| 1. |
- Magaletti, F., et al.
(författare)
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The sharp-interface limit of the Cahn-Hilliard/Navier-Stokes model for binary fluids
- 2013
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Ingår i: Journal of Fluid Mechanics. - : Cambridge University Press (CUP). - 0022-1120 .- 1469-7645. ; 714, s. 95-126
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Tidskriftsartikel (refereegranskat)abstract
- The Cahn-Hilliard model is increasingly often being used in combination with the incompressible Navier-Stokes equation to describe unsteady binary fluids in a variety of applications ranging from turbulent two-phase flows to microfluidics. The thickness of the interface between the two bulk fluids and the mobility are the main parameters of the model. For real fluids they are usually too small to be directly used in numerical simulations. Several authors proposed criteria for the proper choice of interface thickness and mobility in order to reach the so-called 'sharp-interface limit'. In this paper the problem is approached by a formal asymptotic expansion of the governing equations. It is shown that the mobility is an effective parameter to be chosen proportional to the square of the interface thickness. The theoretical results are confirmed by numerical simulations for two prototypal flows, namely capillary waves riding the interface and droplets coalescence. The numerical analysis of two different physical problems confirms the theoretical findings and establishes an optimal relationship between the effective parameters of the model.
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| 2. |
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| 3. |
- Battista, F., et al.
(författare)
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Turbulence-combustion interaction in H2/CO/air Bunsen flame
- 2020
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Ingår i: ETC 2013 - 14th European Turbulence Conference. - : Zakon Group LLC.
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Konferensbidrag (refereegranskat)abstract
- In last decades, the increasing care to environmental safeguard and costs in the hydrocarbon fuel supplying have prompted in the development of alternative fuels, namely hydrogen based fuels as syngas. Syngas consists in a mixture of hydrogen and carbon monoxide (CO) in different relative concentration, in some cases with small concentration of methane. The aim of this work is to address the dynamics of turbulent hydrogen/carbon-monoxide/air Bunsen flames by means of Direct Numerical Simulation. The main issue is to understand how the thermo-diffusive instabilities occurring in pure hydrogen/air flame [7] are influenced by the presence of the carbon-monoxide. It is well known that the thermo-diffusive instabilities are mainly induced by the high hydrogen diffusivity leading to local quenching and temperature peaks in the flame with consequent increase of pollutant formation (e.g. NOx). The presence of carbon monoxide in the fuel mixture has significant effects in flame dynamics where we observe a damping of the H2/air flame instabilities with less apparent quenching and high temperature peaks.
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| 4. |
- Cimarelli, A., et al.
(författare)
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Sources and fluxes of scale energy in the overlap layer of wall turbulence
- 2015
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Ingår i: Journal of Fluid Mechanics. - : Cambridge University Press (CUP). - 0022-1120 .- 1469-7645. ; 771, s. 407-423
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Tidskriftsartikel (refereegranskat)abstract
- Direct numerical simulations of turbulent channel flows at friction Reynolds numbers (Re) of 550, 1000 and 1500 are used to analyse the turbulent production, transfer and dissipation mechanisms in the compound space of scales and wall distances by means of the Kolmogorov equation generalized to inhomogeneous anisotropic flows. Two distinct peaks of scale-energy source are identified. The first, stronger one, belongs to the near-wall cycle. Its location in the space of scales and physical space is found to scale in viscous units, while its intensity grows slowly with Re, indicating a near-wall modulation. The second source peak is found further away from the wall in the putative overlap layer, and it is separated from the near-wall source by a layer of significant scale-energy sink. The dynamics of the second outer source appears to be strongly dependent on the Reynolds number. The detailed scale-by-scale analysis of this source highlights well-defined features that are used to make the properties of the outer turbulent source independent of Reynolds number and wall distance by rescaling the problem. Overall, the present results suggest a strong connection of the observed outer scale-energy source with the presence of an outer region of turbulence production whose mechanisms are well separated from the near-wall region and whose statistical features agree with the hypothesis of an overlap layer dominated by attached eddies. Inner-outer interactions between the near-wall and outer source region in terms of scale-energy fluxes are also analysed. It is conjectured that the near-wall modulation of the statistics at increasing Reynolds number can be related to a confinement of the near-wall turbulence production due to the presence of increasingly large production scales in the outer scale-energy source region.
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| 5. |
- Gualtieri, P., et al.
(författare)
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Clustering and turbulence modulation in particle-laden shear flows
- 2013
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Ingår i: Journal of Fluid Mechanics. - : Cambridge University Press (CUP). - 0022-1120 .- 1469-7645. ; 715, s. 134-162
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Tidskriftsartikel (refereegranskat)abstract
- Turbulent fluctuations induce the common phenomenon known as clustering in the spatial arrangement of small inertial particles transported by the fluid. Particles spread non-uniformly, and form clusters where their local concentration is much higher than in nearby rarefaction regions. The underlying physics has been exhaustively analysed in the so-called one-way coupling regime, i.e. negligible back-reaction of the particles on the fluid, where the mean flow anisotropy induces preferential orientation of the clusters. Turbulent transport in suspensions with significant mass in the disperse phase, i.e. particles back-reacting in the carrier phase (the two-way coupling regime), has instead been much less investigated and is still poorly understood. The issue is discussed here by addressing direct numerical simulations of particle-laden homogeneous shear flows in the two-way coupling regime. Consistent with previous findings, we observe an overall depletion of the turbulent fluctuations for particles with response time of the order of the Kolmogorov time scale. The depletion occurs in the energy-containing range, while augmentation is observed in the small-scale range down to the dissipative scales. Increasing the mass load results in substantial broadening of the energy cospectrum, thereby extending the range of scales driven by anisotropic production mechanisms. As discussed throughout the paper, this is due to the clusters which form the spatial support of the back-reaction field and give rise to a highly anisotropic forcing, active down to the smallest scales. A certain impact on two-phase flow turbulence modelling is expected from the above conclusions, since the frequently assumed small-scale isotropy is poorly recovered when the coupling between the phases becomes significant.
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| 6. |
- Gualtieri, P., et al.
(författare)
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Exact regularized point particle method for multiphase fows in the two-way coupling regime
- 2015
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Ingår i: Journal of Fluid Mechanics. - : Cambridge University Press (CUP). - 0022-1120 .- 1469-7645. ; 773, s. 520-561
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Tidskriftsartikel (refereegranskat)abstract
- Particulate flows have mainly been studied under the simplifying assumption of a one-way coupling regime where the disperse phase does not modify the carrier fluid. A more complete view of multiphase flows can be gained calling into play two-way coupling effects, i.e. by accounting for the inter-phase momentum exchange, which is certainly relevant at increasing mass loading. In this paper we present a new methodology rigorously designed to capture the inter-phase momentum exchange for particles smaller than the smallest hydrodynamical scale, e.g. the Kolmogorov scale in a turbulent flow. The momentum coupling mechanism exploits the unsteady Stokes flow around a small rigid sphere, where the transient disturbance produced by each particle is evaluated in a closed form. The particles are described as lumped point masses, which would lead to the appearance of singularities. A rigorous regularization procedure is conceived to extract the physically relevant interactions between the particles and the fluid which avoids any 'ad hoc' assumption. The approach is suited for high-efficiency implementation on massively parallel machines since the transient disturbance produced by the particles is strongly localized in space. We will show that hundreds of thousands of particles can be handled at an affordable computational cost, as demonstrated by a preliminary application to a particle-laden turbulent shear flow.
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| 7. |
- Gualtieri, P., et al.
(författare)
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Exact regularized point particle method for particle-laden flows in the two-way coupling regime
- 2012
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Ingår i: Proceedings of the Seventh International Symposium On Turbulence Heat and Mass Transfer. - : Begell House. ; , s. 1342-1351
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Konferensbidrag (refereegranskat)abstract
- In this paper we present a new methodology which is proved to capture the momentum exchange between a carrier turbulent flow and thousands of sub-Kolmogorov inertial particles. The velocity disturbance produced by the disperse phase is described in terms of exact regularized unsteady Stokes solutions. The approach is validated by addressing the motion of a single particle in still fluid and by comparing data of actual turbulent spatially homogeneous flows against results provided by the classical particle-in-cell method.
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| 8. |
- Picano, F., et al.
(författare)
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Particle-laden jets : Particle distribution and back-reaction on the flow
- 2011
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Ingår i: Journal of Physics: Conference Series. - : IOP Publishing. - 1742-6596.
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Konferensbidrag (refereegranskat)abstract
- DNS data of particle-laden jets are discussed both in the one- and two-way coupling regimes. Dynamics of inertial particles in turbulent jets is characterized by an anomalous transport that leads to the formation of particle concentration peaks along the jet axis. Larger is the particle inertia farther the peak location occurs. The controlling parameter is found to be the local large-scale Stokes number which decreases quadratically with the axial distance and is order one in coincidence of the peaks. The centerline mean particle velocity is characterized by two scaling laws. The former occurs upstream the location where the Stokes number is order one, and is linear in the axial distance with negative coefficient. The latter, occurring downstream where the local Stokes number is small, coincides with that of the centerline mean fluid velocity. This behavior affects the development of the particle-laden jet when the mass load of the particulate phase increases and two-way coupling effects become relevant. Two distinct behaviors for the jet development are found behind and beyond the location of unity local Stokes number leading to different scaling laws for the mean centerline fluid velocity.
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| 9. |
- Sardina, Gaetano, et al.
(författare)
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Statistics of Particle Accumulation in Spatially Developing Turbulent Boundary Layers
- 2014
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Ingår i: Flow Turbulence and Combustion. - : Springer Science and Business Media LLC. - 1386-6184 .- 1573-1987. ; 92:1-2, s. 27-40
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Tidskriftsartikel (refereegranskat)abstract
- We present the results of a Direct Numerical Simulation of a particle-laden spatially developing turbulent boundary layer up to Re (theta) = 2500. Two main features differentiate the behavior of inertial particles in a zero-pressure-gradient turbulent boundary layer from the more commonly studied case of a parallel channel flow. The first is the variation along the streamwise direction of the local dimensionless parameters defining the fluid-particle interactions. The second is the coexistence of an irrotational free-stream and a near-wall rotational turbulent flow. As concerns the first issue, an inner and an outer Stokes number can be defined using inner and outer flow units. The inner Stokes number governs the near-wall behavior similarly to the case of channel flow. To understand the effect of a laminar-turbulent interface, we examine the behavior of particles initially released in the free stream and show that they present a distinct behavior with respect to those directly injected inside the boundary layer. A region of minimum concentration occurs inside the turbulent boundary layer at about one displacement thickness from the wall. Its formation is due to the competition between two transport mechanisms: a relatively slow turbulent diffusion towards the buffer layer and a fast turbophoretic drift towards the wall.
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| 10. |
- Sardina, Gaetano, et al.
(författare)
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Statistics of particle accumulation in spatially developing turbulent boundary layers
- 2012
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Ingår i: THMT-12. Proceedings of the Seventh International Symposium On Turbulence Heat and Mass Transfer. - : Begell House. ; , s. 1715-1723
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Konferensbidrag (refereegranskat)abstract
- A Direct Numerical Simulation of a particle-laden spatially developing turbulent boundary layer up to Reθ = 2500 has been performed. The peculiar feature of a boundary layer flow seeded with heavy particles is the variation of the local dimensionless parameters defining the fluid-particle interactions along the streamwise direction. An inner and an outer Stokes number can be defined using inner and outer flow units. These two parameters show different decay rates in the streamwise direction so that it is possible to find a decoupled particle dynamics between the inner and the outer region of the boundary layer. Preferential near-wall particle accumulation is similar to that observed in turbulent channel flow, while a self-similar behavior characterizes the outer region.
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| 11. |
- Sardina, G., et al.
(författare)
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Wall accumulation and spatial localization in particle-laden wall flows
- 2012
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Ingår i: Journal of Fluid Mechanics. - : Cambridge University Press (CUP). - 0022-1120 .- 1469-7645. ; 699, s. 50-78
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Tidskriftsartikel (refereegranskat)abstract
- We study the two main phenomenologies associated with the transport of inertial particles in turbulent flows, turbophoresis and small-scale clustering. Turbophoresis describes the turbulence-induced wall accumulation of particles dispersed in wall turbulence, while small-scale clustering is a form of local segregation that affects the particle distribution in the presence of fine-scale turbulence. Despite the fact that the two aspects are usually addressed separately, this paper shows that they occur simultaneously in wall-bounded flows, where they represent different aspects of the same process. We study these phenomena by post-processing data from a direct numerical simulation of turbulent channel flow with different populations of inertial particles. It is shown that artificial domain truncation can easily alter the mean particle concentration profile, unless the domain is large enough to exclude possible correlation of the turbulence and the near-wall particle aggregates. The data show a strong link between accumulation level and clustering intensity in the near-wall region. At statistical steady state, most accumulating particles aggregate in strongly directional and almost filamentary structures, as found by considering suitable two-point observables able to extract clustering intensity and anisotropy. The analysis provides quantitative indications of the wall-segregation process as a function of the particle inertia. It is shown that, although the most wall-accumulating particles are too heavy to segregate in homogeneous turbulence, they exhibit the most intense local small-scale clustering near the wall as measured by the singularity exponent of the particle pair correlation function.
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| 12. |
- Troiani, G., et al.
(författare)
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Curvature and velocity strain dependencies of burning speed in a turbulent premixed jet flame
- 2020
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Ingår i: ETC 2013 - 14th European Turbulence Conference. - : Zakon Group LLC.
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Konferensbidrag (refereegranskat)abstract
- In this work the dependency of the turbulent burning speed on flame stretch in a premixed jet flame is analyzed. Considering a reference system attached to the front, the flame stretch is split into three contributions based on flame front curvature, normal fluid velocity and divergence of tangential velocity. The turbulent burning velocity is derived from the measure of the divergence of the mean unconditioned velocity field, that is taken as an estimate of the mean reaction rate in the context of flamelet hypothesis. The results are in a reasonable agreement with the literature data on turbulent combustion rates. Though the present methodology is more complex than the usual one based on reactant consumption rate, it provides the local burning speed and not the overall one. Combining these measurements with the local flame stretch, we show that, for a given flame, it exists a wide region along the flame height where the increase of the local flame speed in respect to the laminar unstretched one (stretching factor) is constant. Since the Reynolds number controls the small-scale behavior of turbulence, these findings denote a direct connection between the local, turbulence-induced, flame front deformation and the increase of the local flame propagation speed. The aim of this work is to establish correlations between the three different terms of flame stretch and the turbulent combustion speed that can lead to the definition of suitable closure models for turbulent combustion numerical simulations.
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