Paper Submission
ETC2019 17th European Turbulence Conference





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14:00   Multiphase Flows 2
14:00
15 mins

#443
DROPLET NUCLEATION IN TURBULENT STEAM JETS
Andrea Gallegati, Francesco Battista, Paolo Gualtieri, Carlo Massimo Casciola
Abstract: Homogeneous nucleation of liquid droplets occurs when an hot vapour stream mixes with a cooler and dry external envi- ronment. Many applications could benefit from a better understanding of such a complex turbulent flow. Up to date, the sensitivity of nucleation rates to the underlying turbulent flow has been extensively investigated with experiments [3, 4], using Dibutyl phthalate (DBP) or water as condensing species. Only recently some numerical simulations have been carried out, especially RANS/LES [1, 5] and some DNS [6], but a steam jet DNS is still missing despite the multitude of experiments. The nonlinear interplay between homogeneous nucleation and turbulent fluctuations within a multiphase flow, leads to a non-trivial physical situation, where to take in account cross-coupling phenomena reveals to be crucial. Classical Nucle- ation Theory (CNT) prescribes rates and critical diameters at which droplets nucleate, strongly depending on the local thermodynamical state, e.g. saturation ratio, temperature and surface tension. DNS allows to capture without any mod- elling the underlying turbulence, while additional effects such as the disperse phase back-reaction are accounted. In the so called two-way coupling regime coalescence and droplet-droplet collisions can be neglected (dilute regime), while because of the mass load the particles back-reaction can not. In this study a full description of the droplet nucleation in a steam turbulent jet is provided by means of DNS, which accounts for all the coupling effects i.e. mass, momentum and energy transfer between the different phases. The DNS, in the low Mach number formulation, is carried out exploiting an hybrid Eulerian-Lagrangian approach and accounting for the disperse droplets under the point-particle approximation. Each conservation equation has a source term representing the particles back-reaction on the fluid and an equal (although opposite) contribution is found on the equations for the droplets dynamics. These singular forcing terms are regularized and time-delayed, in a physically consistent way [2]. During the talk, the relevance of the effects arising in the two-way coupling regime will be discussed, comparing results accounting for the particles back-reaction with those obtained neglecting it (one-way coupling regime). We have already seen that the disperse phase affects the carrier phase dynamics, modulating turbulence, stretching the shape of the jet, but always preserving (the statuary) self-similarity we are used to. Because of the particles back-reaction on the thermody- namics, directly affecting the nucleation process, we are able to account for the whole effect of the phase change process. Moreover, following a Lagrangian description of the disperse phase dynamics, each particle trajectory is drawn and every observable can be fully characterized from a statistical point of view, contrary to the approaches adopted in literature [1, 6], evolving an equivalent Eulerian field in the so called PBE-PDF approach. This original contribution constitutes an absolute novelty in this community, whereas the high density ratio shed light on the necessity to investigate aerosol formation in the two way coupling regime. References [1] G.Y. Di Veroli and S. Rigopoulos. Modeling of aerosol formation in a turbulent jet with the transported population balance equation-probability density function approach. Physics of Fluids, 23(4):043305, 2011. [2] P. Gualtieri, F. Picano, G. Sardina, and C.M. Casciola. Exact regularized point particle method for multiphase flows in the two-way coupling regime. Journal of Fluid Mechanics, 773:520–561, 2015. [3] T.K. Lesniewski and S.K. Friedlander. Particle nucleation and growth in a free turbulent jet. In Proceedings of the Royal Society of London A: Mathematical, Physical and Engineering Sciences, 454, pages 2477–2504. The Royal Society, 1998. [4] S. Lim, J. Cha, H. Lee, T. Kim, and W.G. Shin. Understanding the condensation process of turbulent steam jet using the pdpa system. International Journal of Multiphase Flow, 98:168–181, 2018. [5] I. Pesmazoglou, A.M. Kempf, and S. Navarro-Martinez. Aerosol nucleation in a turbulent jet using large eddy simulations. Chemical Engineering Science, 116:383–397, 2014. [6] K. Zhou, A. Attili, A. Alshaarawi, and F. Bisetti. Simulation of aerosol nucleation and growth in a turbulent mixing layer. Physics of Fluids, 26(6):065106, 2014.
14:15
15 mins

#463
Bubble break-up in turbulence
Luc Deike, Daniel J. Ruth, Stephane Perrard, Wouter Mostert
Abstract: We present experiments and direct numerical simulations of the breakup of bubbles in a turbulent flow. In both the experiments and the numerical simulations, an air cavity of controlled size is released in a three-dimensional homogeneous and isotropic turbulent flow, where it breaks and deforms. It permits to characterize the break-up dynamics for a wide range of Weber numbers (defined as the ratio between turbulent and surface tension forces) and the resulting distribution of child bubble size distribution, together with the final pinch-off dynamics in a turbulent flow.
14:30
15 mins

#613
Experimental investigation of bubble breakup in strong turbulence
Rui Ni, Ashik Ullah Mohammad Masuk, Ashwanth Salibindla, Shiyong Tan
Abstract: A persistent theme throughout the study of multiphase flows is the need to model and predict the detailed behaviors of all involved phases and the phenomena that they manifest at multiple length and time scales. When combined with background turbulent flows with similar multiscale nature, they pose a formidable challenge, even in the dilute dispersed regime. For many applications, from nuclear thermal hydraulics to bubble-mediated air-sea gas exchange, the dispersed phase often consists of many bubbles, bounded by surface tension and separated from the surrounding fluid by a deformable interface. Although many analytic and empirical models of multiphase flows have been formulated strictly for spherical or spheroidal particles with fixed shapes, in turbulent flows, finite-sized bubbles are constantly deforming with altogether different dynamics and momentum couplings over a wide range of scales. In this talk, I will share some ongoing efforts on developing new experimental facilities and techniques to simultaneously measure both the bubble deformation and surrounding turbulent flows in a Lagrangian framework. In particular, we are trying to determine the bubble breakup mechanisms when their size close to the Hinze scale.
14:45
15 mins

#432
EFFECT OF SOLUBLE SURFACTANT ON TURBULENT BUBBLY CHANNEL FLOW UNDERGOING TOPOLOGY CHANGES
Zaheer Ahmed, Daulet Izbassarov, Outi Tammisola, Jiacai Lu, Gretar Tryggvason, Metin Muradoglu
Abstract: The effects of soluble surfactant on the structure of turbulent bubbly flow in a pressure-driven vertical channel are examined using particle-resolved direct numerical simulations. The flow equations are fully coupled with the bulk and interfacial surfactant concentration evolution equations and solved using a fully parallel front-tracking method [4, 3]. The topology changes are handled using the algorithm developed by Lu and Tryggvason [2]. Simulations are first performed to examine the effects of soluble surfactant on turbulent bubbly channel flow without topology change, using the same computational setup as Lu et al. [1] for Re τ = 127.3 where the Reynolds number is based on friction velocity and half width of the channel. The results are shown in Fig. 1 for both clean and contaminated cases. As seen, the bubbles migrate toward the walls and form wall layers for the clean case, whereas the contaminated bubbles are more homogeneously distributed across the channel, which is qualitatively in good agreement with the results of Lu et al. [1]. The flow rates and the average wall shear stresses are also plotted in Fig. 1 for the clean and contaminated cases. The formation of wall layers in the clean case decreases the average liquid flow rate in the channel significantly, but the addition of surfactant prevents this reduction. The perfect agreement between the present results and the results of Lu et al. [1] for the clean case shows the accuracy of the present simulations. After the validation of the method, extensive simulations are performed for two types of surfactant, i.e., Triton X-100 and 1-Pentanol, for a range of bulk surfactant concentrations to examine the effects of soluble surfactant on turbulent bubbly flow as studied experimentally by Takagi et al. [5]. Topology changes such as bubble breakup and coalescence are fully taken into account in all the simulations. The preliminary results are in good qualitative agreement with the experimental observations of Takagi et al. [5]: The clean bubbles migrate toward the wall when coalesce is prevented but tend to coalesce to form larger deformable bubbles and thus migrate toward centerline while the bubble coalesce is largely inhibited by addition of surfactant and the contaminated bubbles tend to migrate away from the wall leaving bubble-devoid liquid layer on the wall.
15:00
15 mins

#217
DYNAMICS AND FRAGMENTATION OF SMALL FLEXIBLE FIBERS IN TURBULENCE
Sofia Allende, Christophe Henry, Jérémie Bec
Abstract: The dynamics of small flexible, inextensible fibers in a turbulent flow is found to follow most of the time that of a stiff rod. Deviations occur when the fibers experience a strong-enough compression and buckle. Such events are very rare and intermittent because of the long-term Lagrangian correlations of turbulent velocity gradients. We investigate the consequence of such a dynamics on fiber fragmentation. Two mechanisms are considered: tensile failure, corresponding to a too strong stretching by the flow, and flexural failure, when the fiber breaks because of a too strong curvature. Clearly, flexural failure can only occur when the fiber buckles. Surprisingly, large tension excursions are found to also be dominated by buckling rather than strong stretching. Fragmentation processes are hence determined by the most excited buckling mode and thus have an intricate dependence on the fiber flexibility.
15:15
15 mins

#82
FRAGMENTATION OF FIBRES IN TURBULENT FLOWS
Christophe Brouzet, Benjamin Favier, Marie-Julie Dalbe, Nicolas Vandenberghe, Gautier Verhille
Abstract: Marine plastic pollution is a global environmental issue as it can affect organisms ranging from small invertebrates to whales and cause the propagation of invasive species [2, 5]. While plastic debris mainly accumulate in the center of the five oceanic gyres, the fragmentation processes and the fate of plastics in oceans remain open questions. Indeed, plastic debris are weathered by several mechanisms, such as UV radiation, chemicals, microorganisms and fluid forces, but there is no model to explain the evolution of the fragment size distribution, which is crucial in the transport of such debris [2]. Nevertheless, as seen in Figure 1(a), the robustness of the fragment size distributions measured across the globe suggests that a general mechanism is at the origin of plastic fragmentation. Here, we investigate the scenario where plastic fragmentation in oceans is caused by mechanical forces resulting from the advection by a turbulent flow. As the Kolmogorov length in oceans is of the order of 0.6 to 6 mm, or even smaller during storms, most of the collected plastic fragments are in the inertial range. We therefore study experimentally and numerically the deformation and breakup of brittle fibres in the inertial range of a turbulent flow. In such a situation, a flexible fibre deforms because of the inhomogeneity of the fluid forces it experiences and the scale of its deformation depends on the fluid properties, the intensity of turbulence and the elastic properties of the fibre [1, 4]. In order to describe the fragmentation mechanisms involved and to model the evolution of the fragment size distribution with time, we focus on large deformation events where fragmentation occurs. The experiments are performed in a von Kármán turbulent flow, illustrated in Figure 1(b), with glass fibres of diameter 20 μm. The imaging of fibre deformations and breaking events are performed using a Schlieren technique and a high-speed camera. An example of fibre deformations observed with this technique is given in Figure 1(c). The numerical study is performed using Kinematic Simulations for the flow and a 1D elastica equation for the fibre [4]. Both experimental and numerical results exhibit a similar trend for the evolution of fragment size distribution and allow us to understand the main features of the fragment size distributions shown in Figure 1(a).
15:30
15 mins

#620
Droplet size distribution in surfactant-laden turbulent channel flow
Alfredo Soldati, Giovanni Soligo, Alessio Roccon
Abstract: In this work, we numerically investigate the dynamics of a swarm of surfactant-laden droplets in wall bounded turbulence. Simulations are based on the direct solution of the Navier-Stokes equations, coupled with a phase field method to describe droplets dynamics and surfactant concentration [2]. We consider a turbulent channel flow at a constant shear Reynolds number, Reτ = 300, in which a swarm of large and deformable surfactant-laden droplets is released. Different values of the surface tension, set via the Weber number (ratio between inertial and surface tension forces), and different types of surfactant with increasing strength (set with the elasticity number, βs, which quantifies the surface tension reduction) have been considered. Specifically, we consider two Weber numbers, W e = 1.50 and W e = 3.00, and four elasticity numbers, from βs = 0.50 (weaker surfactant) up to βs = 4.00 (stronger surfactant). The surfactant distribution over the droplet interface is determined by the interplay between inertial, viscous and surface tension forces (normal and tangential to the interface). In particular, we observe a distribution strongly influenced by the Marangoni (tangential) stresses. The surfactant distribution and the accompanying surface tension reduction results in an increase of both coalescence and breakage rates, and thus a modification of the dispersed phase morphology. Specifically, for stronger surfactants (higher elasticity numbers), a larger number of droplets is found in the channel. This modification in the dispersed phase morphology is reflected in the droplet size distribution as well and, for larger surfactant strengths, the presence of smaller droplets is favored. Indeed, the droplet size distribution shifts towards smaller values when increasing the surfactant strength. In addition, the droplet size distribution is in fair agreement with the power law distribution proposed by [1].