arXiv subjects

Nehal Dash

Publications and source records attributed to Nehal Dash.

3 recordsLinked to original sources

arXiv2026

A numerical study to analyze the interplay of Weissenberg number and viscosity ratio in a log-strain tensorial model for viscoelastic fluids

We present a computational study aimed at exploring the different and independent roles of the Weissenberg number and of the ratio between the polymeric and solvent viscosity contributions in a viscoelastic fluid model. The tensorial model under investigation, recently proposed, is based on a logarithmic relation between the elastic (or recoverable) strain and the elastic stress. In this model, the elastic strain plays the role of a conformation tensor and its evolution equation inherently preserves its determinant and positive definiteness. These properties are also enforced in the computational method employed in the study. A finite-difference discretization in time is combined with a stabilized mixed finite element formulation based on the Variational Multiscale method for the spatial discretization and with a generalized Lie derivative approach for the advection terms. The behavior of the model is analyzed in paradigmatic pressure-driven flows and we find that the value of the viscosity ratio is crucial in determining to which extent non-Newtonian flow profiles are observed upon increasing the Weissenberg number. By comparing the solutions of the log-strain tensorial model with those of a suitable Generalized Newtonian Fluid model, we show that flow-type dependence plays a significant role even in the simple planar flow past a cylinder.

cond-mat.soft↗

arXiv2023

Nonspherical oscillations of an encapsulated microbubble with interface energy under the acoustic field

The practical applications of gas-filled encapsulated microbubbles involve inherent nonspherical oscillations under acoustic fields. The gas-encapsulation and encapsulation-liquid interfaces significantly affect the mechanics of the bubbles, especially of smaller radii, and their consideration is vital for mimicking the experimental setting. In this paper, we apply the interface energy model [N. Dash and G. Tamadapu, J. Fluid Mech. 932, A26 (2022)] to examine the nonspherical oscillations of an encapsulated microbubble with a radius of $2$$\mu$m and $5$$\mu$m under an acoustic field. Using the Lagrangian energy formulation, the coupled dynamical governing equations for spherical and nonspherical modes are derived, incorporating the effects of interface energy at the interfaces, shell elasticity, and viscosity. Through a perturbation analysis based on the Krylov-Bogoliubov method of averaging, a set of first-order differential (slow-time) equations is obtained to conduct steady-state and conditional-stability analysis. The stability analysis helped in determining the excitation pressure and frequency of the acoustic field required for smaller radii bubbles to exhibit finite amplitude shape oscillations. Direct numerical simulations of the governing equations revealed that the parametrically forced even mode ($n=2$) excites even modes, while the odd modes ($n=3$) excite both even and odd modes. For smaller radii bubbles, we observe shape mode oscillations of finite non-zero amplitudes only in the presence of interface parameters. The initial size-dependent interface parameter and shell viscoelastic parameters are identified as the key parameters that play a critical role in exhibiting finite shape mode oscillations of the bubble.

physics.flu-dyn↗

arXiv2021

Radial dynamics of an encapsulated microbubble with interface energy

In this work, a mathematical model based on interface energy is proposed within the framework of surface continuum mechanics to study the dynamics of encapsulated bubbles. The interface model naturally induces a residual stress field into the bulk of the bubble, with possible expansion/shrinkage from a stress-free configuration to a natural equilibrium configuration. The significant influence of interface area strain and the coupled effect of stretch and curvature is observed in the numerical simulations based on constrained optimization. Due to the bending rigidity related to additional terms, the dynamic interface tension can become negative, but not due to the interface area strain. The coupled effect of interface strain and curvature term observed is new and plays a dominant role in the dominant compression behavior of encapsulated bubbles observed in the experiments. We validate our model by fitting experimental data of $1.7\,\mu$m, $1.4\,\mu$m, and $1\,\mu$m radii bubbles by calculating the optimized parameters. Our work also highlights the role of interface parameters and natural configuration gas pressure in estimating the size-independent viscoelastic material properties of encapsulated bubbles with interesting future developments.

physics.flu-dyn↗