Liquid films coating vertical cylinders can form annular liquid collars which translate downwards under gravity. We investigate the dynamics of a thin viscoplastic liquid film coating the interior or exterior of a vertical cylindrical tube, quantifying how the yield stress modifies both the Rayleigh-Plateau instability leading to collar formation and the translation of collars down the tube. We use thin-film theory to derive an evolution equation for the layer thickness, which we solve numerically to examine the nonlinear dynamics. Instability and collar formation occur when gravity is sufficiently strong to make the fluid yield initially. We use matched asymptotics to derive a model describing the quasisteady translation of a slender liquid collar when the Bond number is small. The structure of the asymptotic solution for a viscoplastic collar shares some features with the Newtonian version, but there are several novel asymptotic regions that emerge at the two ends of the collar. The global force balance, which determines the collar's speed, is modified by a leading-order contribution from viscous drag in the collar when the liquid is viscoplastic. We use the asymptotic model to describe slow changes in collar volume when the film thicknesses ahead of, and behind, the collar are unequal. When the film thickness ahead of the collar is less than a critical value that we determine, viscoplastic collars adjust their volume and reach a steadily translating state. This contrasts with the Newtonian problem, where the only state in which steady translation occurs is unstable to small changes in the film thickness.

Accurate modeling of ice accretion is important for safe and efficient design of aircraft and wind turbine systems. Heat transfer predictions obtained from fluid flow solvers are used as input in ice accretion codes. In glaze ice conditions, freezing rates and resulting ice shapes are highly sensitive to input values of the heat transfer coefficient. Hence, accurate prediction of heat transfer on iced airfoils is crucial for correctly predicting the ice accretion process. In this study, we perform conjugate heat transfer (CHT) simulations using wall-modeled large-eddy simulation (WMLES) over surfaces characterized by ice roughness. The results show that WMLES with CHT accurately captures surface temperature distributions and heat fluxes across a range of roughness geometries. For cases considered, large roughness-to-boundary-layer thickness ratios disrupt outer-layer similarity, leading to substantial errors in estimating equivalent sandgrain roughness when applying traditional empirical models based on surface statistics. The simulations further show that local heat fluxes vary significantly across roughness elements due to low thermal conductivity of the solid; in particular, roughness crests exhibit reduced fluxes in contrast to slopes and valleys. Notably, as roughness height increases, wall heat flux at the crest diminishes, even leading to heat flux reversal in some cases, where thermal energy is transferred from fluid to solid. These effects are not captured in isothermal wall simulations, which overestimate the Stanton number, especially at roughness peaks. By enabling calculation of Stanton number using heat flux distributions, not directly available in experiments, the present simulations augment experimental results and highlight the importance of including solid conduction effects for accurately modeling heat transfer over rough, low-conductivity surfaces such as ice.

A mechanism by which the surface zonal flows of giant planets can be gradually attenuated with depth is explored. The zonal flow is driven by an imposed forcing in a thin layer near the surface. A meridional circulation is set up, analogous to the Ferrel-like cells observed in Jupiter's atmosphere. Acting on a stably stratified thin surface layer, the meridional flow induces a horizontal temperature anomaly which leads to a gradual reduction of the zonal winds with depth, governed by the thermal wind equation. Our model is a Boussinesq plane layer, with gravity acting parallel to the rotation axis. A suite of fully three-dimensional time-dependent numerical simulations has been performed to investigate the model behavior. Below the forced stable layer, convection is occurring, typically in the form of tall thin cells. The fluctuating components of the three-dimensional flow can be driven by either the convection or the Reynolds stresses associated with the jet shear flow. When fluctuations are mainly driven by convection in the form of tall thin columns and the forcing is relatively weak, the horizontal temperature anomaly persists much deeper into the interior than when it is driven by shear flow. The zonal jets can therefore extend deep into the interior, consistent with the Juno gravity data.

Granular flows of non-spherical particles exhibit complex dynamics that challenge classical rheological descriptions. Using discrete element method (DEM) simulations, we show that superballs display distinct flow behaviors, including enhanced boundary effects, modified velocity profiles, and increased bulk viscosity, compared to spheres. We develop a particle-shape-dependent constitutive model incorporating dimensionless granular temperature to capture nonlocal effects, validated through continuum simulations. This framework enables accurate prediction of flow behaviors across quasistatic and inertial regimes, advancing the modeling of granular systems with complex particle geometries.

We propose an alternative surface tension adjustment approach in the pseudopotential lattice Boltzmann (LB) model, which can be easily and straightforwardly incorporated into different widely used collision operators, such as single relaxation time (SRT or LBGK), multiple relaxation time (MRT) and entropic-MRT (KBC) operators. Benefiting from the proposed surface tension adjustment method, a remarkable tunable surface tension range of 140 times can be achieved. We have also successfully modeled the droplet impact and splashing dynamics on thin liquid films with a Weber number up to 10,500, achieving one order of magnitude higher than LB simulations reported in the literature.

In this work, we semianalytically investigate the influence of an imposed spatially linearly varying viscosity field on near wall motion of a model microswimmer (squirmer). We explore its associated phase portraits and swimming trajectories for different swimming gaits and compare them with their constant viscosity analogs. The results indicate that even simplistic ambient viscosity gradient has substantial role on near-wall squirmer motility, which provides valuable insights for understanding and controlling microswimmer motion in relatively complex biological and microfluidic systems.

The finite thickness of cell membranes is an often-overlooked detail in continuum models that can change their physics in fundamental ways. We show that when the bilayer bends, shear flows arise at its two surfaces, producing flow reversal, pressure inversion, and stagnation points that standard two-dimensional models cannot capture. Our findings highlight a missing element in existing continuum theories: resolving coupling at the membrane's two surfaces requires accounting for thickness. This refinement predicts new nanoscale dissipation mechanisms and suggests experimental signatures in fluctuation spectra, pointing toward a more unified description of membrane dynamics.

Fluid--fluid interfacial instability and subsequent fluid mixing are ubiquitous in nature and engineering. The hydrodynamic instability of fluid interfaces has long centered on the pressure gradient-driven long-wavelength Rayleigh--Taylor instability and the resonance-induced short-wavelength Faraday instability. However, neither instability alone can explain the dynamics when both mechanisms are present. We identify a previously unseen multi-modal instability emerging from their coexistence. When the denser fluid is polydimethylsiloxane, the mixed region at a high density contrast (Atwood number=0.9) spans a vibration amplitude range approximately twice the gravitational acceleration. Using Floquet stability analysis, we show how vibrations govern transitions between the RT and Faraday instabilities, leading to contention between these instabilities rather than resonant enhancement. The initial transient growth is represented by the exponential modal growth of the most unstable Floquet exponent, along with its accompanying periodic behavior. Direct numerical simulations validate these findings and track interface breakup into the multiscale and nonlinear regimes. Specifically, we show that growing RT modes nonlinearly suppress Faraday responses even when the initial growth rate of the Faraday instability is 3.63 times that of RT, so a bidirectional competition hinders their sustained coexistence.