| 1. |
- Vanna, Francesco De, et al.
(author)
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Effect of convective schemes in wall-resolved and wall-modeled LES of compressible wall turbulence
- 2023
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In: Computers & Fluids. - : Elsevier BV. - 0045-7930 .- 1879-0747. ; 250, s. 105710-
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Journal article (peer-reviewed)abstract
- The current study discusses how numerical schemes and discretization approaches affect wall-resolved and wall-modeled LES outcomes. A turbulent boundary layer setup over a flat plate in both super-and hypersonic conditions is used to illustrate the effect of different numerical discretization strategies. In particular, six convective methods are examined, as well as various degrees of hybridization between shock-capturing and centered approaches: The former introducing non-negligible numerical viscosity, the latter being virtually dissipation-free. The analysis reveals that injected numerical viscosity due to upwinding procedures consid-erably alters wall dynamics for both wall-resolved and especially wall-modeled arrangements. In particular, if low-order or pure shock-capturing schemes are used, wall modeling fails in heading the system dynamics due to a strong modulation of main turbulent features. Conversely, realistic turbulence patterns are recovered if hybrid and/or high-order shock-capturing methods are employed. Thus, the paper establishes criteria for selecting a suitable numerical setup in wall-modeled LES, providing suggestions for grid resolution levels and convective scheme selection/hybridization. An overview perspective concerning numerical diffusion coupling with turbulent stresses in wall-resolved and wall-modeled LES is also provided.
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| 2. |
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| 3. |
- Battista, F., et al.
(author)
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Fractal scaling of turbulent premixed flame fronts : Application to LES
- 2015
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In: International Journal of Heat and Fluid Flow. - : Elsevier BV. - 0142-727X .- 1879-2278. ; 51, s. 78-87
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Journal article (peer-reviewed)abstract
- The fractal scaling properties of turbulent premixed flame fronts have been investigated and considered for modeling sub-grid scales in the Large-Eddy-Simulation framework. Since the width of such thin reaction fronts cannot be resolved into the coarse mesh of LES, the extent of wrinkled flame surface contained in a volume is taken into account. The amount of unresolved flame front is estimated via the "wrinkling factor" that depends on the definition of a suitable fractal dimension and the scale at which the fractal scaling is lost, the inner cut-off length e. In this context, the present study considers laboratory experiments and one-step reaction DNS of turbulent premixed jet flames in different regimes of turbulent premixed flames. Fractal dimension is found to be substantially constant and well below that typical of passive scalar fronts. The inner cut-off length shows a clear scaling with the dissipative scale of Kolmogorov for the regimes here considered. These features have been exploited performing Large Eddy Simulations. Good model performance has been found comparing the LES against a corresponding DNS at moderate Reynolds number and experimental data at higher Reynolds numbers.
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| 4. |
- Battista, F., et al.
(author)
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Turbulence-combustion interaction in H2/CO/air Bunsen flame
- 2020
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In: ETC 2013 - 14th European Turbulence Conference. - : Zakon Group LLC.
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Conference paper (peer-reviewed)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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| 5. |
- Bäbler, Matthäus, 1977-, et al.
(author)
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Breakup of small aggregates in bounded and unbounded turbulent flows
- 2020
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In: ETC 2013 - 14th European Turbulence Conference. - : Zakon Group LLC.
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Conference paper (peer-reviewed)abstract
- Breakup of small tracer-like aggregates is studied by means of numerical simulations in four different flows, namely homogeneous isotropic turbulence, smooth stochastic flow, turbulent channel flow, and developing boundary layer flow. Aggregate breakup occurs when the local hydrodynamic stress σ ∼ ε1/2, where ε is the local energy dissipation, overcomes a given threshold value σcr [or equivalently εcr ∼ σcr2 ] characteristic for a given type of aggregates. Following the aggregate trajectory upon release and detecting the first occurrence of local energy dissipation exceeding the predefined threshold allows for estimating the breakup rate as a function of εcr. Results show that the breakup rate decreases with increasing threshold. For small values of the threshold, this decrease assumes consistent scaling among the different flows which is explained by universal small scale flow properties.
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| 6. |
- Bäbler, Matthäus, et al.
(author)
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Numerical simulations of aggregate breakup in bounded and unbounded turbulent flows
- 2015
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In: Journal of Fluid Mechanics. - : Cambridge University Press (CUP). - 0022-1120 .- 1469-7645. ; 766
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Journal article (peer-reviewed)abstract
- Breakup of small aggregates in fully developed turbulence is studied by means of direct numerical simulations in a series of typical bounded and unbounded flow configurations, such as a turbulent channel flow, a developing boundary layer and homogeneous isotropic turbulence. The simplest criterion for breakup is adopted, whereby aggregate breakup occurs when the local hydrodynamic stress sigma similar to epsilon(1/2), with epsilon being the energy dissipation at the position of the aggregate, overcomes a given threshold sigma(cr), which is characteristic for a given type of aggregate. Results show that the breakup rate decreases with increasing threshold. For small thresholds, it develops a scaling behaviour among the different flows. For high thresholds, the breakup rates show strong differences between the different flow configurations, highlighting the importance of non-universal mean-flow properties. To further assess the effects of flow inhomogeneity and turbulent fluctuations, the results are compared with those obtained in a smooth stochastic flow. Furthermore, we discuss the limitations and applicability of a set of independent proxies.
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| 7. |
- Chiara, Luigi Filippo, et al.
(author)
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Suspensions of deformable particles in Poiseuille flows at finite inertia
- 2020
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In: Fluid Dynamics Research. - : IOP Publishing. - 0169-5983 .- 1873-7005. ; 52:6
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Journal article (peer-reviewed)abstract
- We analyze a suspension of deformable particles in a pressure-driven flow. The suspension is composed of neutrally buoyant initially spherical particles and a Newtonian carrier fluid, and the flow is solved by means of direct numerical simulations, using a fully Eulerian method based on a one-continuum formulation. The solid phase is modeled with an incompressible viscous hyperelastic constitutive relation, and the flow is characterized by three main dimensionless parameters, namely the solid volume fraction, the Reynolds and capillary numbers. The dependency of the effective viscosity on these three quantities is investigated to study the inertial effects on a suspension of deformable particles. It can be observed that the suspension has a shear-thinning behavior, and the reduction in effective viscosity for high shear rates is emphasized in denser configurations. The separate analysis of the Reynolds and capillary numbers reveal that the effective viscosity depends more on the capillary than on the Reynolds number. In addition, our simulations exhibit a consistent tendency for deformable particles to move toward the center of the channel, where the shear rate is low. This phenomenon is particularly marked for very dilute suspensions, where a whole region near the wall is empty of particles. Furthermore, when the volume fraction is increased this near-wall region is gradually occupied, because of higher mutual particle interactions. Deformability also plays an important role in the process. Indeed, at high capillary numbers, particles are more sensitive to shear rate variations and can modify their shape more easily to accommodate a greater number of particles in the central region of the channel. Finally, the total stress budgets show that the relative particle-induced stress contribution increases with the volume fraction and Reynolds number, and decreases with the particle deformability.
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| 8. |
- Costa, Pedro, et al.
(author)
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Effects of the finite particle size in turbulent wall-bounded flows of dense suspensions
- 2018
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In: Journal of Fluid Mechanics. - : CAMBRIDGE UNIV PRESS. - 0022-1120 .- 1469-7645. ; 843, s. 450-478
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Journal article (peer-reviewed)abstract
- We use interface-resolved numerical simulations to study finite-size effects in turbulent channel flow of neutrally buoyant spheres. Two cases with particle sizes differing by a factor of two, at the same solid volume fraction of 20% and bulk Reynolds number are considered. These are complemented with two reference single-phase flows: the unladen case, and the flow of a Newtonian fluid with the effective suspension viscosity of the same mixture in the laminar regime. As recently highlighted in Costa etal. (Phys. Rev. Lett., vol.117, 2016, 134501), a particle-wall layer is responsible for deviations of the mesoscale-averaged statistics from what is observed in the continuum limit where the suspension is modelled as a Newtonian fluid with (higher) effective viscosity. Here we investigate in detail the fluid and particle dynamics inside this layer and in the bulk. In the particle-wall layer, the near-wall inhomogeneity has an influence on the suspension microstructure over a distance proportional to the particle size. In this layer, particles have a significant (apparent) slip velocity that is reflected in the distribution of wall shear stresses. This is characterized by extreme events (both much higher and much lower than the mean). Based on these observations we provide a scaling for the particle-to-fluid apparent slip velocity as a function of the flow parameters. We also extend the scaling laws in Costa etal. (Phys. Rev. Lett., vol.117, 2016, 134501) to second-order Eulerian statistics in the homogeneous suspension region away from the wall. The results show that finite-size effects in the bulk of the channel become important for larger particles, while negligible for lower-order statistics and smaller particles. Finally, we study the particle dynamics along the wall-normal direction. Our results suggest that single-point dispersion is dominated by particle-turbulence (and not particle-particle) interactions, while differences in two-point dispersion and collisional dynamics are consistent with a picture of shear-driven interactions.
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| 9. |
- Costa, Pedro, et al.
(author)
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Interface-resolved simulations of small inertial particles in turbulent channel flow
- 2020
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In: Journal of Fluid Mechanics. - : Cambridge University Press. - 0022-1120 .- 1469-7645. ; 883
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Journal article (peer-reviewed)abstract
- We present a direct comparison between interface-resolved and one-way-coupled point-particle direct numerical simulations (DNS) of gravity-free turbulent channel flow laden with small inertial particles, with high particle-to-fluid density ratio and diameter of approximately three viscous units. The most dilute flow considered, solid volume fraction O(10(-5)), shows the particle feedback on the flow to be negligible, whereas differences with respect to the unladen case, notably a drag increase of approximately 10 %, are found for a volume fraction O(10(-4)). This is attributed to a dense layer of particles at the wall, caused by turbophoresis, flowing with large particle-to-fluid apparent slip velocity. The most dilute case is therefore taken as the benchmark for assessing the validity of a widely used point-particle model, where the particle dynamics results only from inertial and nonlinear drag forces. In the bulk of the channel, the first- and second-order moments of the particle velocity from the point-particle DNS agree well with those from the interface-resolved DNS. Close to the wall, however, most of the statistics show major qualitative differences. We show that this difference originates from the strong shear-induced lift force acting on the particles in the near-wall region. This mechanism is well captured by the lift force model due to Saffman (J. Fluid Mech., vol. 22 (2), 1965, pp. 385-400), while other widely used, more elaborate, approaches aiming at extending the lift model for a wider range of particle Reynolds numbers can actually underpredict the magnitude of the near-wall particle velocity fluctuations for the cases analysed here.
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| 10. |
- Costa, Pedro, et al.
(author)
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Near-wall turbulence modulation by small inertial particles
- 2021
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In: Journal of Fluid Mechanics. - : CAMBRIDGE UNIV PRESS. - 0022-1120 .- 1469-7645. ; 922
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Journal article (peer-reviewed)abstract
- We use interface-resolved simulations to study near-wall turbulence modulation by small inertial particles, much denser than the fluid, in dilute/semi-dilute conditions. We considered three bulk solid mass fractions, , and , with only the latter two showing turbulence modulation. The increase of the drag is strong at , but mild in the densest case. Two distinct regimes of turbulence modulation emerge: for smaller mass fractions, the turbulence statistics are weakly affected and the near-wall particle accumulation increases the drag so the flow appears as a single-phase flow at slightly higher Reynolds number. Conversely, at higher mass fractions, the particles modulate the turbulent dynamics over the entire flow, and the interphase coupling becomes more complex. In this case, fluid Reynolds stresses are attenuated, but the inertial particle dynamics near the wall increases the drag via correlated velocity fluctuations, leading to an overall drag increase. Hence, we conclude that, although particles at high mass fractions reduce the fluid turbulent drag, the solid phase inertial dynamics still increases the overall drag. However, inspection of the streamwise momentum budget in the two-way coupling limit of vanishing volume fraction, but finite mass fraction, indicates that this trend could reverse at even higher particle load.
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