Browsing by Subject "Viscoelasticity"

Now showing 1 - 12 of 12

Open Access
The effect of fluid viscoelasticity in lubricated contacts in the presence of cavitation
(Elsevier, 2021-03-27) Gamaniel, Samuel Shari; Dini, D.; Biancofiore, Luca
In this work we study the influence of fluid viscoelasticity on the performance of lubricated contacts in the presence of cavitation. Several studies of viscoelastic lubricants have been carried out, but none of them have considered the possibility of the presence of cavitation. To describe the effect of viscoelasticity, we use the Oldroyd-B model. By assuming that the product between ϵ, i.e. the ratio between vertical and horizontal length scales, and the Weissenberg number (Wi), i.e. the ratio between polymer relaxation time and flow time scale, is small, we can linearise the viscoelastic thin film equations, following the approach pioneered by "Tichy, J., 1996, Non-Newtonian lubrication with the convected Maxwell model." Consequently, the zeroth-order in ϵWi corresponds to a Reynolds equation modified to describe also the film cavitation through the mass-conserving Elrod-Adams model. We consider the flow of viscoelastic lubricants using: (i) a cosine profile representing a journal bearing unwrapped geometry, and (ii) a pocketed profile to model a textured surface in lubricated contacts. The introduction of viscoelasticity decreases the length of cavitated region in the cosine profile due to the increasing pressure distribution within the film. Consequently, the load carrying capacity increases with Wi by up to 50% in the most favorable condition, confirming the beneficial influence of the polymers in bearings. On the other hand for the pocketed profile, results show that the load can increase or decrease at higher Wi depending on the texture position in the contact. The squeeze flow problem between two plates is also modeled for viscoelastic lubricants considering an oscillating top surface. For this configuration a load reduction is observed with increasing Wi due to the additional time needed to reform the film at high Wi. Furthermore, if viscoelastic effects increase, the cavitation region widens until reaching a value of Wi for which a full-film reformation does not occur after the initial film rupture.
Open Access
Effect of magnetic field on the radial pulsations of a gas bubble in a non-Newtonian fluid
(Elsevier Ltd, 2015) Behnia, S.; Mobadersani F.; Yahyavi, M.; Rezavand, A.; Hoesinpour, N.; Ezzat, A.
Dynamics of acoustically driven bubbles' radial oscillations in viscoelastic fluids are known as complex and uncontrollable phenomenon indicative of highly active nonlinear as well as chaotic behavior. In the present paper, the effect of magnetic fields on the non-linear behavior of bubble growth under the excitation of an acoustic pressure pulse in non-Newtonian fluid domain has been investigated. The constitutive equation [Upper-Convective Maxwell (UCM)] was used for modeling the rheological behaviors of the fluid. Due to the importance of the bubble in the medical applications such as drug, protein or gene delivery, blood is assumed to be the reference fluid. It was found that the magnetic field parameter (B) can be used for controlling the nonlinear radial oscillations of a spherical, acoustically forced gas bubble in nonlinear viscoelastic media. The relevance and importance of this control method to biomedical ultrasound applications were highlighted. We have studied the dynamic behavior of the radial response of the bubble before and after applying the magnetic field using Lyapunov exponent spectra, bifurcation diagrams and time series. A period-doubling bifurcation structure was predicted to occur for certain values of the parameters effects. Results indicated its strong impact on reducing the chaotic radial oscillations to regular ones. © 2015 Elsevier Ltd. All rights reserved.
Open Access
Growth-induced instabilities of an elastic film on a viscoelastic substrate: Analytical solution and computational approach via eigenvalue analysis
(Mathematical Sciences Publishers, 2018) Valizadeh, I.; Steinmann, P.; Javili, Ali
The objective of this contribution is to study for the first time the growth-induced instabilities of an elastic film on a viscoelastic substrate using an analytical approach as well as computational simulations via eigenvalue analysis. The growth-induced instabilities of a thin film on a substrate is of particular interest in modeling living tissues such as skin, brain, and airways. The analytical solution is based on Airy's stress function adopted to viscoelastic constitutive behavior. The computational simulations, on the other hand, are carried out using the finite deformation continuum theory accounting for growth via the multiplicative decomposition of the deformation gradient into elastic and growth parts. To capture the critical growth of elastic films and the associated folding pattern, eigenvalue analysis is utilized, in contrast to the commonly used perturbation strategy. The eigenvalue analysis provides accurate, reliable, and reproducible solutions as contrasted to the perturbation approach. The numerical results obtained from the finite element method show an excellent agreement between the computational simulations and the proposed analytical solution.
Open Access
Heterogeneous multifrequency direct inversion (HMDI) for magnetic resonance elastography with application to a clinical brain exam
(Elsevier B.V., 2018) Barnhill, E.; Davies, P. J.; Ariyurek, C.; Fehlner, A.; Braun, J.; Sack, I.
A new viscoelastic wave inversion method for MRE, called Heterogeneous Multifrequency Direct Inversion (HMDI), was developed which accommodates heterogeneous elasticity within a direct inversion (DI) by incorporating first-order gradients and combining results from a narrow band of multiple frequencies. The method is compared with a Helmholtz-type DI, Multifrequency Dual Elasto-Visco inversion (MDEV), both on ground-truth Finite Element Method simulations at varied noise levels and a prospective in vivo brain cohort of 48 subjects ages 18-65. In simulated data, MDEV recovered background material within 5% and HMDI within 1% of prescribed up to SNR of 20 dB. In vivo HMDI and MDEV were then combined with segmentation from SPM to create a fully automated “brain palpation” exam for both whole brain (WB), and brain white matter (WM), measuring two parameters, the complex modulus magnitude |G*|, which measures tissue “stiffness” and the slope of |G*| values across frequencies, a measure of viscous dispersion. |G*| values for MDEV and HMDI were comparable to the literature (for a 3-frequency set centered at 50 Hz, WB means were 2.17 and 2.15 kPa respectively, and WM means were 2.47 and 2.49 kPa respectively). Both methods showed moderate correlation to age in both WB and WM, for both |G*| and |G*| slope, with Pearson's r ≥ 0.4 in the most sensitive frequency sets. In comparison to MDEV, HMDI showed better preservation of recovered target shapes, more noise-robustness, and stabler recovery values in regions with rapid property change, however summary statistics for both methods were quite similar. By eliminating homogeneity assumptions within a fast, fully automatic, regularization-free direct inversion, HMDI appears to be a worthwhile addition to the MRE image reconstruction repertoire. In addition to supporting the literature showing decrease in brain viscoelasticity with age, our work supports a wide range of inter-individual variation in brain MRE results.
Open Access
Modeling lubricants enhanced by finite elasticity polymers
(2023-12) Ahmed, Humayun
Lubrication is essential for the longevity of mechanical and biological surfaces in relative motion and susceptible to friction and wear. A well designed lubricant, for example a base oil enhanced with polymer additives, can effectively reduce both energetic and material losses. However, difficulty arises when modeling these lubricant mixtures exhibiting complex rheological behavior, in particular, a dependence of the viscosity on pressure (piezoviscosity), temperature (thermal thinning), shear rate (shear thinning) and the onset of viscoelasticity. Accurate estimates of the load carrying capacity of the thin lubricating film requires careful modeling of shear thinning. Available models such as the generalized Reynolds equation (GR) and the approximate shear distribution (ASD) have drawbacks such as large computational time and poor accuracy, respectively. In this work, we present a new approach, i.e. the modified viscosity (MV) model. We investigate, for both MV and GR, the load, the maximum pressure and the computational time, for (i) sliding (non-cavitating) contacts, (ii) cavitating and (iii) squeezing contacts. We observe that the computational time is reduced (i) considerably for non-cavitating sliding and rolling contacts and (ii) by several order of magnitudes for cavitating and squeezing contacts. For strongly elastic lubricants, the viscoelastic Reynolds (VR) approach (Ahmed & Biancofiore, Journal of Non-Newtonian Fluid Mechanics, 292, 104524, 2021.) has been shown to be effective in modeling (i) the pressure distribution and (ii) the load carrying capacity of a viscoelastic lubricating film for mechanical contacts for the Oldroyd-B constitutive relation. In this work, we have extended the VR approach to the non-linear finitely extensible non-linear elastic (FENE) type constitutive relations that account for the (i) finite extension of the polymer chains and (ii) shear thinning. We have validated the VR approach against DNS, showing an excellent agreement over a wide range of the Weissenberg number W i, i.e. the ratio between the polymer relaxation time and the flow time scale, and finite extensibility parameter L, using FENE-CR and FENE-P. Following a thorough validation, the pressure distribution and the load carrying capacity of a journal bearing, whose channel height is governed by the journal eccentricity ratio e, is considered. It is observed that the load carrying capacity of the film portrays a strongly non-linear dependence on W i, L and e: while it increases for small values of W i, limited greatly by the capacity of the polymer to stretch, a saturation and a subsequent decline is observed for highly viscoelastic regimes. Additionally, a weakly (strongly) eccentric configuration plays an important role in promoting (hindering) the growth in load versus both W i and L. These effects are significant and have to be considered when modeling thin contacts lubricated with a strongly viscoelastic fluid. Additionally, we have extended the VR approach towards three-dimensional lubricated contacts (in cartesian and cylindrical coordinate systems) for several non-linear constitutive relations and have provided a linearized model in De. Owing to the increase in computational requirements, a globally fully-implicit numerical technique was adopted for the efficient solution of the equations. The load and friction response for an extruded journal bearing e = 0.9 (and parabolic slider) showed a strong variation versus the channel aspect ratio (otherwise zero for a Newtonian lubricant), i.e. a = ℓx/ℓz, the ratio between the channel streamwise and spanwise lengths. The effects of transient surface motion on the response of an elastic polymer have also been examined, with a specific focus on the load carrying capacity and the friction, via a second-order perturbation model and the VR approach. We find, the perturbed models only offer a matching prediction (i) once the motion has proceeded from some time and, (ii) the De is small. A simplified look into the influence of polymer elasticity on the temperature distribution of the film showed a weak dependency versus De. The film heating owing to the fluid dissipation remained largely unaffected unless the De was large.
Embargo
Modeling polymeric lubricants with non-linear stress constitutive relations
(Elsevier, 2023-09-16)
Lubricating oils are used to minimize the friction and wear of mechanical components by virtue of a thin lubricant film separating the sliding surfaces. The film’s characteristics, under high pressure, can exhibit non-Newtonian effects, such as viscoelasticity and shear thinning. The strength of these effects are measured using the Weissenberg (Deborah) number 𝑊 𝑖 (𝐷𝑒), i.e., the ratio of the polymer relaxation time to the shear (residence) time scale. Modeling these effects is computationally challenging, especially when relying on the direct numerical simulation (DNS) of the Cauchy momentum and the mass conservation equations. However, the viscoelastic Reynolds (VR) approach (Ahmed & Biancofiore, Journal of Non-Newtonian Fluid Mechanics, 292, 104524, 2021.) has been shown to be effective in modeling (i) the pressure distribution and (ii) the load carrying capacity of a viscoelastic lubricating film for mechanical contacts for the Oldroyd-B constitutive relation, since these contacts operate within the small 𝐷𝑒 limit (but no constraint in 𝑊 𝑖). In this work, we have extended the VR approach to the finitely extensible non-linear elastic (FENE) type constitutive relations that account for the (i) finite extension of the polymer chains and (ii) shear thinning. We have validated the VR approach against DNS, showing an excellent agreement over a wide range of the Weissenberg number 𝑊 𝑖, and finite extensibility parameter 𝐿, using FENE-CR and FENE-P models. Following a thorough validation, the pressure distribution and the load carrying capacity of a journal bearing, whose channel height is governed by the journal eccentricity ratio 𝑒, is considered. It is observed that the load carrying capacity of the film portrays a strongly non-linear dependence on 𝑊 𝑖, 𝐿 and 𝑒: while it increases for small values of 𝑊 𝑖, limited greatly by the capacity of the polymer to stretch, a saturation and a subsequent decline is observed for high 𝑊 𝑖 regimes. Additionally, a weakly (strongly) eccentric configuration plays an important role in promoting (hindering) the growth in load versus both 𝑊 𝑖 and 𝐿. These effects are significant and have to be considered when modeling thin contacts lubricated with a strongly viscoelastic fluid.
Embargo
Molecular rheology of unentangled polymer melts in nanochannels
(2024-08) Yıldırım, Ahmet Burak
We investigate the rheological properties of non-Newtonian melts of short polymer chains at the molecular level, with a focus on nanoscale confinement. Using a combination of all-atom molecular dynamics (MD) and coarse-grained simulations, we examine the behavior of oligomer melts with various topologies under non-equilibrium conditions. Our findings reveal significant deviations in the microscopic stress tensor under steady-state shear compared to predictions from continuum models, highlighting the limitations of the Oldroyd-B and generalized Phan-Thien–Tanner (gPTT) models in capturing nanoscale phenomena. We demonstrate that these deviations result in excess viscoelastic stress, which diminishes with decreasing confinement and vanishes in bulk systems. This excess stress is linked to the spatial orientation of carbon-carbon bonds near surfaces, where adsorbed chains form effective polymer brush-like layers. Additionally, we explore the effects of chain rigidity, surface-oligomer attraction, and molecular variables on rheological responses, providing a comprehensive understanding of the interplay between molecular-scale phenomena and macroscopic behavior in confined polymeric systems.
Open Access
A new approach for modeling viscoelastic thin film lubrication
(Elsevier BV, 2021-03-27) Biancofiore, Luca; Ahmed, Humayun
Lubricants can exhibit significant viscoelastic effects due to the addition of high molecular weight polymers. The overall behavior of the mixture is vastly different from a simpler Newtonian fluid. Therefore, understating the influence of viscoelasticity on the load carrying capacity of the film is essential for lubricated contacts. A new modeling technique based on lubrication theory is proposed to take into account viscoelastic effects. As a result, we obtain a modified equation for the pressure, i.e. the viscoelastic Reynolds (VR) equation. We have first examined a parabolic slider to mimic a roller bearing configuration. An increase of the load carrying capacity is observed when polymers are added to the lubricant. Furthermore, our results are compared with existing models based on the lubrication approximation and direct numerical simulations (DNS). For small Weissenberg number (), i.e. the ratio between the polymer relaxation time and the residence time scale, VR predicts the same pressure of the linearized model, in which is the perturbation parameter ( is the ratio between the vertical length scale and the horizontal length scale). However, the difference grows rapidly as viscoelastic effects become stronger. Excellent quantitative and qualitative agreement is observed between DNS and our model over small to moderate Weissenberg number. While DNS is numerically unstable at high values of the Weissenberg number, VR does not have the same issue allowing to capture the evolution of the stress and pressure also when the viscoelastic effects are strong. It is shown that even in high shear flows, normal stresses have the largest impact on load carrying capacity and thus cannot be neglected. Furthermore, the additional pressure due to viscoelasticity comprises two components, the first one due to the normal stress and the second one due to the shear stress. Afterwards, the methodology used for the parabolic slider is extended to a plane slider where, instead, the load decreases by adding polymers to the fluid. In particular, under the effect of the polymers surface slopes enhance the rate at which pressure gradients increase, whereas curvature opposes this along the contact. Therefore, the increase of the load carrying capacity observed for viscoelastic lubricants is due to its shape close to the inlet, which is steeper than the plane slider.
Open Access
Nonlinear modeling of non-Newtonian hydrodynamic lubrication
(2019-08) Ahmed, Humayun
Lubrication is essential to improve the performance of sliding surfaces. Power transmission in mechanical and biological systems rely on proper lubrication to minimize wear and energy losses. However, most practical applications involve conditions that cause or require the lubricant to exhibit non-Newtonian behavior, namely shear thinning and viscoelasticity. In this study a novel non-linear 1D Reynolds equation is proposed for modeling shear thinning and a 1D viscoelastic Reynolds equation is proposed to model viscoelastic effects. The models are compared with the direct numerical simulation (DNS) of thin films for different geometries. The results are in good qualitative and quantitative agreement indicating the simplified models are valid. The pressure presents strong variations as lubricant elasticity becomes significant, but stagnates as the polymer relaxation time becomes slow compared to the characteristic ow time. The net film pressure is shown to be a superposition of a Newtonian and viscoelastic component. The viscoelastic pressure varies as contact geometry changes. Surfaces with constant slope (plane slider) exhibit a constant decrease in film pressure whereas parabolic surfaces can enhance pressure for low relaxation times.
Open Access
Supramolecular nanostructure formation of coassembled amyloid inspired peptides
(American Chemical Society, 2016-06) Cinar, G.; Orujalipoor, I.; Su, C.-J.; Jeng, U.-S.; Ide, S.; Güler, Mustafa O.
Characterization of amyloid-like aggregates through converging approaches can yield deeper understanding of their complex self-assembly mechanisms and the nature of their strong mechanical stability, which may in turn contribute to the design of novel supramolecular peptide nanostructures as functional materials. In this study, we investigated the coassembly kinetics of oppositely charged short amyloid-inspired peptides (AIPs) into supramolecular nanostructures by using confocal fluorescence imaging of thioflavin T binding, turbidity assay and in situ small-angle X-ray scattering (SAXS) analysis. We showed that coassembly kinetics of the AIP nanostructures were consistent with nucleation-dependent amyloid-like aggregation, and aggregation behavior of the AIPs was affected by the initial monomer concentration and sonication. Moreover, SAXS analysis was performed to gain structural information on the size, shape, electron density, and internal organization of the coassembled AIP nanostructures. The scattering data of the coassembled AIP nanostructures were best fitted into to a combination of polydisperse core-shell cylinder (PCSC) and decoupling flexible cylinder (FCPR) models, and the structural parameters were estimated based on the fitting results of the scattering data. The stability of the coassembled AIP nanostructures in both fiber organization and bulk viscoelastic properties was also revealed via temperature-dependent SAXS analysis and oscillatory rheology measurements, respectively.
Open Access
Theoretical and experimental analysis of a soft and miniature quadruped
(2020-11) Taşkıran, Tamer
Multi-body dynamic modeling of non-rigid and legged robots is an active research area with the goal of investigating the interaction of a robot with its environment and its resulting locomotion. The results of such studies can be used to build a simulation that will be used for the iterative process of the mechanical design of the robot, and the algorithm design of its high- and low-level controllers. The dynamics of soft robots is more challenging comparing to rigid robots because of the partial derivatives and the shape integrals existing in the dynamic models, and the literature is open to improvement. The subject of this study is S-Quad, a soft and miniature quadruped with c-shaped legs. To analyze the effect of the compliance of its body and legs to its locomotion, a soft-body dynamic model has been developed. The development process starts with the rigid-body dynamic analysis, which will be a base for the soft-body dynamic analysis and used to examine the rigid body - rigid legs (RBRL) version of the robot. The Newton-Euler method has been used to develop this model, in which the contact forces are estimated with the viscoelasticity theory. Scenarios of different model parameters were simulated to estimate the motion of the robot. With the obtained results the effects of the model parameters were discussed, and then appropriate parameters have been selected. The contact analysis of a curved leg inside of the robot dynamics, in which any intersection algorithm is not used, is an advantageous aspect of this study. The compliance of the legs has been incorporated into this dynamic analysis with a non-linear viscoelastic model. The dependency of the leg compliance to the roll angle of the leg was derived from Castigliano’s theorem. Then, the motion of the rigid body - soft leg (RBSL) robot was estimated accordingly. The comparison of the RBRL and the RBSL results was utilized to interpret the effect of the leg compliance. The modeling and the integration with robot dynamics of this compliant leg model is a novel aspect of this model. An offline Finite Element Analysis has been conducted to estimate the deflections of the soft body under the contact forces, which were exported from the RBRL simulation. The estimated deflections were put back into this simulation to obtain the simulation of soft body - rigid legs (SBRL) robot. Thus, the capability of the developed model to include body deflections has been proven. Finally, the RBRL and the RBSL robot simulations have been verified with the experiments conducted with a motion capture system. These experimental results were also used to interpret the advantages and disadvantages of using a soft body in the robot.
Open Access
Viscoelastic effects in lubricated contacts in the presence of cavitation
(2020-11) Gamaniel, Samuel Shari
A model is proposed to study the influence of fluid viscoelasticity on the performance of lubricated contacts in the presence of cavitation. Previous studies on viscoelastic lubricants did not consider the presence of cavitation, rather reported negative pressures in regions where cavitation was expected to occur. The proposed model uses the Oldroyd-B constitutive model to describe the presence of cavitation and assumes that the Deborah number (De), the ratio between polymer relaxation time and flow time scale, is small. In doing so, the viscoelastic thin film equations can be linearised in a similar approach to what was pioneered by ”Tichy, J., 1996, Non-Newtonian lubrication with the convected Maxwell model.” The zeroth order solution in De corresponds to the Reynolds equation and has been modified to describe also the film cavitation through the mass-conserving Elrod-Adams model. We model several bearing configurations for the flow of viscoelastic lubricants using (i) a cosine/parabolic profile representing a journal bearing unwrapped geometry, and (ii) a pocketed profile to model a textured surface in lubricated contacts. Introducing viscoelasticity to the cavitating journal bearing decreases the length of the non-active (cavitation) region due to an increasing pressure distribution in the lubricant film. This results in an increase to the load carrying capacity with increasing De corroborating the beneficial influence of the polymers in fluid film bearings. The pocket profile is shown to either increase or decrease the load carrying capacity with increasing viscoelastic effects, depending on the location of surface texturing at the contact. An oscillating squeeze flow problem is modeled for viscoelastic lubricants between two flat plates with motion only at the top surface. A reduction in the load carrying capacity at larger values of De is observed as film reformation is seen to be retarded with increasing viscoelastic effects. As viscoelastic effects become stronger, the nonactive region is grows continuously until reaching a value of De beyond which a full film reformation does not occur upon the inception of cavitation. The study is extended to a direct numerical simulations using the openFoam toolbox. A model that couples a solver for incompressible, isothermal, two phase flow with interaction between the phases and a solver for viscoelastic fluids is proposed. However, DNS are only valid for lower values of De as instabilities occur as a result of the non-linear coupling.