The effect of bubbles on developed turbulence (original) (raw)
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Journal of Turbulence, 2006
The effect of bubbles on fully developed turbulent flow is investigated numerically and experimentally, summarizing the results of our previous papers (Mazzitelli et al., 2003, Physics of Fluids 15, L5. and Journal of Fluid Mechanics 488, 283;, Journal of Fluid Mechanics 538, 153). On the numerical side, we simulate Navier-Stokes turbulence with a Taylor-Reynolds number of Re λ ≈ 60, a large large-scale forcing, and periodic boundary conditions. The point-like bubbles follow their Lagrangian paths and act as point forces on the flow. As a consequence, the spectral slope is less steep as compared to the Kolmogorov case. The slope decrease is identified as a lift force effect. On the experimental side, we do hot-film anemometry in a turbulent water channel with Re λ ≈ 200 in which we have injected small bubbles up to a volume percentage of 3%. Here the challenge is to disentangle the bubble spikes from the hot-film velocity signal. To achieve this goal, we have developed a pattern recognition scheme. Furthermore, we injected microbubbles up to a volume percentage of 0.3%. Both in the counter flowing situation with small bubbles and in the co-flow situation with microbubbles, we obtain a less spectral slope, in agreement with the numerical result.
On bubble clustering and energy spectra in pseudo-turbulence
Journal of Fluid Mechanics, 2010
Three-dimensional particle tracking velocimetry (PTV) and phase-sensitive constant temperature anemometry in pseudo-turbulence -i.e. flow solely driven by rising bubbles -were performed to investigate bubble clustering and to obtain the mean bubble rise velocity, distributions of bubble velocities and energy spectra at dilute gas concentrations (α 6 2.2 %). To characterize the clustering the pair correlation function G(r, θ ) was calculated. The deformable bubbles with equivalent bubble diameter d b = 4-5 mm were found to cluster within a radial distance of a few bubble radii with a preferred vertical orientation. This vertical alignment was present at both small and large scales. For small distances also some horizontal clustering was found. The large number of data points and the non-intrusiveness of PTV allowed wellconverged probability density functions (PDFs) of the bubble velocity to be obtained. The PDFs had a non-Gaussian form for all velocity components and intermittency effects could be observed. The energy spectrum of the liquid velocity fluctuations decayed with a power law of −3.2, different from the ≈ − 5/3 found for homogeneous isotropic turbulence, but close to the prediction −3 by Lance & Bataille (J. Fluid Mech., vol. 222, 1991, p. 95) for pseudo-turbulence.
International Journal of Multiphase Flow, 2011
Direct numerical simulations (DNS) are performed to study the behavior of a swarm of rising air bubbles in water, employing the front tracking method, which allows to handle finite-size bubbles. The swarms consist of monodisperse deformable 4 mm bubbles with a gas fraction of 5% and 15%. This paper focuses on the comparison of the liquid energy spectra and bubble velocity probability density functions (PDFs) with experimental data obtained by phase-sensitive constant-temperature anemometry (CTA) and threedimensional particle tracking velocimetry (PTV), respectively.
Statistical analysis of small bubble dynamics in isotropic turbulence
Physics of Fluids, 2007
The dynamics and dispersion of small air bubbles in isotropic turbulence are analyzed computationally. The flow field is simulated using a pseudo-spectral code, while the bubble dynamics are analyzed by integration of a Lagrangian equation of motion that accounts for buoyancy, added mass, pressure, drag, and lift forces. Probability density functions (pdfs) of bubble velocities, lift and drag forces, and of field velocities and vorticities along bubble trajectories are used to analyze bubble dynamics. Lagrangian bubble trajectories are also employed to determine dispersion characteristics, following the theoretical development of Cushman and Moroni . Consistent with available experimental data, bubble rise velocities are increasingly suppressed with increasing turbulence intensity. The analysis also reveals that the vertical bubble velocities are characterized by asymmetric pdfs that are positive or negative-skewed dependent upon the non-dimensional turbulence intensity and the Taylor length scale. The role of the lift force in moving the bubbles to the down-flow side of turbulent eddies, and consequently retarding their rise, is consistently observed in all analysis. The dispersion of 40 µm bubbles and transition to Fickian behavior is shown to be weakly affected by the turbulence level. Larger, 400 µm bubbles are shown to be more sensitive to turbulence level with transition to Fickian behavior delayed in low turbulence fields.
This paper presents an analysis of measurements of mean flow and turbulence statistics in bubble plumes conducted in a large experimental tank (digester) at a wastewater treatment plant. Profiles of dissipation rates of turbulent kinetic energy are presented for the first time, together with distributions for the turbulent kinetic energy and Kolmogorov length scales. Dissipation rates obtained from time velocity series and SCAMP measurements are also compared.
1 Dancing Bubbles in Turbulent Flows: PIV Measurements and Analysis
2015
Two-phase bubbly flows are widely applied in engineering and environmental processes. The interaction of the dispersed phase with the continuous phase has a great effect on transfer processes between the phases. The interstitial relative velocities between the phases and the interfacial area and the shape of the dispersed phase are the key dependent parameters in the drag, heat and mass transfer between the phases. Although the physical understanding of bubbles rise in a liquid is a significant practical importance in many areas of engineering, neither the interactions between bubbles in clusters nor the bubble-induced pseudo-turbulence (i.e., the generation of velocity fluctuations by bubbles and their wakes in a laminar flow) are fully understood. The modeling of bubbly flows with the Computational Fluid Dynamics (CFD) codes requires detailed information about the full field velocity close to the bubble and its wake. Such information is not widely available. Experimental data exis...
Bubble Induced Turbulence in Bubble Plumes
In bubbly flow, bubbles and the surrounding fluid interact through both force and turbulence coupling. The effects of flow turbulence on bubble trajectory are reflected in turbulent dispersion. Bubbles will introduce extra turbulence into the fluid through wake effects (so-called pseudo turbulence). The single phase k-epsilon model does not incorporate the bubble-induced turbulence [1]. This study aimed to develop and implement a model to account for these effects. The gas-stirred-ladle experiments of [10] and [1] were employed for validation. The model framework combines a volume of fluid (VOF) and discrete phase model (DPM). VOF is used to capture the fountain shape formed by the bubble plume reaching the surface, while DPM is a parcel-based Lagrangian approach to track bubbles.
Power spectral distributions of pseudo-turbulent bubbly flows
Physics of Fluids, 2013
An experimental study was carried out to determinate the power spectral density (PSD) of mono-dispersed bubbly flows in a vertical channel using flying hot-film anemometry. To improve bubble detection, optical fibers were installed in close proximity to the anemometer sensing element; in this way, the collisions of bubbles with the probe can be detected and removed from the signal. Measurements were performed with gas fractions up to 6%. The PSD distributions were found to decay with a power of −3, in agreement with previous studies, but for a much wider range of Reynolds and Weber numbers. Our measurements indicate that the power decay does not depend strongly on the nature of hydrodynamic interactions among bubbles. C 2013 AIP Publishing LLC. [http://dx.
Hot-film anemometry in bubbly flow I: bubble–probe interaction
2005
The interaction between a bubble, which is rising in a descending water flow, and a hot-film anemometer was experimentally investigated using stereoscopic high-speed imaging. The mean downward water velocity varied from 0 up to 0.15 m/s, i.e., relatively low, allowing for an extended bubble-probe interaction. Moreover, the direction of the water causes the wake of the probe to play a role before the bubble touches the probe. The equivalent bubble radii were 0.4-2.8 mm and the bubble velocities relative to the probe ranged from 0.04 to 0.38 m/s. Image processing techniques were applied to reconstruct the bubblesÕ path, shape, and orientation during the interaction process. As a result, three types of interactions were found, namely penetrating, bouncing, and splitting interactions. The image sequences were compared with the corresponding time series of the hot-film anemometer. From the time series the type of interaction cannot be deduced, at least not for the analyzed flow situation. Furthermore, we demonstrated that the residence time estimate from the hot-film data is systematically biased in our type of experiments. Finally, it was found that the velocity of a bubble may be altered considerably due to the interaction.