arXiv subjects

Kai Mu

Publications and source records attributed to Kai Mu.

5 recordsLinked to original sources

arXiv2026

Influence of van der Waals forces on the instability of a liquid film in a tube

The instability of a liquid film in a nanotube is significantly influenced by van der Waals forces. A theoretical framework based on the axisymmetric Stokes equations is developed to investigate their effects through linear stability analysis. The model reveals that van der Waals forces markedly enhance perturbation growth, reduce the dominant wavelength, and lower the critical film thickness that distinguishes collapse from non-collapse regimes. Direct numerical simulations of the Navier-Stokes equations both confirm these theoretical predictions and extend the analysis into the nonlinear regime. In this regime, van der Waals forces are found to alter the interfacial morphology and suppress the formation of satellite lobes. Both rupture and collapse follow a universal temporal scaling law with exponent 1/3 and exhibit self-similar behavior near the singularity.

physics.flu-dyn↗

arXiv2026

Spatial instability analysis and mode transition of a viscoelastic jet in a co-flowing gas stream

Spatial linear instability analysis is employed to investigate the instability of a viscoelastic liquid jet in a co-flowing gas stream. The theoretical model incorporates a non-uniform axial base profile represented by a hyperbolic tangent, capturing the shear layer. The Oldroyd-B model discretized with Chebyshev polynomials is employed, and energy budget analysis is used to interpret underlying mechanisms.At low Weber numbers, the jet evolves axisymmetrically and the instability is governed by interfacial gas-pressure fluctuations; as the Weber number increases, the growing inertia drives a transition of the predominant mode from axisymmetric to helical. At weak elasticity, the instability is also primarily governed by gas-pressure fluctuations. As elasticity increases, the predominant mode transitions from axisymmetric to helical. This transition is accompanied by a migration of disturbance structures from the interface toward the jet interior and an enhanced coupling between velocity perturbation and the basic flow. These trends reveal a new predominant instability mechanism -- the elasticity-enhanced shear-driven instability -- which is distinct from capillary or Kelvin-Helmholtz instabilities in Newtonian jets. A We-El phase diagram delineates the boundary between predominant modes, and experimental results obtained in a flow-focusing configuration validate the theoretical predictions.

physics.flu-dyn↗

arXiv2024

Capillary-driven migration of droplets on conical fibers

A droplet placed on a hydrophilic conical fiber tends to move toward the end of larger radii due to capillary action. Experimental investigations are performed to explore the dynamics of droplets with varying viscosities and volumes on different fibers at the microscale. Droplets are found to accelerate initially and subsequently decelerate during migration. A dynamic model is developed to capture dynamics of the droplet migration, addressing the limitations of previous equilibrium-based scaling laws. Both experimental results and theoretical predictions indicate that droplets on more divergent fibers experience a longer acceleration phase. Additionally, gravitational effects are pronounced on fibers with small cone angles, exerting a substantial influence on droplet migration even below the capillary scale. Moreover, droplets move more slowly on dry fibers compared to those prewetted with the same liquid, primarily attributed to the increased friction. The experiments reveal the formation of a residual liquid film after droplet migration on dry fibers, leading to considerable volume loss in the droplets. To encompass the intricacies of migration on dry fibers, the model is refined to incorporate a higher friction coefficient and variable droplet volumes, providing a more comprehensive depiction of the underlying physics.

physics.flu-dyn↗

arXiv2024

Inertia and slip effects on the instability of a liquid film coated on a fibre

To investigate the influence of inertia and slip on the instability of a liquid film on a fibre, a theoretical framework based on the axisymmetric Navier-Stokes equations is proposed via linear instability analysis. The model reveals that slip significantly enhances perturbation growth in viscous film flows, whereas it exerts minimal influence on flows dominated by inertia. Moreover, under no-slip boundary conditions, the dominant instability mode of thin films remains unaltered by inertia, closely aligning with predictions from a no-slip lubrication model. Conversely, when slip is introduced, the dominant wavenumber experiences a noticeable reduction as inertia decreases. This trend is captured by an introduced lubrication model with giant slip. Direct numerical simulations of the Navier-Stokes equations are then performed to further confirm the theoretical findings at the linear stage. For the nonlinear dynamics, no-slip simulations show complex vortical structures within films, driven by fluid inertia near surfaces. Additionally, in scenarios with weak inertia, a reduction in the volume of satellite droplets is observed due to slip, following a power-law relationship.

physics.flu-dyn↗

arXiv2020

Intrinsic Relationship Reasoning for Small Object Detection

The small objects in images and videos are usually not independent individuals. Instead, they more or less present some semantic and spatial layout relationships with each other. Modeling and inferring such intrinsic relationships can thereby be beneficial for small object detection. In this paper, we propose a novel context reasoning approach for small object detection which models and infers the intrinsic semantic and spatial layout relationships between objects. Specifically, we first construct a semantic module to model the sparse semantic relationships based on the initial regional features, and a spatial layout module to model the sparse spatial layout relationships based on their position and shape information, respectively. Both of them are then fed into a context reasoning module for integrating the contextual information with respect to the objects and their relationships, which is further fused with the original regional visual features for classification and regression. Experimental results reveal that the proposed approach can effectively boost the small object detection performance.

cs.CV↗