Darish Jeswin Dhas

We investigate how breaking the geometric axisymmetry of fibers alters their dynamics when suspended in wall turbulence, using intrinsic curvature as a controlled proxy for symmetry loss. The study is based on a series of large-scale direct numerical simulations of turbulent channel flow laden with flexible fibers with varying lengths and intrinsic curvatures. We find that curved fibers tend to accumulate in the near-wall region of the flow. Analyzing their orientation dynamics, we recorded a marked shift in dynamics in comparison to their straight counterparts as they tend to have a preferential alignment even in the near-isotropic region of the flow. They also tend to tumble at larger rates with increasing curvatures. Furthermore, we also find that fiber dynamics become less sensitive to their curvature with increasing length.

In this paper, we study numerically the role of fiber length and flexibility on the orientational dynamics of slender fibers in turbulent channel flow. We consider fibers of different flexibility at varying aspect ratio, up to lengths being comparable to the channel height. These fibers are constructed by constraining a large number of sub-Kolmogorov rods in a single chain, alongside a bending stiffness torque that allows to prescribe a finite value of the fiber rigidity. To perform our analysis, we carried out a series of one-way coupled direct numerical simulations of a fiber-laden channel flow at fixed shear Reynolds number: Reτ=300, based on the half height of the channel. By calculating the orientational statistics of the suspended fibers, we find that shorter fibers, with length O(10−1) when normalized by the channel half height, tend to exhibit a nearly-isotropic orientation distribution near the channel center, as would fibers suspended in homogeneous isotropic turbulence. As the fiber length is increased (up to values comparable to the channel half height), however, deviations from the isotropic orientation distribution become more and more significant. When the fibers are more rigid, these deviations are dampened and it is also observed that the tumbling rate of the fiber is lowered on average.

We study the effects of buoyancy, surface-tension gradients and phase boundary on the stability of a layer of water that is confined between air at the top and a layer of ice at the bottom. The temperature of the overlying air and flux condition at the free surface of the water layer are such that the layer is susceptible to both thermal and thermocapillary instabilities. We perform a linear stability analysis to identify these modes of instability and investigate the effects of the phase boundary on them. We find that with increasing thickness of the ice layer, the critical Rayleigh and Marangoni numbers for the instabilities are found to first decrease and then asymptote to constant values for ice thicknesses much larger than the thickness of the water layer. In the case of thermocapillary instability, we find that the thickness of the ice layer has negligible influence on the stability threshold for dimensionless wavenumber . Furthermore, the inclusion of Marangoni stresses reduces the stability threshold of the thermal instability.

Fluid flows over phase-changing boundaries find relevance in the study of many problems in astrophysical, geophysical, and engineering settings. A detailed understanding of the nonlinear coupling between fluid flow and the moving boundary is often critical to the evolution of the system. Growth of sea ice in the Earth's polar regions and the dynamics of ice sheets and the Earth's interior are a few examples that highlight their ubiquity across scales.Here, we consider the stability of a layer of liquid that is bounded by its solid phase at the bottom and a free surface at the top. The liquid has a temperature of maximum density, Tm, and convective motions in the liquid layer ensue when the temperature of the free surface is greater than Tm. We use a combination of linear and asymptotic analyses to explore the effects of the phase-changing boundary, density anomaly, and the free surface on the thermal and thermocapillary instabilities in this system.

In this paper, we study the linear stability and the subsequent formation of nonlinear waves in a colloidal, gravity-driven falling film flow. We first investigate the system's stability by performing a linear stability analysis and observe that the presence of colloidal particles stabilizes both the long-wave surface and the short-wave shear instability modes. The stabilization is attributed to Brownian diffusion equilibrating the leading order particle volume fraction, causing a uniform increase in viscosity. With the particle evolution equation decoupled from the momentum equation in the linear limit, we analytically study the damped yet intricate evolution of perturbations to the particle concentration field. The particle mode decays via three different asymptotic routes: Brownian diffusion, anomalous diffusion, and Taylor dispersion. To study the nonlinear waves, we then derive nonlinear models in the framework of long-wave theory using Benney's gradient expansion approach and the integral-boundary-layer approach. Subsequently, we use a central-manifold approach to derive a depth-averaged equation for the particle volume fraction evolution, incorporating Brownian diffusion and Taylor dispersion. Comparisons of the linear predictions of the nonlinear models with the linear stability calculations show good agreement in the limit of small wave numbers.