arXiv subjects

Hugo N. Ulloa

Publications and source records attributed to Hugo N. Ulloa.

9 recordsLinked to original sources

arXiv2026

Two-phase Temperature Reconstruction in Ice-Water Systems

What controls heat transport when liquid water interacts with ice? Answering this question is essential for understanding water bodies undergoing phase change. Yet experimental progress remains limited by the lack of a minimally invasive methodology for simultaneously resolving temperature fields in coupled liquid water and non-isothermal ice systems. Here, we introduce a physics-based data-assimilation method for reconstructing temperature fields in buoyancy-driven flows interacting with non-isothermal ice. Particle tracking velocimetry provides the liquid velocity field, while thermal and kinematic boundary conditions constrain the inverse problem. The method couples advection--diffusion in liquid water with conduction in ice to reconstruct simultaneous mean temperature fields and quantify heat transport across the water--ice interface, while remaining minimally invasive and compatible with free-surface systems. We demonstrate the method in laboratory experiments in which the temperature range across the liquid water in contact with ice drives cabbeling-induced convection. This framework enables investigation of coupled thermo-fluid dynamics in cryospheric aquatic systems, including heat exchange at the ice-water interface and liquid-phase energetics, with broader applications to phase-change processes in food and energy industries.

physics.flu-dyn↗

arXiv2026

Shear-driven mixing of segregated granular materials

As granular materials flow and settle, interactions among particles of different sizes or properties drive mixing and segregation, producing dynamics that shape systems ranging from silos to asteroids. A hallmark of polydisperse granular flows is shear-driven size segregation, in which larger grains tend to rise above smaller ones through particle-scale rearrangements. Despite substantial progress in modeling granular flow and segregation, the complementary process of granular mixing remains less well understood. Here, we investigate the evolution of initially segregated dense granular materials driven out of equilibrium by imposed shear. We ask: what controls the extent and rate of mixing and restratification in a sheared bidisperse granular flow? Addressing this question is essential for understanding how external forcing disrupts or reinforces particle-size organization, and for optimizing processes that require controlled mixing. Using theoretical analysis and numerical simulations, we develop a framework that quantifies the degree of mixing and segregation dynamics of dense bidisperse granular flows. Our results identify the controlling roles of particle-size ratio and the P\'eclet number, clarify the conditions under which segregated states persist or are transiently homogenized, and provide a basis for improved prediction and control of granular mixtures in natural and industrial settings.

cond-mat.soft↗

arXiv2026

Effects of temperature-dependent material properties in coldwater Rayleigh-B\'enard convection

Water exhibits an anomalous nonlinear density equation of state (EOS) as it approaches freezing, along with an increase in viscosity ($\mu$) and a decrease in thermal conductivity ($k$). Here we ask: how do these temperature-dependent material properties affect thermal convection in coldwater? We examine these effects within the canonical Rayleigh-B\'enard convection (RBC) system, performing direct numerical simulations with coldwater between $0^{\circ}$C and the temperature of maximum density $3.98^{\circ}$C. To disentangle the effects of these properties, we consider two main cases: RBC with a quadratic EOS and constant $\mu$ and $k$; and RBC with a quadratic EOS and temperature-dependent $\mu(T)$ and $k(T)$. We show that the most significant consequence is a shift in the mean fluid temperature relative to `standard RBC'. The nonlinear EOS and the temperature-dependent $\mu$ and $k$ drive this shift in opposite directions, with the former dominating and producing a net decrease in the mean temperature. This decrease varies with the Rayleigh number $Ra$, reaching up to $0.08^{\circ}$C at $Ra=10^{8}$. Additionally, the onset of convection is affected, but the influence from the EOS and $\mu(T)$ and $k(T)$ compensate, resulting in a negligible net effect. Despite these changes, the Nusselt and the Reynolds numbers follow classical scalings in all cases. Our results establish how the anomalous material properties of coldwater affect local and global features of convection, evidencing the implications of model selection for studies of cryospheric water bodies.

physics.flu-dyn↗

arXiv2025

Scaling particle-size segregation in wide-ranging sheared granular flows

Scaling relationships have been proposed to describe shear-driven size segregation based on intruder experiments and simulations. While these models have shown agreement with experimental and numerical results under uniform shear rate, their validity across varying shear-rate conditions remains uncertain. Here, we employ Discrete Element Method (DEM) simulations to investigate particle size segregation in sheared granular flows under wide-ranging shear-rate conditions. We find that the scaling between segregation velocity and local rheological conditions holds only within a moderate inertial number range ($0.01 < I < 0.1$), and breaks down in both quasi-static and collisional regimes. Furthermore, we show that this discrepancy leads continuum models to mispredict segregation rates in bidisperse mixtures. These findings emphasize the need for more generalized scaling laws capable of capturing segregation dynamics across a broader spectrum of shear-rate conditions and regimes.

cond-mat.soft↗

arXiv2024

A Method for Calculating Attenuation in Creeping Materials

The phase lag between an applied forcing and a response to that forcing is a fundamen tal parameter in geophysical signal processing. For solid deforming materials, the phase lag between an oscillatory applied stress and the resulting strain response encapsulates information about the dynamical behavior of materials and attenuation. The phase lag is not directly measured and must be extracted through multiple steps by carefully comparing two time-series signals. The extracted value of the phase lag is highly sensitive to the analysis method, and often there are no comparable values to increase confidence in the calculated results. In this study, we propose a method for extracting the phase lag between two signals when either one or both include an underlying nonlinear trend, which is very common when measuring attenuation in creeping materials. We demonstrate the robustness of the method by analyzing artificial signals with known phases and quantifying their absolute and relative errors. We apply the method to two experimental datasets and compare our results with those of previous studies

physics.geo-ph↗

arXiv2024

Energetics of particle-size segregation

We introduce a continuum framework for the energetics of particle-size segregation in bidisperse granular flows. Building on continuum segregation equations and a recent segregation flux model, the proposed framework offers general analytical expressions to study the physics of granular flows from a mechanical energy perspective. To demonstrate the framework's applicability, we examined the energetics in shear-driven flows. Numerical experiments with varying frictional coefficients and particle-size ratios revealed two distinct phases in the associated energetics with particle segregation and diffusive remixing, and that the potential energy to the kinetic energy ratio in the steady state follows the scaling relationship $\hat{E}^{(s)}_{gp} / \hat{E}^{(s)}_{k} \propto Pe^{-1/2}_{sr}$ for $0.4 \leq Pe_{sr} \leq 300$, the segregation-rheology Péclet number. Our findings hint that the bulk segregation-mixing state can be predicted and controlled using $Pe_{sr}$, determined from known system parameters, providing a impactful tool for engineering and geophysical applications.

cond-mat.soft↗

arXiv2024

Floating active carpets drive transport and aggregation in aquatic ecosystems

Communities of swimming microorganisms often thrive near liquid-air interfaces. We study how such `active carpets' shape their aquatic environment by driving biogenic transport in the water column beneath them. The hydrodynamic stirring that active carpets generate leads to diffusive upward fluxes of nutrients from deeper water layers, and downward fluxes of oxygen and carbon. Combining analytical theory and simulations, we examine the biogenic transport by studying fundamental metrics, including the single and pair diffusivity, the first passage time for particle pair encounters, and the rate of particle aggregation. Our findings reveal that the hydrodynamic fluctuations driven by active carpets have a region of influence that reaches orders of magnitude further in distance than the size of the organisms. These nonequilibrium fluctuations lead to a strongly enhanced diffusion of particles, which is anisotropic and space-dependent. Fluctuations also facilitate encounters of particle pairs, which we quantify by analysing their velocity pair correlation functions as a function of distance between the particles. We found that the size of the particles plays a crucial role in their encounter rates, with larger particles situated near the active carpet being more favourable for aggregation. Overall, this research broadens our comprehension of aquatic systems out of equilibrium and how biologically driven fluctuations contribute to the transport of fundamental elements in biogeochemical cycles.

cond-mat.soft↗

arXiv2024

Active Carpets in floating viscous films

Earth's aquatic environments are inherently stratified layered systems where interfaces between layers serve as ecological niches for microbial swimmers, forming colonies known as Active Carpet (AC). Previous theoretical studies have explored the hydrodynamic fluctuations exerted by ACs in semi-infinite fluid media, demonstrating their capability to enhance thermal diffusion and mass transport in aquatic systems. Yet, little is understood about the fluid dynamics and impact of ACs residing in confined layered environments, like slicks floating on water bodies. In this study, we report novel solutions for the hydrodynamic fluctuations induced by ACs geometrically confined between a free surface and a fluid-fluid interface characterized by a jump in fluid viscosity. Combining theory and numerical experiments, we investigate the topology of the biogenic hydrodynamic fluctuations in a confined, thin fluid environment. We reveal that within this thin layer, ACs gives shape to three characteristic regions: Region I is the closest zone to the AC and the fluid-fluid interface, where hydrodynamic fluctuations are dominantly vertical; Region II is further up from the AC and is characterized by isotropic hydrodynamic fluctuations; Region III is the furthest region, near the free surface and is dominated by horizontal flow fluctuations. We demonstrate that the extent of these regions depends strongly on the degree of confinement, i.e. the layer thickness and the strength of the viscosity jump. Lastly, we show that confinement fosters the emergence of large-scale flow structures within the layer housing the ACs--not previously reported. Our findings shed light on the complex interplay between confinement and hydrodynamics in floating viscous film biological systems, providing valuable insights with implications spanning from ecological conservation to bio-inspired engineering.

physics.flu-dyn↗

arXiv2023

Towards Understanding Underwater Weather Events in Rivers Using Autonomous Surface Vehicles

Climate change has increased the frequency and severity of extreme weather events such as hurricanes and winter storms. The complex interplay of floods with tides, runoff, and sediment creates additional hazards -- including erosion and the undermining of urban infrastructure -- consequently impacting the health of our rivers and ecosystems. Observations of these underwater phenomena are rare, because satellites and sensors mounted on aerial vehicles cannot penetrate the murky waters. Autonomous Surface Vehicles (ASVs) provides a means to track and map these complex and dynamic underwater phenomena. This work highlights preliminary results of high-resolution data gathering with ASVs, equipped with a suite of sensors capable of measuring physical and chemical parameters of the river. Measurements were acquired along the lower Schuylkill River in the Philadelphia area at high-tide and low-tide conditions. The data will be leveraged to improve our understanding of changes in bathymetry due to floods; the dynamics of mixing and stagnation zones and their impact on water quality; and the dynamics of suspension and resuspension of fine sediment. The data will also provide insight into the development of adaptive sampling strategies for ASVs that can maximize the information gain for future field experiments.

cs.RO↗