Journal of Non-Newtonian Fluid Mechanics最新文献

Understanding polymer dynamics under shear flow is crucial for studying their rheological behavior in diverse applications. However, conventional micro analyses provide limited insights into polymer elongation and conformation. To address this, we propose a hybrid model combining the Lattice Boltzmann method and Langevin Dynamics technique, which captures the multiscale nature of polymer dynamics. Using the coarse-grain bead-spring method, we optimize computational efficiency and model polymers as chains with specific mass and charge. Our hybrid model integrates Navier-Stokes equations with external drag force modified based on segment velocities from Brownian Dynamics simulations.

In our study, we investigated the effects of chain structure and solvent properties on polymer solutions under shear flow through numerical simulations. We observed that in high shear rate flows, a viscous solvent promotes polymer elongation, while low shear rate flows lead to chain insolubility in the base oil. Longer chains have a greater overall impact on the fluid due to increased contact points with the solvent. The size of the polymer coil over time is influenced by shear rate, chain length, and solvent viscosity. Moreover, solvent density, particle mass, and radius locally affect fluid flow. The higher viscosity fluids result in amplified hydrodynamic and random forces acting on the chains. These findings have implications for applications involving polymer additives that alter the properties of the host solvent in natural and artificial processes. Our study represents an initial step towards a comprehensive understanding of polymer dynamics, taking into account the diverse factors that influence them.

This work builds upon the previously published analysis of a lid-driven cavity filled with viscoplastic fluid. We extend the study from a two-dimensional case to a three-dimensional one, employing the moment representation of the lattice Boltzmann method to obtain numerical results. The findings expand the existing dataset, which can potentially serve as benchmark results for inertial regimes of viscoplastic flows. In this study, we investigate the Reynolds and Bingham numbers until the flow transition from stationary to a transient regime. The results reveal that, similarly to the Newtonian case, there is an effective Reynolds number for the bifurcation, approximately Re^⁎ = Re₀ (1 + Bn), where Re₀ represents the bifurcation point for a Newtonian fluid. Like the Newtonian cases, there were instances where the Taylor-Görtler-like vortices moved toward the cavity's side periodically. In other cases, more than two vortices simultaneously formed, with their number changing over time. Finally, similar to the two-dimensional case, the bifurcation initiated after the Moffat eddies in the downstream corner broke down into plugs.

We succeeded in developing a multiscale simulation (MSS) method for a spinning process of a polymer melt. A previous work by Sato and Taniguchi (2017) developed a MSS method where the microscopic model and macroscopic model for the spinning process are respectively modeled by using a slip-link model and a continuous fluid mechanics model. Here we replace the microscopic model with the Kremer–Grest coarse-grained (CG) model, and investigate the state of the polymer chains at steady state in the spinning process, by changing the draw ratio Dr. Unlike the previous MSS, where the microscopic simulator is a slip-link model, in which polymer chains are simulated in virtual space and entanglements are treated by virtual links, in the present MSS, a real space molecular dynamics simulator is used as the microscopic simulator. The replacement brings the advantage that we can obtain more information on the state of polymer chains, but also brings two computational difficulties, (I) the requirement of a huge computational cost, and (II) the simulation box problem related to the periodic boundary condition in the microscopic system. To deal with (I), we considered a micro-macro coupling method different from previous MSS. To resolve problem (II), we used the UEF (uniform extensional flow) method developed by Nicholson and Rutledge (2016) and Murashima et al. (2018) for a polymer melt system. By using these two ideas, we performed MSS simulations, and established a correspondence between the macroscopic flow and the microscopic polymer conformations at any position along the spinning line. Furthermore, we investigated the influence of Dr on the stretching and orientation of polymers chains and the spatial correlation between polymer chains.

This study focuses on the two-dimensional numerical investigation of complex fluid flows through a micro-contraction geometry in the creeping flow regime, specifically examining elastoviscoplastic (EVP) fluids. These fluids exhibit a combination of viscous, elastic, and plastic behaviors. The governing equations, including mass and momentum, are solved using a finite volume method-based discretization technique. Saramito’s constitutive model is utilized to accurately represent the viscous, elastic, and plastic responses of the EVP fluid. The present results demonstrate significant differences in flow dynamics, such as vortex dynamics and transitions between flow regimes (e.g., steady to unsteady), when compared to simple Newtonian and non-Newtonian viscoelastic (VE) or viscoplastic (VP) fluids. This study reveals that when the yield strain $\left({ϵ}_y\right)$ exceeds a critical value, approximately ranging from 0.79 to 0.89, the flow transits from a steady to an unsteady state for the EVP fluids. Importantly, the present study shows that EVP fluids exhibit intensified chaotic flow dynamics and increased instability compared to VE and VP fluids under similar flow conditions. However, the presence of shear-thinning behavior in EVP fluids suppresses this instability. The analysis of local velocity fields and flow deformation in this study highlights the impact on the stretching of fluid microstructure and elastic stresses, which ultimately contribute to the origin of this intensified unstable flow condition for EVP fluids. The finding from this study holds significant potential for enhancing heat or mass transfer rates and mixing efficiency in micro-scale systems, where the prevailing steady and laminar flow conditions often hinder these transport processes.

The effect of molecular weight distribution of polyacrylamide (PAAm) on drag reduction was studied in two flow geometries. Commercial PAAm with different weight averaged molecular weights (M_w = 5 × 10⁵ to 1.8 × 10⁷ g/mol) were investigated in turbulent pipe and rotational flows. Comparison of PAAm with different molecular weight distributions showed that drag reduction is not only a function of the averaged molecular weight. Broader polymer molecular weight distributions provided increased drag reduction over polymers of same average molecular weight but with a more narrow distribution. The role of distribution widths is of significance as polymer degradation in turbulent flows causes narrowing of the molecular weight distributions. Multiple linear regression was employed to connect weight fractions of polyacrylamide with drag reduction. Multiple linear regression was successfully applied to describe drag reduction in turbulent pipe and rotational flows indicating that drag reduction can be quantitatively derived from the molecular weight distribution of PAAm.

Inelastic time-dependent fluids display continuous and reversible changes in viscosity when subjected to a constant shear-rate. These alterations arise from the gradual modification of the material’s microstructure due to shear-induced effects, known as shear rejuvenation. When this process generates smaller structural units, it is termed thixotropy; conversely, if it produces larger units, it is labeled anti-thixotropy. Aging is another characteristic of such fluids, denoting the capacity of the material to regain its original structure in the absence of shear, thus reversing the initial time-dependent change. This phenomenon often results from thermally activated Brownian motion prompting the reorganization of the material’s microconstituents. Consequently, attractive forces between these components can instigate the reconstruction of a network-like structure within the material. This study centers on investigating how variations in fluid microstructure impact the onset of transition to turbulence in a flat plate boundary-layer flow. Specifically, the focus is on cases where larger structural units emerge during the breakdown process (anti-thixotropy). To represent such fluids, the Quemada model, an inelastic structural-kinetic model, is employed. This model effectively captures thixotropy and anti-thixotropy by appropriately configuring model parameters. The analysis begins with obtaining a local similarity solution for the generalized Blasius equation, representing the base flow. Subsequently, the stability of this flow is assessed using linear temporal stability theory. This involves introducing infinitesimally-small normal-mode perturbations to the base flow, yielding the generalized Orr–Sommerfeld equation. Solving this equation using the spectral method provides insights into stability. Results from this study indicate that for low Deborah numbers, the shear-thickening behavior prevails, causing destabilization. In contrast, higher Deborah numbers lead to stability. This implies that anti-thixotropy effectively delays the onset of transition to turbulence and could hold practical applications for flow control.

In this work, we numerically investigate flow instabilities of inertialess circular Couette flow of dilute wormlike micelle solutions. Using the reformulated reactive rod model (RRM-R) (Hommel and Graham, 2021), which treats micelles as rigid Brownian rods undergoing reversible scission and fusion in flow, we study the development and behavior of both vorticity banding and finger-like instabilities. In particular, we focus on solutions that exhibit reentrant constitutive curves, in which there exists some region where the shear stress, $\tau$, has a multivalued relation to shear rate, $\dot{\gamma }$. We find that the radial dependence of the shear stress in circular Couette flow allows for solutions in which parts of the domain lie in the region of the flow curve where $\partial \tau /\partial \dot{\gamma }>0$, while others lie in the region where $\partial \tau /\partial \dot{\gamma }<0$; this mixed behavior can lead to complex flow instabilities that manifest as finger-like structures of elongated and anisotropically-oriented micelles. In 3D simulations we find that the initial instability is 2D in origin, and 3D finger-like structures arise through the axial instability of 2D sheets. Finally, we show that the RRM-R can capture vorticity banding in narrow-gap circular Couette flow and that vorticity bands are linearly stable to perturbations.

The impact of the inlet velocity profile at the die exit on the film dynamics within the air-gap region was investigated using numerical simulations for both Newtonian and viscoelastic Phan-Thien and Tanner (PTT) fluids in isothermal two-dimensional (2-D) film casting processes. In an industrial context, intentional adjustments were made to reduce the inlet velocities at the edge of the casting die, effectively mitigating the edge-beads characterized by a higher edge thickness than the center thickness of the final films. By varying the inlet velocity conditions with decreasing edge velocities, the steady film dynamics were correlated with the onsets of draw resonance instability and frequency responses to a disturbance, which were determined using the transfer function data obtained under tension-controlled conditions. The results revealed that decreasing the inlet velocity at the edge improved not only the formation of films but also the process stability for both Newtonian and viscoelastic fluids. In addition, the sensitivity or frequency response to a disturbance was effectively reduced by decreasing the inlet velocity at the edge. This observed impact of the inlet velocity was closely related to the increased tension levels at take-up and a greater portion of the neck-like deformation type in the air-gap region.