10:45
Multiphase Flows 7
10:45
15 mins
#19
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Particle distribution in a turbulent rough wall pipe
Leon Chan, Tony Zahtila, Jimmy Philip, Andrew Ooi
Abstract: Numerical simulation of a turbulent rough wall pipe laden with particles is conducted at a friction Reynolds number of 180 using direct numerical simulation. The Lagrangian point-particle method is used to track the particles and only the effects of the continuum fluid on the particles are considered. The pipe roughness consist of three-dimensional sinusoidal elements and three cases with varying roughness semi-amplitude of h+ = 5, 10 and 20 were simulated while maintaining a fixed roughness wavelength of lambda+ = 141. For each case, four parcels of particles (each containing 200,000 particles) with different Stokes numbers (St+ = 0.1, 1, 10 and 100) were included in the simulation to systematically investigate the effects of roughness height on the distribution of particles in the pipe. The collision of the heavy particles with the rough-wall significantly changes the distribution of heavy particles (St+ >> 1$) throughout the pipe, which would otherwise accumulate on the smooth wall due to the preference of heavy particles to be attracted to regions of high shear (see figure 1(a)). Lighter particles (St << 1) on the other hand have the tendency to accumulate at regions of intense vorticity. For case with h+ = 20, we observed that the maximum accumulation of light particles (St+ = 0.1) are located within the roughness canopy as reflected in the spike in the normalised concentration plot in figure 1(a)). Figure 1(b) shows that these particles are trapped in the wake/recirculation region of the roughness whereas heavy particles (St+=100) are not affected as it appears to be randomly distributed (figure 1(c)). Further study of the effects of rough walls on the particles such as wall collision frequency, topographical distribution of collision on the wall and trajectory of particles will also be analysed and presented in the conference.
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11:00
15 mins
#273
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Creation of turbulent puff in pipe flow with microbubble suspension
Kotaro Nakamura, Hyun Jin Park, Yuji Tasaka, Yuichi Murai
Abstract: Flow transition scenario in a pipe flow has been identified since the experimental study by O. Reynolds[1]. In a pipe flow, finite amplitude perturbations lead to an abrupt onset of turbulence[2][3]. The onset depends on the amplitude of perturbations[3] and the Reynolds number[4] generally defined as Re = UD/ν; U is the bulk mean velocity, D is the pipe diameter, and ν is the kinematic viscosity. The transition starts around 1,700 ≲ Re ≲ 2,300 with an appearance of localized turbulence named a puff[5]. A puff envelops streamwise vortices accompanying low-speed streaks[6], and these flow elements are key roles to sustain a puff[7]. A turbulent puff travels downstream and finally returns to laminar states stochastically in a finite lifetime[8]. Recent studies reported that flow transition in a pipe is modulated by the presence of neutrally buoyant particles[9][10] and microbubbles (MBs) with high volume fraction[11]. The objective of the present work is to examine the impact of dilute MB suspensions on the creation of a puff at Re = 1,900. Distinguishing flow status based on trends of streamwise velocity profiles on the pipe axis measured by laser Doppler velocimetry, creation probability of a puff was estimated in a flow without and with MBs. Suspended MBs were found to increase the probability at most 40% even though volume fraction of MBs was extremely dilute, around 0.018% as the maximum [12]. This suggests modulation on puff creations. We expect that the results are related to recent works by our groups[13][14] that dilute MBs have sensitive interaction with vortical structures and be accumulated in the vortices, which leads to enhancing vortices. For estimating the preferential concentration of MBs in a puff, bubble positions were visualized experimentally using laser sheet and calculated numerically using Euler-Lagrange simulations coupled with velocity fields of a turbulent puff obtained by direct numerical simulations (DNS)[7].
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11:15
15 mins
#492
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Wall-bounded turbulent flows: particles near surfaces
Christophe Henry
Abstract: This research activity aims at deepening our understanding of the transport of particles in wall-bounded turbulent flows, their interaction with surfaces (deposition) and their interactions within the fluid (agglomeration). Suspensions of particles are present in a wide range of fields, ranging from industrial applications (fouling of pumps by micro-organisms or fouling of combustion engines by soot particles) to environmental issues (dynamics of pollutants in the atmosphere or droplet formation in clouds) with implications even in our every-day life (accumulation of lime-scale in pipes). This accumulation results from the coupling between the transport of suspended particles by the fluid and the adhesion between particles and surfaces {4, 3}. One of the key challenges related to this research is that it involves different fields (including fluid dynamics, interface chemistry and material sciences) which span a wide range of time- and spatial-scales.
With the rapid development of modern computational techniques and efficient numerical solutions, researchers and engineers are increasingly relying on numerical simulations to address these issues. This has led to the development of both direct numerical simulations (that can be seen as numerical experiments) and macroscopic numerical tools (especially CFD codes that are widely used in industrial applications) [1, 6]. This study is focussed on detailing the importance of having coherent models between the approaches used for the fluid phase and the particle phase. This will be achieved by comparing the results obtained using various approaches, including: a) DNS simulations coupled with a Lagrangian tracking of solid particles; b) CFD simulations coupling Rij-epsilon models for turbulent flows and stochastic Lagrangian models for particles [5]; c) coupling of two stochastic Lagrange approaches for both fluid and particle phases [2]. One of the difficulty lies indeed in addressing properly the role of particle-surface interactions in their near-wall concentration in turbulent flows. As transpires from DNS studies, the outcome of particle impaction on a surface can indeed affect the concentration of particles in the near-wall region and, in turns, the deposition rate. Yet, when it comes to performing simulations using large time steps, coherent models are needed between the outcome of particle interactions with the surface and what is actually implemented in the model for near-wall turbulence in the Eulerian simulation. Numerical simulations will also be presented in the presence of surface roughness, since it can have noticeable consequences on the question of particle deposition on surfaces (for pollutant or aerosols in the atmosphere).
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11:30
15 mins
#461
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Transition to turbulence in core-annular pipe flow
Carlos Plana, Baofang Song, Marc Avila
Abstract: Core-Annular flows (CAF) are a particular set of two-phase pipe flow regimes where an inner fluid, the "core", is surrounded by an outer fluid, the "annulus". In the core-annular configuration, the viscous fluid tends towards the center of the pipe while the less viscous one migrates to the high-shear region close to the walls. This structure results in a large reduction in the friction losses when compared with the flow of the single-phase viscous fluid [1]. However, the laminar core-annular flow configuration, in which the interface between the two fluids is parallel to the pipe wall is usually unstable, and therefore, we are interested in the study of the evolution of the system into the nonlinear regime. We use the Cahn–Hilliard (phase-field) method to model the two-phase flow [2]. Our implementation employs the pseudospectral Fourier–Galerkin method in the periodic azimuthal and axial directions and high order finite differences in the radial direction [3]. In this work, we study the transition to turbulence of laminar core-annular flow of kerosene surrounded by water. The flow is driven upward in a vertical pipe and the Reynolds number (based on the average flow speed) is Re = 6000. We find a variety of fluid instabilities, which emerge from the interaction and competition of capillary forces (mainly at low Reynolds number) and interfacial friction due to large shear velocity gradients at the interface. We will discuss both the linear instabilties of the laminar flow, as well as the transition and saturation in the nonlinear regime. A snapshot of the transition process for the case with Re = 6000 is presented in figure 1.
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11:45
15 mins
#517
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Investigation of Interfacial Forces in CFD simulation of Turbulent Bubbly Pipe Flows
Mohsen Shiea, Marco Vanni, Daniele Marchisio, Antonio Buffo
Abstract: Predictions of Eulerian-Eulerian CFD simulations of bubbly flows depend strongly upon the choice of models to describe interfacial forces, i.e. drag, turbulent dispersion, lift and wall lubrication. The present work studies the effect of interfacial forces and relevant models, specifically those to describe the lift and wall lubrication forces, on the predictions of Eulerian-Eulerian CFD simulations of turbulent bubbly flows. The test case under study is a developing turbulent bubbly pipe flow, for which experimental volume fraction and velocity of the phases are available. The simulations are done using OpenFOAM software. The results show how the simulation predictions can be improved by using models that are developed to be employed under turbulent conditions, particularly the geometric approach to consider the wall effect. Eventually, the results will highlight the importance of investigating the interfacial forces under turbulent conditions to draw more general conclusions on the applicability of the models.
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12:00
15 mins
#427
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Fluid/particle momentum coupling in turbulent jets
Francesco Battista, Paolo Gualtieri, Jean-Paul Mollicone, Carlo Massimo Casciola
Abstract: Particle laden turbulent jets are characteristic of many natural phenomena and technological applications. Sprays in internal engines consist in turbulent jets transporting tiny droplets. The typical conditions occurring after the primary and secondary atomisation of the liquid jets [5], correspond to a droplet laden turbulent jet in the so-called two-way coupling regime. How do the particle influence the jet dynamics? In particular, is the jet dynamics sill self-similar? Are the self-similar scaling laws the same or are they changed? These are some questions concerning the considered flow. The answers to them imply fundamental consequences for the transported particles and is certainly relevant for reactive flows. Our purpose is to provide a characterisation of this peculiar flow highlighting its main physical properties.
To this purpose the conservative form of incompressible Navier-Stokes equations are solved in a cylindrical domain by means Direct Numerical Simulation (DNS), see [2, 6] for details, coupled with the the Exact Regularised Point Particle (ERPP) approach, [4, 3, 1] exploiting the fluid/particle momentum coupling. Small spherical particles with density much largerthanthefluiddensity,ρp/ρf ≫1,areconsidered,neglectinginter-particlecollisionsasappropriateforthepresent conditions. Particles are introduced at a fixed rate, which is tuned to obtain the desired mass-loading φ, with homogeneous distribution at the jet inlet section and velocity equal to the fluid velocity. Three particle populations with different inertia are employed. The figure shows the instantaneous fluid axial velocity (colour) and the instantaneous particle configurations (back dots) in the left panel. The middle panel shows the mean axial fluid velocity without (left-half) and with (right-half) the particle. The particles tend to increase the jet velocity and decrease its spreading angle. The right panel shows the inverse of the mean axial centerline fluid velocity, Uc, as a function of the streamwise coordinate. As in the classical Newtonian jet, Uc decreases with the inverse of the streamwise coordinate also in the two-way coupling regime, although with different intensity. The self similar behaviour of the jet and its universal behaviour will be deeper analysed together to the turbulence modulation and particle dynamics as a function of the particle inertia and mass-loading.
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12:15
15 mins
#435
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The Exact Regularised Point Particle method for wall turbulence modulation
Paolo Gualtieri, Francesco Battista, Jean-Paul Mollicone, Carlo Massimo Casciola
Abstract: In wall bounded particle laden flows when the mass flow rate of the disperse phase and of the carrier fluid are comparable a significant inter-phase momentum exchange occurs (two-way coupling regime). Under these conditions a substantial alteration of the turbulent fluctuations is observed.
Many numerical attempts to predict the flow modification are available in the literature even hough the results do not indicate a clear effect of the particles on the fluid. The data in \cite{li2001numerical} show that turbulence intensities were increased for small mass loading but
the inverse behaviour is observed for higher loadings. The data discussed in~\cite{zhao2010turbulence} report an overall drag reduction due to the particles.
In this contribution we present new results of a particle laden turbulent pipe flow in the two-way coupling regime exploiting a novel momentum coupling method named Exact Regularised Point Particle (ERPP) approach~\cite{battista2018exact,gualtieri2015exact} which overcomes the typical difficulties of the Particle In Cell approach. In fact, in the ERPP, the momentum coupling is achieved in a physically consistent manner. The local flow disturbance produced by each small particle is described in closed form by an exact unsteady Stokes solution.
The left panel of figure~\ref{fig:fig1} (see pdf file) highlights the back-reaction field of the particles on the carrier fluid.
The colours correspond to the feedback-intensity which is strongly imprinted by the instantaneous particle geometrical configuration. The average effect of the particles on the fluid is the decrease of the mean flow rate for an
imposed pressure gradient, i.e. the drag is increased by the particles. The right panel of figure~\ref{fig:fig1} shows the mean velocity profile. As apparent from the data, the neat effect of the particles is to reduce the flow rate, hence the overall drag is increased as observed in several experiments~\cite{righetti2004particle,li2012experimental}.
In the extended contribution we will fully document the turbulence modulation and, by exploiting the axial mean momentum equation, we will give reason of the observed drag increase.
The research has received funding from the European Research Council under the ERC Grant Agreement no. 339446. PRACE, under grant no. 2014112647,
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