arXiv subjects

G. P. Raja Sekhar

Publications and source records attributed to G. P. Raja Sekhar.

7 recordsLinked to original sources

arXiv2025

A Two-Phase Model of Early Atherosclerotic Plaque Development with LDL Toxicity Effects

Atherosclerosis is a chronic inflammatory cardiovascular disease in which fatty plaque is built inside an artery wall. Early atherosclerotic plaque development is typically characterized by inflammatory tissues primarily consisting of foam cells and macrophages. We present a biphasic model that explores early plaque growth to emphasize the role of cytokines (particularly, Monocyte Chemoattractant Protein-1) and oxidized low-density lipoprotein (oxLDL) in monocyte recruitment and foam cell production, respectively. The plaque boundary is assumed to move at the same speed as the inflammatory tissues close to the periphery. This study discusses the oxLDL cholesterols recruitment inside intima and their internalization by the inflammatory cells. Excessive intracellular cholesterol accumulation becomes toxic to macrophage foam cells, leading to cell death beyond a threshold. Our findings reveal that initially, the plaque evolves rapidly, and the growth rate eventually reduces because of the cholesterol-induced toxicity. The present study manifests that higher oxLDL cholesterol flux reduces plaque growth rate, while elevated cytokines flux promotes the corresponding plaque growth behaviour. Between oxLDL cholesterol and intracellular cholesterol, the second one is much more effective towards the growth of inflammatory tissue. The cholesterol-based toxicity-induced cell death parameters are crucial in flattening the plaque growth profile. A detailed analysis of the model presented in this article provides critical insights into the various biochemical and cellular mechanisms behind early plaque development.

q-bio.TO↗

arXiv2024

Chiral swimmer with a regular arbitrary active patch

We investigate the low Reynolds number hydrodynamics of a spherical swimmer with a predominantly hydrophobic surface, except for a hydrophilic active patch. This active patch covers a portion of the surface and exhibits chiral activity that varies as a function of $θ$ and $ϕ$. Our study considers two types of active patches: (i) a symmetric active patch (independent of $ϕ$) and (ii) an arbitrary active patch (depends on both $θ$ and $ϕ$). The swimming velocity, rotation rate, and flow field of the swimmer are calculated analytically. The objective of this work is to find the optimal configurations for both patch models to maximize the swimmer's velocity and efficiency. Interestingly, the maximum velocity can be controlled by adjusting the hydrophobicity, patch configuration, and strength of the surface activity. We find that for the symmetric patch model, the swimmer's velocity is $U_{SP} = 1.414 U_s$, where $U_s$ is the velocity of a swimmer whose surface is fully covered with chiral activity as a reference. For the arbitrary patch model, the velocity is $U_{AP} = 1.45 U_s$, which is higher than that of the symmetric patch model. Our results indicate that swimmers with low hydrophobicity exhibit efficient swimming characteristics. Additionally, due to the incomplete coverage of the active patch, the Stokeslet and Rotlet terms appear in the flow field generated by the swimmer, which is a deviation compared to the case of a swimmer whose surface is fully covered with chiral activity. This study provides insights useful for designing synthetic active particles, which can be applied, for example, in targeted drug delivery, chemotaxis, and phototaxis.

physics.flu-dyn↗

arXiv2024

Hydrodynamic instability of shear imposed falling film over a uniformly heated inclined undulated substrate

Linear and weakly nonlinear stability analyses of an externally shear-imposed, gravity-driven falling film over a uniformly heated wavy substrate are studied. The longwave asymptotic expansion technique is utilized to formulate a single nonlinear free surface deflection equation. The linear stability criteria for the onset of instability are derived using the normal mode form in the linearized portion of the surface deformation equation. Linear stability theory reveals that the flow-directed sturdy external shear grows the surface wave instability by increasing the net driving force. On the contrary, the upstream-directed imposed shear may reduce the surface mode instability by restricting the gravity-driving force, which has the consequence of weakening the bulk velocity of the liquid film. However, the surface mode can be stabilized/destabilized by increasing the temperature-dependent density/surface-tension variation. Further, the bottom steepness shows dual behaviour on the surface instability depending upon the wavy wall's portion (uphill/downhill). At the downhill portion, the surface wave becomes more unstable than at the bottom substrate's uphill portion. Moreover, the multi-scale method is incorporated to obtain the complex Ginzburg-Landau equation in order to study the weakly nonlinear stability, confirming the existence of various flow regions of the liquid film. At any bottom portion (uphill/downhill), the flow-directed external shear expands the super-critical stable zones, which causes an amplification in the nonlinear wave amplitude, and the backflow-directed shear plays a counterproductive role. On the other hand, the super-critical stable region decreases or increases as long as the linear variation of density or surface tension increases with respect to the temperature, whereas the sub-critical unstable region exhibits an inverse trend.

physics.flu-dyn↗

arXiv2024

Nonlinear biphasic mixture model: existence and uniqueness results

This paper is concerned with the development and analysis of a mathematical model that is motivated by interstitial hydrodynamics and tissue deformation mechanics (poro-elasto-hydrodynamics) within an in-vitro solid tumor. The classical mixture theory is adopted for mass and momentum balance equations for a two-phase system. A main contribution of this study, we treat the physiological transport parameter (i.e., hydraulic resistivity) as anisotropic and heterogeneous, thus the governing system is strongly coupled and nonlinear. We derived a weak formulation and then formulated the equivalent fixed-point problem. This enabled us to use the Galerkin method, and the classical results on monotone operators combined with the well-known Schauder and Banach fixed point theorems to prove the existence and uniqueness results.

math.AP↗

arXiv2021

Effective medium model for a suspension of active swimmers

Several active organisms in nature tend to reside as a community in a viscous fluid medium. We analyze the variation of swimming characteristics of an active swimmer present in a dilute and disperse suspension, modeled as an effective Brinkman medium. This idealized representation of a collection of active swimmers allows one to distinguish the impact of the interior domain available to an individual swimmer as well as the contribution of its neighbors. Darcy's law along with the analytical solution enables the effective resistivity to be predicted as a function of the volume fraction which is in close agreement with the well-known Carman-Kozeny equation. This facilitates the successive analysis of the propulsion speed, power dissipation, and swimming efficiency of the targeted swimmer as a function of the volume fraction which is decisive in nutrient transport and uptake or reproduction in a collective environment. A stress-jump condition is also imposed across a cell to indicate a mean effective force due to the nearby swimmers. At suitable values of this stress-jump coefficient, the relative increase of the migration velocity and swimming efficiency is noticeably higher at an optimum occupancy.

cond-mat.soft↗

arXiv2019

Forced convection in a fluid saturated anisotropic porous channel with isoflux boundaries

Fully developed forced convective flow in a channel filled with porous material bounded by two impermeable walls subject to constant heat flux is considered. We consider Brinkman-Forchhimer equation to govern the flow inside the porous medium which accounts for the presence of inertial term. We assume that the porous medium is anisotropic in nature and permeability is varying along all the directions, correspondingly, permeability will be a matrix which appear as a positive semi-definite matrix in the momentum equation. The anisotropic nature of the porous medium is characterized by anisotropic ratio and angle. We have obtained the velocity, temperature and Nusselt number numerically, due to the presence of the non-linear quadratic term in the momentum equation. The effect of anisotropic permeability ratio and anisotropic angle on hydrodynamic and heat transfer is shown. We see that Nusselt number is altered significantly with the anisotropic ratio and angle. We present a fresh analysis about the inclusion of the permeability matrix in the Brinkman-Forchhimer extended Darcy momentum equation. To the best of the authors knowledge, this is the first attempt to analyze general anisotropy corresponding to Brinkman-Forchhimer extended Darcy model.

physics.flu-dyn↗

arXiv2016

Effect of surfactant concentration and interfacial slip on the flow past a viscous drop at low surface Péclet number

The motion of a viscous drop is investigated when the interface is fully covered with a stagnant layer of surfactant in an arbitrary unsteady Stokes flow for the low surface Péclet number limit. The effect of the interfacial slip coefficient on the behavior of the flow field is also considered. The hydrodynamic problem is solved by the solenoidal decomposition method and the drag force is computed in terms of Faxen's laws using a perturbation ansatz in powers of the surface Péclet number. The analytical expressions for the migration velocity of the drop are also obtained in powers of the surface Péclet number. Further instances corresponding to a given ambient flow as uniform flow, Couette flow, Poiseuille flow are analyzed. Moreover, it is observed that, a surfactant-induced cross-stream migration of the drop occur towards the centre-line in both Couette flow and Poiseuille flow cases. The variation of the drag force and migration velocity is computed for different parameters such as Péclet number, Marangoni number etc.

physics.flu-dyn↗