arXiv subjects

Marco Vanni

Publications and source records attributed to Marco Vanni.

3 recordsLinked to original sources

arXiv2026

Inferring the Turbulent Breakup of Colloidal Aggregates Using Graph Neural Networks

Solid aggregates in turbulent suspensions may break under the action of shear stresses. We explore the use of Graph Neural Networks (GNN) to infer aggregate fragmentation once the aggregate structure and flow velocity gradients are known. We consider two models: the first GNN is a classifier, trained to distinguish aggregates that break from those that do not; the second GNN is a regression model, trained to predict the maximal tensile force within each aggregate in a given flow condition. We show that both models complete their task with a high statistical accuracy, also generalizing to aggregates of different sizes, and generally performing better than the statistical prediction based on mean field quantities. This work paves the way for future use of GNN to quantify aggregate breakup in a large population of aggregates suspended in complex flow configurations, as it takes place in the wet production of fine powders and in the transport of sediments in environmental flows.

physics.flu-dyn↗

arXiv2020

Intermittency acceleration of water droplet population dynamics inside the interfacial layer between cloudy and clear air environments

We use direct numerical simulation to study the temporal evolution of a perturbation localized on the turbulent layer that typically separates a cloud from the surrounding clear air. Across this shearless layer, a turbulent kinetic energy gradient naturally forms. Here, a finite perturbation in the form of a local initial temperature fluctuation is applied to simulate a hydrodynamic instability inside the background turbulent air flow. A numerical initial value problem for two diametrically opposite types of drop population distributions is then solved. Specifically, we consider a mono-disperse population of droplets of 15 $\mu$m of radius and a poly-disperse distribution with radii in the range 0.6 - 30 $\mu$m. For both distributions, it is observed that the evaporation and condensation have a dramatically different weight inside the homogeneous cloudy region and the interfacial anisotropic mixing region. It is observed that the dynamics of drop collisions is highly effected by the turbulence structure of the host region. The two populations show a common aspect during their energy decay transient. That is the increased probability of collisions in the interfacial layer hat houses intense anisotropic velocity fluctuations. This layer, in fact, induces an enhanced differentiation on droplets kinetic energy and sizes. Both polydisperse and monodisperse initial particle distributions contain $10^7$ droplets, matching an initial liquid water content of $0.8 g/m^3$. An estimate of the turbulent collision kernel for geometric collisions used in the population balance equations is given. A preliminary discussion is presented on the structure of the two unsteady non ergodic collision kernels obtained inside the cloud interface region.

physics.flu-dyn↗

arXiv2014

Internal stresses and breakup of rigid isostatic aggregates in homogeneous and isotropic turbulence

By characterising the hydrodynamic stresses generated by statistically homogeneous and isotropic turbulence in rigid aggregates, we estimate theoretically the rate of turbulent breakup of colloidal aggregates and the size distribution of the formed fragments. The adopted method combines Direct Numerical Simulation of the turbulent field with a Discrete Element Method based on Stokesian dynamics. In this way, not only the mechanics of the aggregate is modelled in detail, but the internal stresses are evaluated while the aggregate is moving in the turbulent flow. We examine doublets and cluster-cluster isostatic aggregates, where the failure of a single contact leads to the rupture of the aggregate and breakup occurs when the tensile force at a contact exceeds the cohesive strength of the bond. Due to the different role of the internal stresses, the functional relationship between breakup frequency and turbulence dissipation rate is very different in the two cases. In the limit of very small and very large values, the frequency of breakup scales exponentially with the turbulence dissipation rate for doublets, while it follows a power law for cluster-cluster aggregates. For the case of large isostatic aggregates it is confirmed that the proper scaling length for maximum stress and breakup is the radius of gyration. The cumulative fragment distribution function is nearly independent of the mean turbulence dissipation rate and can be approximated by the sum of a small erosive component and a term that is quadratic with respect to fragment size.

physics.flu-dyn↗