at Re = 3500. The oscillatory regime is caused by an axisymmetric mode of centrifugal instability of the vertical boundary layer and the vortex breakdown is axisymmetric, being composed of two stationary bubbles. For Reynolds numbers up to Re = 3500, different three-dimensional solutions are identified. At Re = 4000, the flow supports the k = 5 mode of centrifugal instability. By increasing the rotation speed to Re = 4500, the vortex breakdown evolves to an S-shaped type after a long computational time. The structure is asymmetric and gyrates around the axis inducing a new time-dependent regime. At Re = 5500, the structure of the vortex breakdown is more complex: the upper part of the structure takes a spiral form. The maximum rotation speed is reached at Re = 10000 and the flow behaviour is now chaotic. The upper structure of the breakdown can be related to the spiral-type. Asymmetric flow separation on the container wall in the form of spiral arms of different angles is also prominent.
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Meliga, Philippe, François Gallaire i Jean-Marc Chomaz. "A weakly nonlinear mechanism for mode selection in swirling jets". Journal of Fluid Mechanics 699 (16.04.2012): 216–62. http://dx.doi.org/10.1017/jfm.2012.93.
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AbstractGlobal linear and nonlinear bifurcation analysis is used to revisit the spiral vortex breakdown of nominally axisymmetric swirling jets. For the parameters considered herein, stability analyses single out two unstable linear modes of azimuthal wavenumber $m= \ensuremath{-} 1$ and $m= \ensuremath{-} 2$, bifurcating from the axisymmetric breakdown solution. These modes are interpreted in terms of spiral perturbations wrapped around and behind the axisymmetric bubble, rotating in time in the same direction as the swirling flow but winding in space in the opposite direction. Issues are addressed regarding the role of these modes with respect to the existence, mode selection and internal structure of vortex breakdown, as assessed from the three-dimensional direct numerical simulations of Ruith et al. (J. Fluid Mech., vol. 486, 2003, pp. 331–378). The normal form describing the leading-order nonlinear interaction between modes is computed and analysed. It admits two stable solutions corresponding to pure single and double helices. At large swirl, the axisymmetric solution bifurcates to the double helix which remains the only stable solution. At low and moderate swirl, it bifurcates first to the single helix, and subsequently to the double helix through a series of subcritical bifurcations yielding hysteresis over a finite range of Reynolds numbers, the estimated bifurcation threshold being in good agreement with that observed in the direct numerical simulations. Evidence is provided that this selection is not to be ascribed to classical mean flow corrections induced by the existence of the unstable modes, but to a non-trivial competition between harmonics. Because the frequencies of the leading modes approach a strong $2$:$1$ resonance, an alternative normal form allowing interactions between the $m= \ensuremath{-} 2$ mode and the first harmonics of the $m= \ensuremath{-} 1$ mode is computed and analysed. It admits two stable solutions, the double helix already identified in the non-resonant case, and a single helix differing from that observed in the non-resonant case only by the presence of a slaved, phase-locked harmonic deformation. On behalf of the finite departure from the $2$:$1$ resonance, the amplitude of the slaved harmonic is however low, and the effect of the resonance on the bifurcation structure is merely limited to a reduction of the hysteresis range.
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Rovig, J. "THE EVOLUTION OF STABLE FOAM AS A DRILLING MEDIUM". APPEA Journal 36, nr 1 (1996): 557. http://dx.doi.org/10.1071/aj95033.
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In the late 1960s Stable Foam was developed by Chevron USA as a lightweight circulation medium to clean out production sand in depleted (depleting) wells. In employing this new medium they found that Stable Foam's compressible bubble structure provided up to 10 times the carrying capacity of many common liquid based circulating fluids.These early successes led the industry to expand the use of Stable Foam from cased hole production clean outs to drill-ins, gravel packing, drilling in lost circulation zones, coil tubing, and today its utilisation as an Underbalanced Drilling fluid in depleted reservoirs.Through this evolution Stable Foam has had its successes and failures as well as unique operational concerns which has affected its consideration and utilisation.The following is a breakdown of the major hurdles which have been addressed and overcome, consequently positioning Stable Foam as a viable option for wider use in Underbalanced Drilling:Foam compatibility in water, oil, salt and high temperature environments.MWD and Directional Drilling performance with a compressible fluid.Surface pressure control while drilling.Surface separation and processing of a three phase well return fluid to provide a zero discharge closed loop circuit.
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COULL, JOHN D., i HOWARD P. HODSON. "Unsteady boundary-layer transition in low-pressure turbines". Journal of Fluid Mechanics 681 (1.07.2011): 370–410. http://dx.doi.org/10.1017/jfm.2011.204.
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This paper examines the transition process in a boundary layer similar to that present over the suction surfaces of aero-engine low-pressure (LP) turbine blades. This transition process is of significant practical interest since the behaviour of this boundary layer largely determines the overall efficiency of the LP turbine. Modern ‘high-lift’ blade designs typically feature a closed laminar separation bubble on the aft portion of the suction surface. The size of this bubble and hence the inefficiency it generates is controlled by the transition between laminar and turbulent flow in the boundary layer and separated shear layer. The transition process is complicated by the inherent unsteadiness of the multi-stage machine: the wakes shed by one blade row convect through the downstream blade passages, periodically disturbing the boundary layers. As a consequence, the transition to turbulence is multi-modal by nature, being promoted by periodic and turbulent fluctuations in the free stream and the inherent instabilities of the boundary layer. Despite many studies examining the flow behaviour, the detailed physics of the unsteady transition phenomena are not yet fully understood. The boundary-layer transition process has been studied experimentally on a flat plate. The opposing test-section wall was curved to impose a streamwise pressure distribution typical of modern high-lift LP turbines over the flat plate. The presence of an upstream blade row has been simulated by a set of moving bars, which shed wakes across the test section inlet. Further upstream, a grid has been installed to elevate the free-stream turbulence to a level believed to be representative of multi-stage LP turbines. Extensive particle imaging velocimetry (PIV) measurements have been performed on the flat-plate boundary layer to examine the flow behaviour. In the absence of the incoming bar wakes, the grid-generated free-stream turbulence induces relatively weak Klebanoff streaks in the boundary layer which are evident as streamwise streaks of low-velocity fluid. Transition is promoted by the streaks and by the inherent inflectional (Kelvin–Helmholtz (KH)) instability of the separation bubble. In unsteady flow, the incoming bar wakes generate stronger Klebanoff streaks as they pass over the leading edge, which convect downstream at a fraction of the free-stream velocity and spread in the streamwise direction. The region of amplified streaks convects in a similar manner to a classical turbulent spot: the leading and trailing edges travel at around 88% and 50% of the free-stream velocity, respectively. The strongest disturbances travel at around 70% of the free-stream velocity. The wakes induce a second type of disturbance as they pass over the separation bubble, in the form of short-span KH structures. Both the streaks and the KH structures contribute to the early wake-induced transition. The KH structures are similar to those observed in the simulation of separated flow transition with high free-stream turbulence by McAuliffe & Yaras (ASME J. Turbomach., vol. 132, no. 1, 2010, 011004), who observed that these structures originated from localised instabilities of the shear layer induced by Klebanoff streaks. In the current measurements, KH structures are frequently observed directly under the path of the wake. The wake-amplified Klebanoff streaks cannot affect the generation of these structures since they do not arrive at the bubble until later in the wake cycle. Rather, the KH structures arise from an interaction between the flow disturbances in the wake and localised instabilities in the shear layer, which are caused by the weak Klebanoff streaks induced by the grid turbulence. The breakdown of the KH structures to small-scale turbulence occurs a short time after the wake has passed over the bubble, and is largely driven by the arrival of the wake-amplified Klebanoff streaks from the leading edge. During this process, the re-attachment location moves rapidly upstream. The minimum length of the bubble occurs when the strongest wake-amplified Klebanoff streaks arrive from the leading edge; these structures travel at around 70% of the free-stream velocity. The bubble remains shorter than its steady-flow length until the trailing edge of the wake-amplified Klebanoff streaks, travelling at 50% of the free-stream velocity, convect past. After this time, the reattachment location moves aft on the surface as a consequence of a calmed flow region which follows behind the wake-induced turbulence.
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Sarkar, S., i Peter R. Voke. "Large-Eddy Simulation of Unsteady Surface Pressure Over a Low-Pressure Turbine Blade due to Interactions of Passing Wakes and Inflexional Boundary Layer". Journal of Turbomachinery 128, nr 2 (1.02.2005): 221–31. http://dx.doi.org/10.1115/1.2137741.
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The unsteady pressure over the suction surface of a modern low-pressure (LP) turbine blade subjected to periodically passing wakes from a moving bar wake generator is described. The results presented are a part of detailed large-eddy simulation (LES) following earlier experiments over the T106 profile for a Reynolds number of 1.6×105 (based on the chord and exit velocity) and the cascade pitch to chord ratio of 0.8. The present LES uses coupled simulations of cylinder for wake, providing four-dimensional inflow conditions for successor simulations of wake interactions with the blade. The three-dimensional, time-dependent, incompressible Navier-Stokes equations in fully covariant form are solved with 2.4×106 grid points for the cascade and 3.05×106 grid points for the cylinder using a symmetry-preserving finite difference scheme of second-order spatial and temporal accuracy. A separation bubble on the suction surface of the blade was found to form under the steady state condition. Pressure fluctuations of large amplitude appear on the suction surface as the wake passes over the separation region. Enhanced receptivity of perturbations associated with the inflexional velocity profile is the cause of instability and coherent vortices appear over the rear half of the suction surface by the rollup of shear layer via Kelvin-Helmholtz (KH) mechanism. Once these vortices are formed, the steady-flow separation changes remarkably. These coherent structures embedded in the boundary layer amplify before breakdown while traveling downstream with a convective speed of about 37% of the local free-stream speed. The vortices play an important role in the generation of turbulence and thus to decide the transitional length, which becomes time dependent. The source of the pressure fluctuations on the rear part of the suction surface is also identified as the formation of these coherent structures. When compared with experiments, it reveals that LES is worth pursuing as an understanding of the eddy motions and interactions is of vital importance for the problem.
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Bodstein, Gustavo C. R., Albert R. George i C. Y. Hui. "The three-dimensional interaction of a streamwise vortex with a large-chord lifting surface: theory and experiment". Journal of Fluid Mechanics 322 (10.09.1996): 51–79. http://dx.doi.org/10.1017/s0022112096002704.
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The three-dimensional vortex flow that develops around a close-coupled canard-wing configuration is characterized by a strong interaction between the vortex generated at the canard and the aircraft wing. In this paper, a theoretical potential flow model is devised to uncover the basic structure of the pressure and velocity distributions on the wing surface. The wing is modelled as a semi-infinite lifting-surface set at zero angle of attack. It is assumed that the vortex is a straight vortex filament, with constant strength, and lying in the freestream direction. The vortex filament is considered to be orthogonal to the leading-edge, passing a certain height over the surface. An incompressible and steady potential flow formulation is created based on the three-dimensional Laplace's equation for the velocity potential. The boundary-value problem is solved analytically using Fourier transforms and the Wiener-Hopf technique. A closed-form solution for the velocity potential is determined, from which the velocity and pressure distributions on the surface and a vortex path correction are obtained. The model predicts an anti-symmetric pressure distribution along the span in region near the leading-edge, and a symmetric pressure distribution downstream from it. The theory also predicts no vertical displacement of the vortex, but a significant lateral displacement. A set of experiments is carried out to study the main features of the flow and to test the theoretical model above. The experimental results include helium-soap bubble and oil-surface flow pattern visualization, as well as pressure measurements. The comparison shows good agreement only for a weak interaction case, whereas for the case where the interaction is strong, secondary boundary-layer separation and vortex breakdown are observed to occur, mainly owing to the strong vortex-boundary layer interaction. In such a case the model does not agree well with the experiments.
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Althaus, W., E. Krause, J. Hofhaus i M. Weimer. "Vortex breakdown: Transition between bubble- and spiral-type breakdown". Meccanica 29, nr 4 (grudzień 1994): 373–82. http://dx.doi.org/10.1007/bf00987572.
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Qadri, Ubaid Ali, Dhiren Mistry i Matthew P. Juniper. "Structural sensitivity of spiral vortex breakdown". Journal of Fluid Mechanics 720 (27.02.2013): 558–81. http://dx.doi.org/10.1017/jfm.2013.34.
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AbstractPrevious numerical simulations have shown that vortex breakdown starts with the formation of a steady axisymmetric bubble and that an unsteady spiralling mode then develops on top of this. We investigate this spiral mode with a linear global stability analysis around the steady bubble and its wake. We obtain the linear direct and adjoint global modes of the linearized Navier–Stokes equations and overlap these to obtain the structural sensitivity of the spiral mode, which identifies the wavemaker region. We also identify regions of absolute instability with a local stability analysis. At moderate swirls, we find that the $m= - 1$ azimuthal mode is the most unstable and that the wavemaker regions of the $m= - 1$ mode lie around the bubble, which is absolutely unstable. The mode is most sensitive to feedback involving the radial and azimuthal components of momentum in the region just upstream of the bubble. To a lesser extent, the mode is also sensitive to feedback involving the axial component of momentum in regions of high shear around the bubble. At an intermediate swirl, in which the bubble and wake have similar absolute growth rates, other researchers have found that the wavemaker of the nonlinear global mode lies in the wake. We agree with their analysis but find that the regions around the bubble are more influential than the wake in determining the growth rate and frequency of the linear global mode. The results from this paper provide the first steps towards passive control strategies for spiral vortex breakdown.
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Rusak, Zvi. "Axisymmetric swirling flow around a vortex breakdown point". Journal of Fluid Mechanics 323 (25.09.1996): 79–105. http://dx.doi.org/10.1017/s0022112096000857.
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The structure of an axisymmetric and inviscid swirling flow around a vortex breakdown point is analysed. The model assumes that a free axisymmetric bubble surface is developed in the flow with a stagnation point at its nose. The classical Squire-Long equation for the stream function ψ(x,y) (where y = r2/2) is transformed into a free boundary problem for the solution of y(x, ψ). The development of the flow is studied in three regions: the approaching flow ahead of the bubble, around the bubble nose and around the separated bubble surface. Asymptotic expansions are constructed to describe the flow ahead of and behind the stagnation point in terms of the radial distance from the vortex axis and from the bubble surface, respectively. In the intermediate region around the stagnation point, the flow is approximated by an asymptotic series of similarity terms that match the expansions in the other regions. The analysis results in two possible matching processes. Analytical expressions are given for the leading term of the intermediate expansion for each of these processes. The first solution describes a swirling flow around a constant-pressure bubble surface, over which the flow is stagnant. The second solution represents a swirling flow around a pressure-varying bubble surface, where the flow expands along the bubble nose. In both solutions, the bubble nose has a parabolic shape, and both exist only when H’ > 0 (where H’ is the derivative at the vortex centre of the total head H with the stream function ψ, and can be determined from the inlet conditions). This result is shown to be equivalent to Brown & Lopez's (1990) criterion for vortex breakdown. Good agreement is found in the region around the stagnation point between the pressure-varying bubble solution and available experimental data for axisymmetric vortex breakdown.
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Guo, Xu, Ying Sun, Chen-Lei Liu, Lin Jing, Yuan-Tao Zhang, Xiao-Long Wang i Igor Timoshkin. "The guiding effect of artificially injected gas bubble on the underwater pulsed spark discharge and its electrical and acoustic parameters after breakdown". Physics of Plasmas 29, nr 11 (listopad 2022): 113504. http://dx.doi.org/10.1063/5.0122080.
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The presence of a low density area is beneficial to the facilitation of the underwater pulsed spark discharge, which can be achieved by artificially injecting gas bubble in between the inter-electrode gap. The generation of intensive acoustic waves by such gas-bubble-guided spark discharges makes them promising underwater acoustic sources in multiple practical applications. This paper is aimed at comprehensive investigation of the guiding effect of the injected bubble on the pre-breakdown process of underwater pulsed spark discharges and potential correlations between their subsequent electrical and acoustic parameters with the purpose of optimizing the acoustic emission. The breakdown probability and the pre-breakdown delay were used to evaluate the general facilitation effect brought by the injected bubble. Experimental and numerical works have been conducted and allow observation on the dynamics of the injected bubble under the influence of the applied voltage. Different guiding modes of the injected bubble for plasma streamers' propagation have been observed regarding its relative position. The characteristics of the electrical properties of gas-bubble-guided spark discharges, including the plasma resistance and the plasma energy density, were analyzed by relating them with the breakdown voltage. The dependency of the acoustic wave amplitude and the acoustic efficiency on these electrical parameters was verified, which provides solid regulation principles for the optimization of the plasma-acoustic system for target practical applications.
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Hummel, Mathias, Christoph Garth, Bernd Hamann, Hans Hagen i Kenneth I. Joy. "Illustrative Visualization of a Vortex Breakdown Bubble". Computer Graphics Forum 30, nr 1 (24.02.2011): 235–36. http://dx.doi.org/10.1111/j.1467-8659.2010.01850.x.
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Koide, Teruaki, i Hide S. Koyama. "Vortex Breakdown in a Differentially Rotating Cylindrical Container". Journal of Fluids Engineering 127, nr 2 (1.03.2005): 358–66. http://dx.doi.org/10.1115/1.1852482.
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Numerical studies are reported on the vortex breakdown in a differentially-rotating cylindrical container in which the top endwall rotates at a high angular velocity Ωt and the cylinder and bottom endwall rotate at a low angular velocity Ωsb. Critical boundaries and the location and size of the vortex breakdown bubble are quite different from the case when the top endwall rotates and the cylinder and the bottom endwall are stationary. As |Ωsb/Ωt| is increased, the breakdown bubble moves toward downstream for Ωsb/Ωt<0, whereas the bubble moves toward upstream for Ωsb/Ωt>0. The Brown and Lopez criterion is extended to a differentially-rotating container.
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Pérez-Torró, Rafael, i Jae Wook Kim. "A large-eddy simulation on a deep-stalled aerofoil with a wavy leading edge". Journal of Fluid Mechanics 813 (17.01.2017): 23–52. http://dx.doi.org/10.1017/jfm.2016.841.
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A numerical investigation on the stalled flow characteristics of a NACA0021 aerofoil with a sinusoidal wavy leading edge (WLE) at chord-based Reynolds number $Re_{\infty }=1.2\times 10^{5}$ and angle of attack $\unicode[STIX]{x1D6FC}=20^{\circ }$ is presented in this paper. It is observed that laminar separation bubbles (LSBs) form at the trough areas of the WLE in a collocated fashion rather than uniformly/periodically distributed over the span. It is found that the distribution of LSBs and their influence on the aerodynamic forces is strongly dependent on the spanwise domain size of the simulation, i.e. the wavenumber of the WLE used. The creation of a pair of counter-rotating streamwise vortices from the WLE and their evolution as an interface/buffer between the LSBs and the adjacent fully separated shear layers are discussed in detail. The current simulation results confirm that an increased lift and a decreased drag are achieved by using the WLEs compared to the straight leading edge (SLE) case, as observed in previous experiments. Additionally, the WLE cases exhibit a significantly reduced level of unsteady fluctuations in aerodynamic forces at the frequency of periodic vortex shedding. The beneficial aerodynamic characteristics of the WLE cases are attributed to the following three major events observed in the current simulations: (i) the appearance of a large low-pressure zone near the leading edge created by the LSBs; (ii) the reattachment of flow behind the LSBs resulting in a decreased volume of the rear wake; and, (iii) the deterioration of von-Kármán (periodic) vortex shedding due to the breakdown of spanwise coherent structures.
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Naumov, Igor V., i Irina Yu Podolskaya. "Topology of vortex breakdown in closed polygonal containers". Journal of Fluid Mechanics 820 (5.05.2017): 263–83. http://dx.doi.org/10.1017/jfm.2017.211.
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The topology of vortex breakdown in the confined flow generated by a rotating lid in a closed container with a polygonal cross-section geometry has been analysed experimentally and numerically for different height/radius aspect ratios $h$ from 0.5 to 3.0. The locations of stagnation points of the breakdown bubble emergence and corresponding Reynolds numbers were determined experimentally and numerically by STAR-CCM+ computational fluid dynamics software for square, pentagonal, hexagonal and octagonal cross-section configurations. The flow pattern and velocity were observed and measured by combining seeding particle visualization and laser Doppler anemometry. The vortex breakdown size and position on the container axis were identified for Reynolds numbers ranging from 500 to 2800 in steady flow conditions. The obtained results were compared with the flow structure in the closed cylindrical container. The results allowed revealing regularities of formation of the vortex breakdown bubble depending on $Re$ and $h$ and the cross-section geometry of the confined container. It was found in a diagram of $Re$ versus $h$ that reducing the number of cross-section angles from eight to four shifts the breakdown bubble location to higher Reynolds numbers and a smaller aspect ratio. The vortex breakdown bubble area for octagonal cross-section was detected to correspond to the one for the cylindrical container but these areas for square and cylindrical containers do not overlap in the entire range of aspect ratio.
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Nath, A., i A. Khare. "Measurement of charged particles and cavitation bubble expansion velocities in laser induced breakdown in water". Laser and Particle Beams 26, nr 3 (8.08.2008): 425–32. http://dx.doi.org/10.1017/s0263034608000438.
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AbstractThe measurement of charged particles and cavitation bubble expansion velocity is reported in a laser induced breakdown in water using beam deflection set-up. Effect of laser power on charged particles, cavitation bubble velocities and higher order bubble oscillations is also studied.
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Nault, Isaac, Sorin M. Mitran, Georgy Sankin i Pei Zhong. "Multiscale model of cavitation bubble formation and breakdown". Journal of the Acoustical Society of America 136, nr 4 (październik 2014): 2192. http://dx.doi.org/10.1121/1.4899947.
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Spall, Robert E. "Transition from spiral‐ to bubble‐type vortex breakdown". Physics of Fluids 8, nr 5 (maj 1996): 1330–32. http://dx.doi.org/10.1063/1.868902.
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Althaus, W., E. Krause, J. Hofhaus i M. Weiner. "Bubble- and spiral-type breakdown of slender vortices". Experimental Thermal and Fluid Science 11, nr 3 (październik 1995): 276–84. http://dx.doi.org/10.1016/0894-1777(95)00050-v.
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Meunier, P., i K. Hourigan. "Mixing in a vortex breakdown flow". Journal of Fluid Mechanics 731 (14.08.2013): 195–222. http://dx.doi.org/10.1017/jfm.2013.226.
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AbstractIn this paper we present experimental and theoretical results on the mixing inside a cylinder with a rotating lid. The helical flow that is created by the rotation of the disc is well known to exhibit a vortex breakdown bubble over a finite range of Reynolds numbers. The mixing properties of the flow are analysed quantitatively by measuring the exponential decay of the variance as a function of time. This homogenization time is extremely sensitive to the asymmetries of the flow, which are introduced by tilting the rotating or the stationary disc and accurately measured using particle image velocimetry (PIV). In the absence of vortex breakdown, the homogenization time is strongly decreased (by a factor of 10) with only a moderate tilt angle of the rotating lid (of the order of $1{5}^{\circ } $). This phenomenon can be explained by the presence of small radial jets at the periphery which create a strong convective mixing. A simple model of exchange flow between the periphery and the bulk correctly predicts the scaling laws for the homogenization time. In the presence of vortex breakdown, the scalar is trapped inside the vortex breakdown bubble, and thus increases substantially the time needed for homogenization. Curiously, the tilt of the rotating lid has a weak effect on the mixing, but a small tilt of the stationary disc (of the order of ${2}^{\circ } $) strongly decreases (by a factor of 10) the homogenization time. Even more surprising is that the homogenization time diverges when the size of the bubble vanishes. All of these features are recovered by applying the Melnikov theory to calculate the volume of the lobes that exit the bubble. It is the first time that this technique has been applied to a three-dimensional stationary flow with a non-axisymmetric perturbation and compared with experimental results, although it has been applied often to two-dimensional flows with a periodic perturbation.
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Mununga, L., D. Lo Jacono, J. N. Sørensen, T. Leweke, M. C. Thompson i K. Hourigan. "Control of confined vortex breakdown with partial rotating lids". Journal of Fluid Mechanics 738 (29.11.2013): 5–33. http://dx.doi.org/10.1017/jfm.2013.596.
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AbstractExperiments were conducted to determine the effectiveness of controlling vortex breakdown in a confined cylindrical vessel using a small rotating disk, which was flush-mounted into the opposite endwall to the rotating endwall driving the primary recirculating flow. The results show that the control disk, with relatively little power input, can modify the azimuthal and axial flow significantly, changing the entire flow structure in the cylinder. Co-rotation was found to precipitate vortex breakdown onset whereas counter-rotation delays it. Furthermore, for the Reynolds-number range over which breakdown normally exists, co-rotation increases the bubble radial and axial dimensions, while shifting the bubble in the upstream direction. By contrast, counter-rotation tends to reduce the size of the bubble, or completely suppress it, while shifting the bubble in the downstream direction. These effects are amplified substantially by the use of larger control disks and higher rotation ratios. A series of numerical simulations close to the onset Reynolds number reveals that the control disk acts to generate a rotation-rate-invariant local positive or negative azimuthal vorticity source away from the immediate vicinity of the control disk but upstream of breakdown. Advection of this source along streamlines modifies the strength of the azimuthal vorticity ring, which effectively controls whether the flow reverses on the axis, and thus, in turn, whether vortex breakdown occurs. The vorticity source generated by the control disk scales approximately linearly with rotation ratio and cubically with disk diameter; this allows the observed variation of the critical Reynolds number to be approximately predicted.
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Nath, A., i A. Khare. "Transient evolution of multiple bubbles in laser induced breakdown in water". Laser and Particle Beams 29, nr 1 (22.12.2010): 1–9. http://dx.doi.org/10.1017/s0263034610000662.
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AbstractPulsed laser induced plasma in water produces multiple bubbles with the passage of laser pulse. Shadowgraphy and beam deflection set-up is used to study the temporal and spatial evolution of these bubbles as a function of distance from the laser focus. The formation of multiple bubbles, bubble coalescence, and their effect onto cavity dynamics is reported. Bubble radius and the corresponding velocities from shadowgraphy is used to calculate the maximum gas pressure inside the bubble using Neppiras model. The maximum pressure inside the cavity is found to be 0.4 MPa at the laser focus.
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BRØNS, M., M. C. THOMPSON i K. HOURIGAN. "Dye visualization near a three-dimensional stagnation point: application to the vortex breakdown bubble". Journal of Fluid Mechanics 622 (10.03.2009): 177–94. http://dx.doi.org/10.1017/s0022112008005107.
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An analytical model, based on the Fokker–Planck equation, is constructed of the dye visualization expected near a three-dimensional stagnation point in a swirling fluid flow. The model is found to predict dye traces that oscillate in density and position in the meridional plane in which swirling flows are typically visualized. Predictions based on the model are made for the steady vortex breakdown bubble in a torsionally driven cylinder and compared with computational fluid dynamics predictions and experimental observations. Previous experimental observations using tracer visualization techniques have suggested that even for low-Reynolds-number flows, the steady vortex breakdown bubble in a torsionally driven cylinder is not axisymmetric and has an inflow/outflow asymmetry at its tail. Recent numerical and theoretical studies show that the asymmetry of the vortex breakdown bubble, and consequently its open nature, can be explained by the very small imperfections that are present in any experimental rig. Distinct from this, here it is shown that even for a perfectly axisymmetric flow and breakdown bubble, the combined effect of dye diffusion and the inevitable small errors in the dye injection position lead to the false perception of an open bubble structure with folds near the lower stagnation point. Furthermore, the asymmetries in the predicted flow structures can be remarkably similar to those observed in flow observations and computational predictions with geometric asymmetries of the rig. Thus, when interpreting dye-visualization patterns in steady flow, even if axisymmetric flow can be achieved, it is important to take into account the relative diffusivity of the dye and the accuracy of its injection.
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Hara, M., Zhen-chao Wang i H. Saito. "Thermal bubble breakdown in liquid nitrogen under nonuniform fields". IEEE Transactions on Dielectrics and Electrical Insulation 1, nr 4 (1994): 709–15. http://dx.doi.org/10.1109/94.311714.
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Vassenden, F., i T. Holt. "Experimental Foundation for Relative Permeability Modeling of Foam". SPE Reservoir Evaluation & Engineering 3, nr 02 (1.04.2000): 179–85. http://dx.doi.org/10.2118/62506-pa.
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Summary Optimization of foam treatments requires modeling or simulation of the process using a foam model that is capable of reproducing the critical properties of foam. In this work we present a simple model that is based on the relative permeability concept, and is well suited for simulation of foam processes with conventional or slightly modified numerical simulators. The model can unify understanding of a number of phenomena that characterizes foam flow. The validity of the model is substantiated by experimental data. A consistent interpretation of foam breakdown, shear thinning effects, permeability effects, and effects of system pressure is possible with the model for gas relative permeability in foam presented. Introduction Foam can be used in reservoir engineering for control of gas flow in reservoirs. By injection of gas and surfactant-containing water, foam can be generated in a formation, and the mobility of the gas be reduced. The macroscopic effect of a large number of foam films in the pore system is determined by the properties of the individual foam films, the number of foam films, and the properties of the three-dimensional network of open pores where gas can flow. One previous effort to model foam flow was the mechanistic approach, where these entities were accounted for explicity.1 Another approach was to identify all factors that influence macroscopic foam flow, and parameterize them in an empirical foam mobility function that reproduces experimental foam behavior.2 Important factors are flow rate, oil saturation, absolute permeability, foam quality, and surfactant concentration. The model presented here falls somewhere in between the mechanistic and the completely empirical approaches. Here, physical arguments are used to advocate a particular shape of the gas relative permeability curve to be used for foam flow, and physically reasonable rules for how this shape should change with changing flow conditions (rates, permeability, system pressure, etc.). The model presented is empirical in the sense that it requires calibration to laboratory data for a specific foam system. Its predictive power is improved compared to a purely empirical model; because a physically reasonable dependence of foam properties on flow conditions is built in, the number of degrees of freedom is reduced. Theory The assumption forming the basis for this work is that the relative permeability concept is equally accurate for the flow of the foam constituents, gas and surfactant solution, as it is for the flow of any other pair of phases. Because foam films contain water, moving films could potentially constitute an extra path for transport for water at a given water saturation. This would change the water relative permeability curve relative to that in the no-foam case. This does not seem to be the case however. Bernard et al.3 have shown that the relative permeability curve for surfactant solution in foam is equal to the curve for water in ordinary water-gas flow. This must mean that the water flows in the same channels whether there is foam in the porous medium or not, and that foam films transport only insignificant amounts of water, either because they do not move, or because they contain too little water. The foam films do influence the mobility of gas however. Foam films span the pore throats in the same manner as films span the ring of a children's toy bubble generator. Gas cannot flow through pore channels that contain films, unless the films move or rupture. Macroscopically, the mobility reduction of gas in porous media by foams is determined by the interplay among many films in many pore channels. The number of films per unit volume is believed to be given by a dynamic balance between film generation and rupture processes.1 The resulting effects on the gas relative permeability curve for foam is discussed next. Single Foam Films. Single foam films form the basis for understanding the behavior of foam in a porous medium. In the present work, the effects of both capillary pressure and of viscous pressure gradients on single foam films are considered. Capillary Pressure Effects. The theory for static film stability considers the forces acting between the two surfaces on each side of a water film. These forces are described by the disjoining pressure,4 ?, which varies with film thickness. Parallel to the definition of capillary pressure, Pc as the difference between pressures in the gas and water phases over a curved interface, the disjoining pressure is the difference between gas and water pressures over a flat film surface, caused by interactions between the two film surfaces. The water pressure in a thin film can be expressed by pg ?? where pg is the gas pressure outside the film. Elsewhere in the pore, the water pressure is pg?P c. As long as Pc>? the water pressure in the film exceeds that of the surrounding water, and the resulting pressure gradient drives water out of the film. A pressure balance is obtained at the thickness where Pc=?.5 In this theory, foam films are supposed to rupture whenever the capillary pressure in the porous medium exceeds the maximum disjoining pressure that the films exhibit. Rupture of thin liquid films by the application of high capillary pressures can be observed in studies of single films.5 Viscous Pressure Gradients. A foam film that is stable against capillary drainage will not necessarily block gas flow. Such films may flow or rupture because of the viscous pressure gradient, like a film in a bubble toy that is blown too hard. This is because there is a geometrical limit to the pressure drop a static film can support. Over a single film that blocks gas flow, there will be a pressure difference, pg1? pg2, generated by gas flow elsewhere in the porous medium. The pressure drop can be related to the pore geometry and the surface tension of the surfactant solution, ?, through the Young-Laplace equation, which states that p g 1 − p g 2 = 4 γ s i n α / r . ( 1 ) The geometrical parameters, r and ?, are defined in Fig. 1. The film is in mechanical equilibrium when the angle between the film and the pore wall is 90°. The group ?/r will have a local maximum in every pore. If the pressure drop exceeds the value corresponding to the maximum ?/r the film cannot remain in that pore, and it will flow and jump to the next pore. This will lead to flow of gas in the pore: either the flow that accompanies film movement, or, if the film ruptures during the jump, the flow that will take place in an open pore. Foams at the Limiting Capillary Pressure. From the existence of a maximum disjoining pressure for foam films, consequences for the macroscopic behavior of foams in porous media can be deduced.6
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Tsai, Feng Chin, i Rong Fung Huang. "Topological Flow Structures of Annular Swirling Jets". Journal of Mechanics 17, nr 3 (wrzesień 2001): 131–38. http://dx.doi.org/10.1017/s1727719100004494.
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AbstractThe effects of blockage and swirl on the macro flow structures of the annular jet past a circular disc are experimentally studied through the time-averaged streamline patterns. In the blockage-effect regime, the flows present multiple modes, single bubble, dual rings, vortex breakdown, and triple rings, in different regimes of blockage ratio and swirl number. The topological models of the flow structures are proposed and discussed according to the measured flow fields to manifest the complex flow structures. The single bubble is a closed recirculation bubble with a stagnation point on the central axis. The dual-ring flow is an open-top recirculsation zone, in which a pair of counter-rotating vortex rings exists in the near wake. The fluids in the dual rings are expelled downstream through a central jet-like swirling flow. A vortex breakdown may occur in the central jet-like swirling flow if the exit swirl number exceeds critical values. When the vortex breakdown interacts with the dual rings, a complex triple-ring flow structure forms. Axial distributions of the local swirl number are presented and discussed. The local swirl number increases with the increase of the exit swirl number and attains the maximum in the dual-ring mode. At large exit swirl numbers where the vortex breakdown occurs, the local swirl number decreases drastically to a low value.
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SUEMATSU, Yoshikazu, Tadaya ITO i Toshiyuki HAYASE. "Vortex breakdown phenomena in a circular pipe (4th report, Mechanism of axisymmetric bubble type breakdown)". Transactions of the Japan Society of Mechanical Engineers Series B 51, nr 471 (1985): 3488–96. http://dx.doi.org/10.1299/kikaib.51.3488.
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SUEMATSU, Yoshikazu, Tadaya ITO i Toshiyuki HAYASE. "Vortex Breakdown Phenomena in a Circular Pipe : 4th Report, Mechanism of Axisymmetric Bubble Type Breakdown". Bulletin of JSME 29, nr 253 (1986): 2086–94. http://dx.doi.org/10.1299/jsme1958.29.2086.
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BRØNS, MORTEN, WEN ZHONG SHEN, JENS NØRKÆR SØRENSEN i WEI JUN ZHU. "The influence of imperfections on the flow structure of steady vortex breakdown bubbles". Journal of Fluid Mechanics 578 (26.04.2007): 453–66. http://dx.doi.org/10.1017/s0022112007005101.
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Vortex breakdown bubbles in the flow in a closed cylinder with a rotating end-cover have previously been successfully simulated by axisymmetric codes in the steady range. However, high-resolution experiments indicate a complicated open bubble structure incompatible with axisymmetry. Numerical studies with generic imperfections in the flow have revealed that the axisymmetric bubble is highly sensitive to imperfections, and that this may resolve the apparent paradox. However, little is known about the influence of specific, physical perturbations on the flow structure. We perform fully three-dimensional simulations of the flow with two independent perturbations: an inclination of the fixed cover and a displacement of the rotating cover. We show that perturbations below a realistic experimental uncertainty may give rise to flow structures resembling those obtained in experiments, that the two perturbations may interact and annihilate their effects, and that the fractal dimension associated with the emptying of the bubble can quantitatively be linked to the visual bubble structure.
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Muthiah, Gopalsamy, i Arnab Samanta. "Transient energy growth of a swirling jet with vortex breakdown". Journal of Fluid Mechanics 856 (2.10.2018): 288–322. http://dx.doi.org/10.1017/jfm.2018.712.
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We investigate the existence of short-time, local transient growth in the helical modes of a rapidly swirling, high-speed jet that has transitioned into an axisymmetric bubble breakdown state. The time-averaged flow consisting of the bubble and its wake downstream constitute the base state, which we show to exhibit strong transient amplification owing to the non-modal behaviour of the continuous eigenspectrum. A pseudospectrum analysis mathematically identifies the so-called potential modes within this continuous spectrum and the resultant non-orthogonality between these modes and the existing discrete stable modes is shown to be the main contributor to such growth. As the swirling flow develops post the collapsed bubble, the potential spectrum moves further toward the unstable half-plane, which along with the concurrent weakening of exponential growth from the discrete unstable modes, increases the dynamic importance of transient growth inside the wake region. The transient amplifications calculated at several locations inside the bubble and wake confirm this, where strong growths inside the wake far outstrip the corresponding modal growths (if available) at shorter times, but especially at the higher helical orders and smaller streamwise wavenumbers. The corresponding optimal perturbations at initial times consist of streamwise streaks of azimuthal velocity, which if concentrated inside the core vortical region, unfold via the classical Orr mechanism to yield structures resembling core (or viscous) Kelvin waves of the corresponding Lamb–Oseen vortex. However, in contrast to that in Lamb–Oseen vortex flow, where critical-layer waves are associated with higher transient gains, here, such core Kelvin modes with the more compact spiral structure at the vortex core are seen to yield the maximum transient amplifications.
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Zong Siguang, 宗思光, 王江安 Wang Jiang′an i 王辉华 Wang Huihua. "Image measure of characters of cavitation bubble by optical breakdown". Acta Optica Sinica 29, nr 8 (2009): 2197–202. http://dx.doi.org/10.3788/aos20092908.2197.
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Paterson, Oliver, Bofu Wang i Xuerui Mao. "Coherent Structures in the Breakdown Bubble of a Vortex Flow". AIAA Journal 56, nr 5 (maj 2018): 1812–17. http://dx.doi.org/10.2514/1.j055329.
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Bruggeman, P. J., C. A. Leys i J. A. Vierendeels. "Electrical breakdown of a bubble in a water-filled capillary". Journal of Applied Physics 99, nr 11 (czerwiec 2006): 116101. http://dx.doi.org/10.1063/1.2199748.
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Gerhold, J. "Evaluation of bubble breakdown limit in LHe below 4.2 K". IEEE Transactions on Electrical Insulation 26, nr 4 (1991): 679–84. http://dx.doi.org/10.1109/14.83689.
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KATO, Kazuhiko, Hirotaka DAN, Ryosuke ADACHI i Yasuaki MATSUDAIRA. "Propagation of Chain-Reacting Bubble Collapse Generating at Cavitation Breakdown". Transactions of the Japan Society of Mechanical Engineers Series B 72, nr 714 (2006): 353–60. http://dx.doi.org/10.1299/kikaib.72.353.
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Schovanec, Petr, Darina Jasikova, Michal Kotek i Vaclav Kopecky. "Evolution and implosion of cavitation bubbles towards solid surface". EPJ Web of Conferences 269 (2022): 01054. http://dx.doi.org/10.1051/epjconf/202226901054.
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Cavitation bubbles generated via laser-induced breakdown are investigated experimentally. The present work focuses on the direction of the first bubble collapse near a solid surface in distilled water. The solid surface is placed first to the right side in a cuvette filled with distilled water and then placed to the top of the cuvette. In this experiment, it is observed in which direction the cavitation bubble collapses. The cavitation bubble is visualized by a high-speed camera of frequency 68kHz.
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Katterbauer, Klemens, Alberto F. Marsala, Virginie Schoepf i Eric Donzier. "A novel artificial intelligence automatic detection framework to increase reliability of PLT gas bubble sensing". Journal of Petroleum Exploration and Production Technology 11, nr 3 (13.02.2021): 1263–73. http://dx.doi.org/10.1007/s13202-021-01098-1.
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AbstractProduction logging tools (PLTs) and formation testing, even in logging while drilling (LWD) conditions during underbalanced drilling, are key technologies for assessing the productivity potential of a gas well and therefore to maximize recovery. Gas bubble detection sensors are key components in determining the fluid phases in the reservoir and accurately quantify recoverable reserves, optimize well placement, geosteering and to qualify the production ability of the well. We present here a new nonlinear autoregressive - breakdown artificial intelligence (AI) detection framework for PLT gas bubble detection sensors that categorize in real-time whether and which sensors become unreliable or have broken down during the logging measurements. AI tools allow the automatization of this method that is critical during data quality control of post-drilling PLT, but it is essential when the measurements are performed in LWD as data assessment and processing need to occur in real-time. This AI framework was validated on both a training and testing dataset, and exhibited strong classification performance. This method enables accurate real-time breakdown detection for gas bubble detection sensors.
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Müller, Miloš, Jan Hujer i Petra Dančová. "Dynamic behaviour of cavitation bubble close to a flexible wall". EPJ Web of Conferences 264 (2022): 01024. http://dx.doi.org/10.1051/epjconf/202226401024.
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The dynamic behaviour of cavitation bubble close to a flexible boundary is investigated experimentally. The cavitation bubble is produced as a consequence of an optical breakdown generated by the focus of 532 nm Nd-Yag laser beam. A gold mirror is used to focus the expanded laser beam into a spherically shaped plasma. A PVDF film sensor is used for the measurement of the interaction magnitude between the bubble and the flexible boundary. As the flexible boundary directly the PVFD film sensor is used. The sensor is flexibly mounted within a specially designed movable frame which enables to modify the elastic properties of the system. The bubble dynamics and the flexible wall movement are recorded by high-speed CCD camera and correlated with the acoustic signal obtained by the PVDF film. Different bubble collapse patters for different bubble wall distances and the corresponding acoustic signals are presented.
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DiGiuseppe, Stephen, Malgorzata Bienkowska-Haba i Martin Sapp. "Human Papillomavirus Entry: Hiding in a Bubble". Journal of Virology 90, nr 18 (13.07.2016): 8032–35. http://dx.doi.org/10.1128/jvi.01065-16.
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Incoming human papillomavirus (HPV) utilize vesicular transport to traffic from the plasma membrane to the trans-Golgi network. Following nuclear envelope breakdown during mitosis, the viral DNA associates with condensed chromosomes utilizing spindle microtubules for delivery. Most intriguingly, the viral DNA resides in a transport vesicle until mitosis is completed and the nuclear envelope has reformed. This finding provides support for the transient existence of nuclear membrane-bound vesicles. Due to their transient nature, it also points to the existence of a cell pathway for the disposal of vesicles ending up fortuitously or purposefully in the nucleus.
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CARDONE, F., i R. MIGNANI. "PIEZONUCLEAR REACTIONS AND LORENTZ INVARIANCE BREAKDOWN". International Journal of Modern Physics E 15, nr 04 (czerwiec 2006): 911–24. http://dx.doi.org/10.1142/s0218301306004600.
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In the last years experiments of cavitating water and of explosions of foils in water have provided possible evidence for production of stable, unstable and artificial nuclides induced by ultrasounds and shock waves, i.e. for nuclear reactions induced by pressure waves (piezonuclear reactions). We propose a possible mechanism for the explanation of these findings, that is constituted by two parts: a classical one, based on the bubble implosion due to cavitation, and a non-classical, related to a possible breakdown of Lorentz invariance for nuclear interactions. Such a mechanism allows one to get precise predictions on the values of energy and power needed to carry out experiments with the commercially available sonotrodes and to get piezonuclear reactions in a reproducible way.