arXiv subjects

James D. Shemilt

Publications and source records attributed to James D. Shemilt.

8 recordsLinked to original sources

arXiv2026

A transition to elasto-viscoplastic turbulence in inertialess channel flow?

We conduct 2D numerical simulations employing a widely used constitutive law for elasto-viscoplastic fluids to show that linear instability leads to spatio-temporal complexity in inertialess channel flow. Fluctuations in the final state are pronounced near and between the yield surfaces that border an unyielded plug spanning the centre of the channel. The instability and transition arise for Weissenberg numbers of order unity and higher.

physics.flu-dyn↗

arXiv2026

Rayleigh-Plateau instability of an elasto-viscoplastic filament

A slender-thread model is used to explore the Rayleigh-Plateau instability of a filament of elasto-viscoplastic fluid. Without elasticity, a finite yield stress suppresses any linear instability for a filament of constant radius. Including sub-yield elastic deformation permits an elastic Rayleigh-Plateau instability above a critical Deborah number. If stresses over the thinner sections of the thread breach the yield threshold, viscoplastic deformations then drive the filament towards pinch-off. The thread consequently evolves to a beads-on-a-string structure. The elasto-plastic anatomy of the beads is explored and categorized.

physics.flu-dyn↗

arXiv2026

Start-up and inertialess instability of elasto-viscoplastic channel flow

An exploration is presented of the start-up and linear stability of pressure-driven channel flow of an elasto-viscoplastic fluid described by Saramito's constitutive law. Streamwise uniform base states are non-unique, depending on the initial stress configuration, and develop discontinuities in the normal stresses and shear rate at the yield surfaces over infinite times. Such stress discontinuities can be eliminated by introducing a sufficient extensional pre-stress; true plugs bordered by stress jumps then become replaced by marginally yielded, plug-like flow, or pseudo-plugs. To examine the stability of all of these state, the linear initial-value problem is solved along with the evolving base states. Because this analysis is performed for finite times, the base states remain continuous and there is no need to perturb any stress discontinuities. Armed with the insights provided, stability is then analyzed as a normal-mode problem for the final states, building in perturbations to the stress discontinuities via certain jump conditions across any yield surfaces. Regardless of whether the base flows contain true plugs or pseudo-plugs, the base states are found to be linearly unstable at zero Reynolds number. The most unstable perturbations possess the highest streamwise wavenumbers and become spatially localized to the regions where stresses lie close to the yield stress.

physics.flu-dyn↗

arXiv2026

Coalescence of Printed Yield Stress Filaments in Direct Ink Writing

In direct ink writing (DIW), neighbouring filaments of yield-stress inks are deposited side-by-side and are expected to merge into smooth, mechanically robust structures. Unlike Newtonian filaments, coalescence can arrest in finite time, leaving a permanent, non-flat ridge set by the competition between capillarity and rheology. Here we study the coalescence of two printed yield-stress filaments, combining scaling theory for the arrested state, direct numerical simulations, and DIW experiments on Carbopol gels imaged by optical coherence tomography. In the viscoplastic limit, we predict and observe an approximately linear decrease of the final bridge height with plastocapillary number and a critical yield stress above which coalescence does not initiate. Simulations further show that elasticity becomes important at high plastocapillary number, enabling larger final bridge heights via a crossover from a rigid Herschel--Bulkley solid to a deformable Kelvin--Voigt response. Our findings provide a framework for predicting deposition profiles and, ultimately, for mitigating residual topography in DIW.

cond-mat.soft↗

arXiv2025

Air-driven dynamics of viscoplastic liquid layers

Airway clearance by coughing is a key mechanism for mucus transport, particularly in obstructive lung diseases associated with altered mucus rheology. We investigate the dynamics of a viscoplastic liquid film driven by flow in a turbulent air layer, which is a model for air-driven mucus transport that incorporates yield-stress effects. Our theoretical analysis is based on a long-wave model for the liquid film flow, and we complement this with experiments, in which layers of Newtonian and yield-stress liquids are exposed to air flow in a rectangular duct. We demonstrate how perturbations to the layer depth can lead to localised yielding and wave generation. Rapid wave growth occurs when the fluid ahead of the oncoming wave is unyielded, so that as the wave propagates, it consumes this static fluid while depositing a much thinner film behind. This mechanism causes dramatic "blow-out" events in experiments, where liquid hits the roof of the tank. By contrast, in Newtonian thin films, multiple surface waves typically form, and blow-out only occurs in experiments when a Newtonian film is sufficiently thick.

physics.flu-dyn↗

arXiv2025

Viscoplasticity can stabilise liquid collar motion on vertical cylinders

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 quasi-steady 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.

physics.flu-dyn↗

arXiv2024

Non-local impact of distal airway constrictions on patterns of inhaled particle deposition

Airway constriction and blockage in obstructive lung diseases cause ventilation heterogeneity and create barriers to effective drug deposition. Established computational particle-deposition models have not accounted for these impacts of disease. We present a new particle-deposition model that calculates ventilation based on the resistance of each airway, such that ventilation responds to airway constriction. The model incorporates distal airway constrictions representative of cystic fibrosis, allowing us to investigate the resulting impact on patterns of deposition. Unlike previous models, our model predicts how constrictions affect deposition in airways throughout the lungs, not just in the constricted airways. Deposition is reduced in airways directly distal and proximal to constrictions. When constrictions are clustered together, central-airways deposition can increase significantly in regions away from constrictions, but distal-airways deposition in those regions remains largely unchanged. We use our model to calculate lung clearance index (LCI), a clinical measure of ventilation heterogeneity, after applying constrictions of varying severities in one lobe. We find an increase in LCI coinciding with significantly reduced deposition in the affected lobe. Our results show how the model provides a framework for development of computational tools that capture the impacts of airway disease, which could significantly affect predictions of regional dosing.

q-bio.TO↗

arXiv2023

Surfactant amplifies yield-stress effects in the capillary instability of a film coating a tube

To assess how the presence of surfactant in lung airways alters the flow of mucus that leads to plug formation and airway closure, we investigate the effect of insoluble surfactant on the instability of a viscoplastic liquid coating the interior of a cylindrical tube. Evolution equations for the layer thickness using thin-film and long-wave approximations are derived that incorporate yield-stress effects and capillary and Marangoni forces. Using numerical simulations and asymptotic analysis of the thin-film system, we quantify how the presence of surfactant slows growth of the Rayleigh-Plateau instability, increases the size of initial perturbation required to trigger instability and decreases the final peak height of the layer. When the surfactant strength is large, the thin-film dynamics coincide with the dynamics of a surfactant-free layer but with time slowed by a factor of four and the capillary Bingham number, a parameter proportional to the yield stress, exactly doubled. By solving the long-wave equations numerically, we quantify how increasing surfactant strength can increase the critical layer thickness for plug formation to occur and delay plugging. The previously established effect of the yield stress in suppressing plug formation [Shemilt et al., J. Fluid Mech., 2022, vol. 944, A22] is shown to be amplified by introducing surfactant. We discuss the implications of these results for understanding the impact of surfactant deficiency and increased mucus yield stress in obstructive lung diseases.

physics.flu-dyn↗