arXiv subjects

Daniel Morón

Publications and source records attributed to Daniel Morón.

5 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↗

arXiv2026

Scaling of the minimal energy for turbulence transition in pipe flow

Predicting the transition of turbulence in pipe flow remains a fundamental problem in fluid dynamics. We use a variational approach to compute nonlinear optimal perturbations to the laminar flow at Reynolds number $Re\leq 5000$. As $Re$ increases, optimal perturbations remain structurally similar, but increasingly localize while their thickness scales as $δ_r \propto Re^{-1/3}$. They grow via the Orr mechanism, followed by a phase of strong nonlinear interaction of oblique waves and a lift-up phase. The energy gain during the Orr phase increases linearly with $Re$ and is independent of the initial perturbation energy, $E_0$. The energy gain during the oblique and lift-up phases is governed by nonlinearities and scales as $\propto Re^2$. We find that regardless of the Reynolds number, transition occurs if the energy of the perturbation exceeds a constant threshold. As a result, the minimum perturbation energy required to cause transition in pipe flow scales as $\mathcal{O}(Re^{-3})$.

physics.flu-dyn↗

arXiv2025

Bayesian minimisation of energy consumption in turbulent pipe flow via unsteady driving

Turbulence accounts for most of the energy losses associated with the pumping of fluids in pipes. Pulsatile drivings can reduce the drag and energy consumption required to supply a desired mass flux, when compared to steady driving. However, not all pulsation waveforms yield reductions. Here, we compute drag- and energy-optimal driving waveforms using direct numerical simulations and a gradient-free black-box optimisation framework. Specifically, we show that Bayesian optimisation is vastly superior to ordinary gradient-based methods in terms of computational efficiency and robustness, due to its ability to deal with noisy objective functions, as they naturally arise from the finite-time averaging of turbulent flows. We identify optimal waveforms for three Reynolds numbers and two Womersley numbers. At a Reynolds number of 8600 and a Womersley number of 10, optimal waveforms reduce total energy consumption by 22 % and drag by 37 %. These reductions are rooted in the suppression of turbulence prior to the acceleration phase, the resulting delay in turbulence onset, and the radial localization of turbulent kinetic energy and production toward the pipe centre. Our results pinpoint that the predominant, steady operation mode of pumping fluids through pipes is far from optimal.

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↗

arXiv2025

Turbulent puffs in transitional pulsatile pipe flow at moderate pulsation amplitudes

We show that, in the transitional regime of pulsatile pipe flow, at moderate-to-high amplitudes 0.5 < A < 1, the first long-lived turbulent structures are localized and take the form of the puffs and slugs observed in statistically steady pipe flow. We perform direct numerical simulations at many pulsation frequencies, amplitudes and Re, and observe different dynamics of puffs and slugs. At certain flow parameters we find, using a causal analysis, that puffs actively make use of linear instabilities in the laminar Sexl-Womersley profile to survive the pulsation. Using all these lessons learned, we extend a low order model by Barkley et al., Nature (2015), to reproduce these dynamics. We find a good agreement between the extended model and our numerical results in a broad parametric space of pulsation amplitudes 0.5 < A < 1, frequencies Wo > 5 and 2100 < Re < 3000. With the help of our numerical results, causal analysis and model, we determine that turbulence production has two sources at these flow parameters: the mean shear as in statistically steady pipe flow, and the instabilities of the instantaneous pulsatile mean profile.

physics.flu-dyn↗