14:00
Multiphase Flows 5
14:00
15 mins
#338
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Inertial effects on the settling and collisions between spheroids in a turbulent flow
Aurore Naso, Sheikh Muhammad Zubair, Alain Pumir, Emmanuel Lévêque
Abstract: We focus on the orientation and collisions between spheroids settling in a turbulent flow, with possible applications to the microphysics of clouds.
We first introduce a parameter allowing to distinguish the regimes in which the rotational motion of spheroidal particles of any aspect ratio is dominated by (fluid) inertial corrections (and the motion is therefore predominantly broadside on) from those in which the fluid inertia effect can be neglected. The relevance of this parameter is first successfully tested for prolate (elongated) and oblate (flat) particles transported in synthetic turbulent flows.
Finally, we present the results of a study of the translational and rotational motion of thin oblate spheroids settling in a turbulent flow, obtained by solving the Navier-Stokes equations using direct numerical simulations. The effect of the first inertial corrections due to the fluid inertia on the orientation of the particles and on their collisions, estimated by using the “ghost collision approximation”, is discussed.
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14:15
15 mins
#580
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Settling of large particles in a turbulence column
Yulia Akutina, Thibaud Revil-Baudard, Julien Chauchat, Olivier Eiff
Abstract: An accurate prediction of the effective fall-velocity of solid particles settling in a turbulent fluid is of major importance fora large number of industrial and geophysical applications. It has been shown that the particle fall-velocity can be either enhanced or hindered by the fluid turbulence [4, 2, 6, 3]. However, the current state-of-the-art research is still far from being able to predict turbulence-induced settling modification. In this work, settling experiments were performed in a turbulence column to investigate the effect of turbulence on the settling of large (larger than the Kolmogorov length scale) solid particles slightly denser than the fluid (ρp/ρf= 1.1−2.6), a regime which can be considered to be of finite size particles [1] for which an analytical form for the fluid-particle forces is unknown and few data is available. Five different particle types (S1−S5, with variable density, shape and size) were tested. The Stokes number based on the Kolmogorov time scale was of order unity. The Particulate Reynolds number varied between 75 and 981. The particles were manually released, one at time, into the turbulence column (a water tank with horizontally oscillating grids to produce statistically homogeneous isotropic turbulence in the middle section). As a particle settled, its instantaneous velocities were measured using particle tracking. 2D particle image velocimetry (PIV) was then performed separately to obtain flow statistics. The turbulence column was run at 5 different stirring frequencies, resulting in a range of turbulence intensities. The local rms velocities of the flow were calculated and the local settling velocity of every particle was then assigned to a local rms velocity of the flow. Since the turbulence column is not perfectly symmetric, and the ideal zero-mean flow is not achievable in practice, the residual mean flow in the tank was subtracted from the instantaneous flow velocities. Figure 1 shows the relation between the particle settling velocity, Ws, and the local vertical turbulence intensity (rms velocity, Wf,rms). In this figure the data were averaged within 8 levels of turbulence intensity with a step of Wf,rms=0.33cm/s. For all types of particles tested, the settling retardation is observed as the turbulence intensity increases. It can be seen that when both the effective settling velocity and the turbulent intensity are non-dimensionilized by the still-fluid terminal velocity, W0, the settling retardation can be described by a unique relation independent of the particle type, Ws/W0 = f(Wf,rms/W0). In this work, using a simplified representation of the flow field, analytical formulations are also derived for the loitering [5] and non-linear drag [2] effects, where WLs and WNLDs are the settling velocities modified by the loitering and non-linear drag effects, respectively. Both formulations show that the settling retardation normalized by the still-fluid settling velocity is a function of the ratio Wf,rms / W0 and is independent of particle parameters, which is the same result as the one obtained from the experimental data. Thus, this scaling law is considered robust for a given parameter regime of relatively large particles settling in a turbulent flow.
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14:30
15 mins
#185
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Multiscale Preferential Sweeping Of Particles Settling In Turbulence
Josin Tom, Andrew Bragg
Abstract: The average settling speed of small heavy particles in turbulent flows is important for many environmental problems such as water droplets in clouds and atmospheric aerosols. In the seminal article, Maxey (1987) developed a theoretical framework for understanding how the particle inertia leads to enhanced settling speeds in turbulence (compared with the Stokes settling speed). This analysis showed that showing that enhanced particle settling speeds in turbulence occur through the preferential sweeping mechanism, which depends on the preferential sampling of the fluid velocity gradient field by the inertial particles. Limitations of this analysis, however, are that it does not describe the case for St = O(1) and that it does not provide insight into potential multiscale contributions to the settling speed. Also, recent Direct Numerical Simulation (DNS) results in Ireland et al. (2016) show that even in a portion of the parameter space where this preferential sampling is absent, the particles nevertheless exhibit enhanced settling velocities. To address these issues, we have developed a new theoretical result, valid for arbitrary Stokes Number, St, using averaging decompositions, Probability Density Function (PDF) methods and coarse-graining procedures. The new result reveals the truly multiscale nature of the mechanism generating the enhanced settling speeds. In particular, it shows how the range of scales at which the preferential sweeping mechanism operates depends on St. This analysis is complemented by results from DNS, where we examine the role of different flow scales on the particle settling speeds by coarse-graining the underlying flow. We also explore the multiscale nature of the preferential sweeping mechanism by considering how particles preferentially sample the fluid velocity gradients coarse-grained at various scales. This explains the findings of Ireland et al. (2016), and further illustrates the truly multiscale nature of the mechanism generating enhanced particle settling speeds in turbulence. Other predictions and results following from the theoretical analysis will also be presented.
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14:45
15 mins
#603
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Effect of turbulence-induced inertial clustering on droplet arrival statistics in a polydisperse droplet field
SHYAM KUMAR M, Chakravarthy S.R, Mathur Manikandan
Abstract: The extent to which inertial clustering affects droplet dynamics in a turbulent flow is of interest in several areas like rain formation, combustion systems etc. Here, we perform an experimental investigation of the effects of inertial clustering on the droplet arrival pattern in a turbulent flow. The experimental set-up (left panel of Fig.1) consists of a settling chamber followed by a contraction, the downstream end of which is fitted with an Active Turbulence Grid (ATG). Water droplets were introduced in the flow through a pressure-based atomizer, placed downstream of the ATG. Different air flow turbulence intensities were achieved by suitably varying the inlet air flow rate and the rotational speeds of the blades in the ATG. Droplet size and velocity were measured using Phase Doppler Interferometer (PDI), and the air flow turbulence was characterized by Laser Doppler Velocimetry (LDV). Measurements were done far downstream of the spray nozzle so that the droplet size distribution and its spatial evolution is mostly due to the background air turbulence and gravity.
From the PDI data, we have investigated the pattern in droplet sizes arriving at the probe volume. In the right panel of Fig.1, the relative occurrence of monotonic patterns, corresponding to patterns of ascending droplet size, with various axial locations for different air turbulent intensities is shown. Up to a maximum air turbulent intensity of 12%, the occurrence of the monotonic pattern decreases with axial location. On the other hand, for air turbulent intensities above 12%, an apparent increase in the occurrence of monotonic pattern with the axial location is observed.
The existence of a threshold turbulent intensity for an increased occurrence of monotonic pattern would be understood in terms of the onset of clustering. The onset of droplet clustering for turbulent intensities greater than the threshold value is confirmed using the Pair Correlation Function (PCF). Further, a 3D Voronoi analysis is used to substantiate the occurrence of inertial clustering upon the background air turbulent intensity passes the threshold value. The effect of inertial clustering on the increased occurrence of the monotonic pattern is understood in terms of the enhanced settling velocity and turbulence induced size-velocity correlation in a polydisperse field.
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15:00
15 mins
#83
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Effect of mass loading on the collison rate of cloud droplets
Bogdan Rosa
Abstract: Small-scale atmospheric turbulence is the primary mechanism that governs the dynamics of cloud processes. Formation of rain drops and onset of precipitation largely depend on the collision rate of small cloud droplets. Quantitative description of the microphysical processes is essential for the development of more accurate subgrid-scale parameterizations in contemporary numerical models for weather forecasting. There are many studies of the processes in scientific literature. The common numerical method to examine drops formation are simulations DNS. The vast majority of previous DNS were performed assuming the point-particle approximation and 1-way momentum coupling between continuous and dispersed phases. This simplified approach is sufficient only for dilute systems with low mass loading. The aim of the present study is to investigate the effect of 2-way momentum coupling on the kinematic and dynamic collision statistics. The new DNS were performed using two different algorithms for computing the interacting force i.e. particle in cell and projection onto neighboring node. To address the effect of gravity, the simulations have been carried out simultaneously both with and without gravitational acceleration.
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15:15
15 mins
#519
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Results from the Zugspitze Experiment: an in-situ cloud-droplet particle-tracking experiment
Guus Bertens, Gholamhossein Bagheri, Haitao Xu, Eberhard Bodenschatz, Jan Moláček
Abstract: It is well-known that rain formation has four phases: nucleation, condensation, turbulent coalescence, and gravitational coalescence. Condensation is effective for droplet sizes of up to ∼ 20 µm, whereas gravitational coalescence is effective for droplet sizes larger than ∼ 100 µm. Turbulence is responsible for bridging the gap between these two, but how exactly it does this, is not known.
One of the first to study the effect of turbulence on rain formation were Saffman & Turner [6]. While beautiful in its simplicity, their theory has some shortcomings: it doesn’t take droplet clustering (e.g. [1]) or the sling effect (e.g. [2]) into account. Many studies have tried to resolve these issues. To keep the problem tractable many theoretical studies assume droplets are monodisperse and/or neglect gravity (e.g. [4, 8]). This makes the results of limited relevance to clouds. Numerical and experimental studies (e.g. [3]) are often limited to low Reynolds numbers, and hence cannot faithfully reproduce cloud conditions. To avoid these issues, one must measure inside clouds.
Here we present an in-situ cloud-droplet tracking experiment. The experiment (Fig. 1, left) is located on top of the environmental research station Schneefernerhaus, at 2650 m altitude, just below the peak of Mt. Zugspitze in the German Alps. At this location clouds occur close to the ground [5], which obviates the need for planes or helicopters. At the heart of the experiment are three high speed cameras, capable of recording 1 Mpx at 10 kHz. They are pointed at a small volume, approximately (2.5 cm)^3 in size, illuminated by a 75 W green laser. The cameras are mounted on rails and can be moved by a linear motor, in order compensate for the mean wind. Images are processed with an in-house particle tracking code, that is remniscent of the Shake-The-Box algorithm [7]. The code is particularly suitable for processing low light imagery (Fig. 1, right), in which many droplet images are out of focus.
We report measurements of the radial distribution function (RDF) for separations of 0.1 mm to 20 mm. Furthermore we can estimate relative radial velocities (RRV), and condition both quantities on approximate droplet size.
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15:30
15 mins
#160
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Accumulation of sedimenting particles in turbulent flows
Alessandro Sozza, Gabor Drotos, Cristobal Lopez, Emilio Hernandez-Garcia
Abstract: We study the effect of turbulence on a sedimenting layer of particle by means of direct numerical simulations. A Lagrangian model of tracers with a downward settling velocity is integrated together with a isotropic homogeneous turbulent flow. Particles initially distributed on the upper plate of the box are released and collected at the bottom. We analyze the distribution of particles at the accumulation plane for different values of settling velocity and Reynolds number. The density of particle at the bottom is the result of geometric properties of the flows, that can be identified in two contributions: the stretching along each particle trajectory and the projection on the accumulation plane. Then, we discuss the degree of clustering and the deviation from homogeneity.
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