arXiv subjects

Alberto Vela-Martín

Publications and source records attributed to Alberto Vela-Martín.

7 recordsLinked to original sources

arXiv2026

A causal model of drop breakup in turbulence

Fragmentation of drops and bubbles in turbulence controls interfacial area generation, mixing, and transport in environmental and engineering flows. Reduced-order models of breakup are highly sought after, but the bidirectional, nonlinear coupling between the interfacial and hydrodynamic stresses is an obstacle to their development. By leveraging a decomposition of the flow into outer and inner regions introduced by Vela-Martín & Avila (2021), we demonstrate that at low Weber numbers breakup is caused by outer eddies that produce extreme events of interfacial stretching. Capillary forces oppose stretching and transfer the interfacial energy back to the velocity field by generating inner eddies as the drop relaxes. Hence, for breakup to occur, outer stretching events must inject energy faster than the interface can convert it into inner eddies. Numerical simulations initialized with ellipsoidal drops reveal that the energy transfer to the inner eddies is governed by the capillary time, whereas the outer forcing acts on the eddy-turnover time scale. Building on these observations and the governing equations, we derive a simple model for the breakup rate. Although the underlying assumptions are strictly valid only in the limit of large surface tension, the resulting equation quantitatively captures both drop and bubble breakup rates over a wide range of Weber numbers using only a single fitting parameter. Our results establish a direct causal link between turbulent intermittency and the memoryless nature of breakup, and suggest a universal mechanism governing drop and bubble breakup at low Weber numbers.

physics.flu-dyn↗

arXiv2025

Probabilistic thresholds of turbulence decay in transitional shear flows

Linearly stable shear flows first transition to turbulence in the form of localised patches. At low Reynolds numbers, these turbulent patches tend to suddenly decay, following a memoryless process typical of rare events. How far in advance their decay can be forecasted is still unknown. We perform massive ensembles of simulations of pipe flow and a reduced order model of shear flows (Moehlis et al. 2004) and determine the first moment in time at which decay becomes fully predictable, subject to a given magnitude of the uncertainty on the flow state. By extensively sampling the chaotic sets, we find that, as one goes back in time from the point of inevitable decay, predictability degrades at greatly varying speeds. However, a well-defined (average) rate of predictability loss can be computed. This rate is independent of the uncertainty and also of the type of rare event, i.e. it applies to decay and to other extreme events. We leverage our databases to define thresholds that approximately separate phase-space regions of distinct decay predictability. Our study has implications for the development of predictive models, in particular it sets their theoretical limits. It also opens avenues to study the causes of extreme events in turbulent flows: a state which is predictable to produce an extreme event, it is causal to it from a probabilistic perspective.

physics.flu-dyn↗

arXiv2021

A low-storage method consistent with second-order statistics for time-resolved databases of turbulent channel flow up to $Re_τ=5300$

In this paper a novel low-storage method for time-resolved databases is presented. This approach reduces the storage cost of time-resolved databases by storing filtered flow fields that target the large and intermediate scales, while retaining all the information needed to fully reconstruct the flow at the level of filtered flow fields and complete second-order statistics. This is done by storing also the filtered turbulent stresses, allowing to recover the exact effect of the small scales on the large and intermediate scales. A significant speed-up of the computations is achieved, first, by relaxing the numerical resolution, which is shown to affect only the dynamics close to the wall, but not the large scales stored in the database, and, second, by exploiting the \change{computing power and efficiency} of GPU co-processors using a new high-resolution hybrid CUDA-MPI code. This speed-up allows running for physically meaningful times to capture the dynamics of the large scales. The resulting temporally resolved large-scale database of a turbulent channel flow up to $Re_τ=5300$, in large boxes for long times, is briefly introduced, showing significant indicators of large-scale dynamics with characteristic times of the order of up to eight eddy turnover times.

physics.flu-dyn↗

arXiv2021

Deformation of drops by outer eddies in turbulence

Drop deformation in fluid flows is investigated here as an exchange between the kinetic energy of the fluid and the surface energy of the drop. We show analytically that this energetic exchange is controlled only by the stretching (or compression) of the drop surface by the rate-of-strain tensor. This mechanism is analogous to the stretching of the vorticity field in turbulence. Leveraging the non-local nature of turbulence dynamics, we introduce a new decomposition that isolates the energetic exchange due to local drop-induced surface effects, from the non-local action of turbulent fluctuations. We perform direct numerical simulations of single inertial drops in isotropic turbulence and show that an important contribution to the increments of the surface energy arises from the non-local stretching of the fluid-fluid interface by eddies far from the drop surface (outer eddies). We report that this mechanism is dominant and independent of surface dynamics in a range of Weber numbers in which drop breakup occurs. These findings shed new light on drop deformation and breakup in turbulent flows, and open the venue for the improvement and simplification of breakup models.

physics.flu-dyn↗

arXiv2021

Evidence for multiplicative cascades as the origin of small-scale intermittency in turbulence

This investigation presents evidence of the relation between the dynamics of intense events in small-scale turbulence and the energy cascade. We use the generalised (Hölder) means to track the temporal evolution of intense events of the enstrophy and the dissipation in direct numerical simulations of isotropic turbulence. We show that these events are modulated by large-scale fluctuations, and that their evolution is consistent with a local multiplicative cascade, as hypothesised by a broad class of intermittency models of turbulence.

physics.flu-dyn↗

arXiv2017

The turbulent cascade in five dimensions

To the naked eye, turbulent flows exhibit whirls of many different sizes. To each size, or scale, corresponds a fraction of the total energy resulting from a cascade in five dimensions: scale, time and three-dimensional space. Understanding this process is critical to modeling strategies of geophysical and industrial flows. By tracking the flow regions containing energy in different scales, we have evidenced the statistical predominance of a cross-scale link whereby fluid lumps of energy at scale $Δ$ appear within lumps of scale $2Δ$ and die within those of scale $Δ/2$. Our approach uncovers the energy cascade in a simple water-like fluid, offering new insights to turbulence models while paving the way to similar analysis in conducting fluids, quantum fluids and plasmas.

physics.flu-dyn↗

arXiv2015

The temporal evolution of the energy flux across scales in homogeneous turbulence

A temporal study of energy transfer across length scales is performed in 3D numerical simulations of homogeneous shear flow and isotropic turbulence. The average time taken by perturbations in the energy flux to travel between scales is measured and shown to be additive. Our data suggests that the propagation of disturbances in the energy flux is independent of the forcing and that it defines a `velocity' that determines the energy flux itself. These results support that the cascade is, on average, a scale-local process where energy is continuously transmitted from one scale to the next in order of decreasing size.

physics.flu-dyn↗