Bulletin of the American Physical Society
70th Annual Meeting of the APS Division of Fluid Dynamics
Volume 62, Number 14
Sunday–Tuesday, November 19–21, 2017; Denver, Colorado
Session D8: Multiphase Flows: Turbulence IIMultiphase Turbulence
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Chair: Roberto Verzisco, Uniroma2 Room: 501 |
Sunday, November 19, 2017 2:15PM - 2:28PM |
D8.00001: V-ONSET: Introducing turbulent multiphase flow facility focusing on Lagrangian interfacial transfer dynamics Ashwanth Salibindla, Ashik Ullah Mohammad Masuk, Rui Ni We have designed and constructed a new vertical water tunnel, V-ONSET, to investigate interfacial mass, momentum and energy transfer between two phases in a Lagrangian frame. This system features an independent control of mean flow and turbulence level. The mean flow opposes the rising/falling velocity of the second phase, ``suspending'' the particles and increasing tracking time in the view area. Strong turbulence is generated by shooting 88 digitally-controlled water jets into the test section. The second phase, either bubbles or oil droplets, can be introduced into the test section through a capillary island. In addition to this flow control system, V-ONSET comes with a 3D two-phase visualization system, consisting of high-speed cameras, two-colored LED system, and in-house Lagrangian particle tracking algorithm. This enables us to acquire the Lagrangian evolution of both phases and the interfacial transfer dynamics in between, paving the way for new closure models for two-phase simulations. [Preview Abstract] |
Sunday, November 19, 2017 2:28PM - 2:41PM |
D8.00002: Lagrangian evolution of deformation of finite-size bubbles in turbulent multiphase flow Ashik Ullah Mohammad Masuk, Ashwanth Salibindla, Rui Ni Finite-size bubbles tend to deform in a strong turbulent environment because of the complex interfacial momentum transfer between them. We have utilized the new V-ONSET turbulence multiphase flow facility to track the deformation and the couplings between two phases in a 3D Lagrangian framework. This rich dataset allows us to understand the roles played by the dynamic pressure and viscous stress, as well as different forces that contribute to the interfacial momentum transfer. [Preview Abstract] |
Sunday, November 19, 2017 2:41PM - 2:54PM |
D8.00003: Visualizing Orientation Fields of Fibers Advected in Turbulent and Chaotic Flows Andrea Masi-Phelps, Bardia Hejazi, Bernhard Mehlig, Greg A. Voth We examine the spatial field of orientations of slender fibers in 3D turbulent and 2D chaotic fluid flows. The spatial field of fiber orientations is dominated by surfaces (in three dimensions) and lines (in two dimensions) across which the fiber orientation changes rapidly and approaches a step change of $\pi$. Using the JHU turbulence database of 3D homogeneous isotropic turbulence at $R_\lambda=418$ and the standard map for 2D chaotic flow, we extract the Cauchy-Green strain tensors whose most extensional eigenvector gives the preferred orientation of fibers at a point in the flow. Visualization of the orientation field reveals the structure of the steps. We also study the moments of the relative orientation of two fibers as a function of the distance between the two points. We observe the anomalous scaling found by Zhao et al (arxiv:1707.06037) which reflects the fractal structure of the orientation field that results from the continuous formation of new steps in the orientation field. [Preview Abstract] |
Sunday, November 19, 2017 2:54PM - 3:07PM |
D8.00004: Interplay Between Surface Energy and Turbulent Kinetic Energy in Multiphase Homogeneous Isotropic Turbulence Robert Chiodi, Olivier Desjardins Interactions between a liquid-gas interface and turbulence can have a significant effect on multiphase flows. Currently, models for this interaction are sparse, largely due to a lack of detailed understanding on how turbulent kinetic energy is exchanged with surface energy through surface tension. This talk will focus on this process through the use of direct numerical simulations of homogeneous isotropic turbulence surrounding an interface between two fluids of unity density and viscosity ratios. In particular, we will discuss the balance between the turbulent kinetic energy and energy stored in the interface surface. The length scales at which surface tension extracts and injects energy into the flow will also be analyzed through the use of a posteriori filtering of the simulation data. Additionally, probability density functions of interface curvature at different Weber numbers will be presented, and indicate that the most probable curvature in homogenous isotropic turbulence is well predicted by the Kolmogorov critical radius/Hinze scale. [Preview Abstract] |
Sunday, November 19, 2017 3:07PM - 3:20PM |
D8.00005: Wavelet energy spectra of multiphase flows Andreas Freund, Antonino Ferrante Classically, the energy spectrum of a turbulent flow is defined using a Fourier transform of the velocity field from the spatial domain to the frequency domain, where the velocity is represented in a basis of sine waves. This analysis works fine for smooth data like that of a single-phase turbulent flow, but consider the case of multiphase flows, in which the velocity is nonsmooth at the interface between the phases. Discontinuities in the derivatives of velocity may be seen at the interface between the carrier and droplet fluids, and these can introduce spurious oscillations in the energy spectra if computed with the Fourier transform. An alternative definition of the energy spectrum uses the wavelet transform, which can handle discontinuities without producing spurious oscillations in the spectra. Thus, we propose to use the wavelet energy spectrum to analyze multiphase flows and apply it for analyzing the DNS data of droplet-laden decaying isotropic turbulence of Dodd \& Ferrante (J.\@ Fluid.\@ Mech.\@ \textbf{806} (2016), 356--412). [Preview Abstract] |
Sunday, November 19, 2017 3:20PM - 3:33PM |
D8.00006: Numerical simulation of two-phase turbulent Taylor-Couette flow with finite-size drops/bubbles Roberto Verzicco, Detlef Lohse, Vamsi Spandan Deformable bubbles/drops larger than the Kolmogorov scale (finite-size) dispersed in a turbulent flow can alter the global and local flow properties in several ways. They can enhance or reduce the net momentum/heat transport, alter the path of drops/bubbles or even the rheological properties of the fluid mixture. A detailed understanding of these systems is still missing therefore we use direct numerical simulations to study a two-phase turbulent Taylor-Couette (TC) flow to gain further insight into this problem. We solve the Navier-Stokes equations for the carrier fluid and an immersed boundary method for the dispersed phase $(10^3 drops)$. The deformation of the dispersed drops is obtained by a multi-physics interaction potential approach tuned for deformation dynamics of any liquid-liquid interface with a given surface tension. Additionally, the drops can collide with each other or against the walls which makes it a fully resolved four-way coupled simulation. Our simulations show that the net drag reduction increases with increasing deformability of bubbles and this is not related to any preferential accumulation. We show that finite-size bubbles block the momentum transfer from the inner to the outer cylinder thus laminarising the bulk and leading to drag reduction. [Preview Abstract] |
Sunday, November 19, 2017 3:33PM - 3:46PM |
D8.00007: Viscosity-modulated breakup and coalescence of large drops in bounded turbulence Alessio Roccon, Francesco Zonta, Marco De Paoli, Alfredo Soldati In this work, we examine the influence of viscosity on breakup and coalescence of a swarm of large drops in a wall-bounded turbulent flow. We consider several values of surface tension and a wide range of drops to fluid viscosity ratios $\lambda=\eta_d/\eta_c$ while we maintain the same density for drops and carrier fluids. Drops can coalesce and break following a complex dynamics that is primarily controlled by the interplay between turbulence fluctuations (measured by $Re_{\tau}$), surface tension (measured by $We$) and $\lambda$. We use Direct Numerical Simulations (DNS) of turbulence coupled with a Phase Field Method (PFM) to describe the drops dynamics. We observe a consistent action of increasing $\lambda$, which, especially for the larger Weber numbers decreases significantly the breakup rate of the drops. Qualitatively, an increase of drop viscosity decreases the breakup rate, very much like an increase of surface tension does. The mechanism by which drop viscosity acts is a modulation of turbulence fluctuations inside the drop, which reduces the work surface tension has to do to preserve drop integrity. We will also consider the case of non-coalescing drops in which a surfactant is able to inhibit the coalescence. [Preview Abstract] |
Sunday, November 19, 2017 3:46PM - 3:59PM |
D8.00008: A combined volume-of-fluid method and low-Mach-number approach for DNS of evaporating droplets in turbulence Michael Dodd, Antonino Ferrante Our objective is to perform DNS of finite-size droplets that are evaporating in isotropic turbulence. This requires fully resolving the process of momentum, heat, and mass transfer between the droplets and surrounding gas. We developed a combined volume-of-fluid (VOF) method and low-Mach-number approach to simulate this flow. The two main novelties of the method are: (i) the VOF algorithm captures the motion of the liquid gas interface in the presence of mass transfer due to evaporation and condensation without requiring a projection step for the liquid velocity, and (ii) the low-Mach-number approach allows for local volume changes caused by phase change while the total volume of the liquid-gas system is constant. The method is verified against an analytical solution for a Stefan flow problem, and the $D^2$ law is verified for a single droplet in quiescent gas. We also demonstrate the schemes robustness when performing DNS of an evaporating droplet in forced isotropic turbulence. [Preview Abstract] |
Sunday, November 19, 2017 3:59PM - 4:12PM |
D8.00009: Effects of droplet size on droplet evaporation rate in isotropic turbulence Antonino Ferrante, Michael Dodd Our objective is to determine the effects of varying the ratio of the droplet diameter to the Kolmogorov lengthscale ($D/\eta$) on droplet evaporation rate in forced isotropic turbulence. We have performed DNS of a single droplet in forced isotropic turbulence using the volume-of-fluid method to fully resolve the process of momentum, heat, and mass transfer between the liquid droplet and the gas. The effect of droplet size on the droplet vaporization rate is investigated by increasing $D/\eta$ from 0.5 to 10 by increasing the Taylor-scale Reynolds number $Re_\lambda$ from 10 to 75. We will present the droplet evaporation rate as a function of $D/\eta$ and compare our results with available experimental data. The DNS results show that increasing $D/\eta$ increases the droplet evaporation rate, which is in agreement with experiments. The physical mechanisms of such behavior will be explained by analyzing the DNS results. [Preview Abstract] |
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