Rheological Stresses Research Articles

Arrhenius constitutive equation and artificial neural network model of flow stress in hot deformation of offshore steel with high strength and toughness

ABSTRACT In this study, the thermal deformation behaviour of a high strength offshore steel at different temperatures and rates was investigated through thermal compression experiments.An Arrhenius constitutive model and a back propagation artificial neural network (BP-ANN) were established to address more complex deformation characteristics. The performances of both models were was evaluated using standard statistical parameters such as the correlation coefficient (R) and average absolute relative error (AARE). The results showed that both models can accurately predict the rheological stresses generated during deformation.The BP-ANN outperforms the Arrhenius equation model with correlation coefficients of fit greater than 99.9% and less than 0.8% relative error. At a strain rate of 0.01 s−1 and 10 s−1, the accuracy of the ANN decreases slightly due to the fact that it exceeds the strain rate range of the training set, as compared to the Arrhenius constitutive equations as these are more accurately predicted.

Role of interfacial rheology on fingering instabilities in lifting Hele-Shaw flows.

The lifting Hele-Shaw cell setup is a popular modification of the classic, fixed-gap, radial viscous fingering problem. In the lifting cell configuration, the upper cell plate is lifted such that a more viscous inner fluid is invaded by an inward-moving outer fluid. As the fluid-fluid interface contracts, one observes the rising of distinctive patterns in which penetrating fingers having rounded tips compete among themselves, reaching different lengths. Despite the scholarly and practical relevance of this confined lifting flow problem, the impact of interfacial rheology effects on its pattern-forming dynamics has been overlooked. Authors of recent studies on the traditional injection-induced radial Hele-Shaw flow and its centrifugally driven variant have shown that, if the fluid-fluid interface is structured (i.e., laden with surfactants, particles, proteins, or other surface-active entities), surface rheological stresses start to act, influencing the development of the viscous fingering patterns. In this paper, we investigate how interfacial rheology affects the stability as well as the shape of the emerging fingered structures in lifting Hele-Shaw flows, at linear and early nonlinear dynamic stages. We tackle the problem by utilizing the Boussinesq-Scriven model to describe the interface and by employing a perturbative mode-coupling scheme. Our linear stability results show that interfacial rheology effects destabilize the interface. Furthermore, our second-order findings indicate that interfacial rheology significantly alters intrinsically nonlinear morphological features of the shrinking interface, inducing the formation of narrow sharp-tip penetrating fingers and favoring enhanced competition among them.

Modeling the Rheological Behavior of a Novel Hot Isostatic Pressed Powder Metallurgy Superalloy

The rheological behavior and deformation mechanisms of a new powder metallurgy (P/M) superalloy at various deformation conditions are researched. The deformation conditions have significant influence on the rheological stress. Increasing the deformation temperature or decreasing the strain rate can decrease the rheological stress. The discontinuous hardening and softening phenomena are observed at the strain rate of 1 s−1, resulting from the complex phase transformation and dynamic recrystallization. Besides, the deformation activity energy (Q) declines with increasing the strain. The phenomenon is attributed to the spheroidization of γ′ phase and the decreased content/aspect ratio of γ′ phase. The deformation mechanisms of the researched superalloy are the accumulation of dislocation, stacking faults shearing, dislocations pinned by γ′ phase, and the formation of microtwins during hot deformation. The strain‐compensated Arrhenius and particle swarm optimization‐based backpropagation artificial neural network (PSO‐BP ANN) models are established to predict the rheological behavior. Compared to the strain‐compensated Arrhenius equation, the developed PSO‐BP ANN model presents the higher accuracy in predicting the rheological behavior of the researched alloy. Furthermore, for the developed PSO‐BP ANN model, the correlation coefficient is 0.9995, and the root mean square error is 1.224 MPa. So, the forecasted rheological stresses are consistent with the measured ones.

Effect of interfacial rheology on fingering patterns in rotating Hele-Shaw cells.

In rotating Hele-Shaw flows, centrifugal force acts, and the interface separating two viscous fluids becomes unstable, driven by the density difference between them. Complex interfacial structures develop where fingers of various shapes and sizes grow, and compete. These patterns have been well studied over the last few decades, analytically, numerically, and experimentally. However, one feature of the pattern-forming dynamics of much current interest has been underappreciated: the role of surface rheological stresses in the deformation, and time evolution of the fluid-fluid interface. In this paper, we employ a perturbative, second-order mode-coupling analysis to investigate how interfacial rheology effects influence centrifugally driven fingering phenomena, beyond the scope of linear stability theory. Describing the viscous Newtonian interface by using a Boussinesq-Scriven model, we derive a nonlinear differential equationthat governs the early linear, and nonlinear time evolution of the system. In this framing, the most prevalent dynamical features of the patterns are described in terms of two dimensionless parameters: the viscosity contrast A (dimensionless viscosity difference between the fluids), and the Boussinesq number Bq which involves a ratio between interfacial and bulk viscosities. At the linear level, our results show that for a given A, surface rheological stresses dictated by Bq have a stabilizing role. Nevertheless, our weakly nonlinear findings reveal a more elaborate scenario in which the shape of the fingers, and their finger competition behavior result from the coupled influence of A and Bq. It is found that, although finger competition phenomena depend on the specific values of A and Bq, the fingers tend to widen as Bq is increased, regardless of the value of A.

Impact of interfacial rheology on finger tip splitting.

Fluid-fluid interfaces, laden with polymers, surfactants, lipid bilayers, proteins, solid particles, or other surface-active agents, often exhibit a rheologically complex response to deformations. Despite its academic and practical relevance to fluid dynamics and various other fields of research, the role of interfacial rheology in viscous fingering remains fairly underexplored. A noteworthy exception is the work by Li and Manikantan [Phys. Rev. Fluids 6, 074001 (2021)2469-990X10.1103/PhysRevFluids.6.074001], who used linear stability analysis to show that surface rheological stresses act to stabilize the development of radial viscous fingering at the linear regime. In this paper, we perform a perturbative, second-order mode-coupling analysis of the system and investigate the influence of interfacial rheology on the morphology of the fingering structures at early nonlinear stages of the dynamics. In particular, we focus on understanding how interfacial rheology impacts the emblematic finger tip-widening and finger tip-splitting phenomena that take place in radial viscous fingering in Hele-Shaw cells. We describe the viscous Newtonian fluid-fluid interface by using a Boussinesq-Scriven model, and derive a generalized Young-Laplace pressure jump condition at the fluid-fluid interface. In this framing, we go beyond the purely linear description and use Darcy's law to obtain a perturbative mode-coupling differential equationwhich describes the time evolution of the perturbation amplitudes, accurate to second order. Our early nonlinear mode-coupling results indicate that regardless of their stabilizing action at the linear regime, interfacial rheology effects favor finger tip widening, leading to the occurrence of enhanced finger tip-splitting events.

Influence of interfacial rheology on viscous fingering

Complex surfactant-laden interfaces exhibit surface rheological stresses when deforming against themselves, which in turn affects the flow and stability of the adjacent bulk fluids. We investigate, for the first time, the impact of interfacial rheology on the Saffman-Taylor or viscous fingering problem and demonstrate the stabilizing role of surface viscosity. We show that surface viscosity slows the growth of unstable protrusions, resulting in thicker fingers. We use these insights to highlight the quantitative changes that occur when a typical surface-viscous surfactant is present in a multiphase fluid displacement problem.

Compression rheological behavior of ultrahighly filled wood flour-polyethylene composites

Ultrahighly filled wood-plactic composites (UF-WPC, filling content exceeds 70 wt%) have high additional value and superior environmental properties due to the substantial reduction in plastic consumption. Studying the flow mode and mechanism of UF-WPC melt in the production process is critical for improving production efficiency and optimizing production equipment. In this paper, the rheological properties of ultrahighly filled wood flour-polyethylene composites (UF-WFPE) under continuous deformation were investigated using a compression rheology method. The plug flow model was introduced to analyse the rheological behavior of UF-WFPE, and was modified for UF-WFPE melt. According to the poor fluidity and strong solid-like characteristics of UF-WFPE, a self-made compression rheometer was prepared to conduct the compression rheology testing. The results indicated that the unmodified plug flow model did not apply to the continuous structural changes of UF-WFPE during compression. During continuous compression, the specimens with WF content of 70 wt%, 75 wt%, and 80 wt% displayed a window period of structural stability before the edge fracture, which indicated the stable rheological behavior. However, as the WF content increased to 90 wt%, the edges of the melt entered the continuous fracture stage after the compaction stage, and no stable rheological behavior period was observed. Based on the modified plug flow model, the rheological stress formula with WF as variable was obtained by fitting rheological stresses of different systems in the steady flow stage, and the reliability of the formula was verified. This study provides an effective method for the rheological analysing of ultrahighly filled wood-plastic composites and lays a theoretical foundation for its practical production and application.

Effects of Surface Viscosity on Breakup of Viscous Threads.

In addition to surface tension lowering and Marangoni stresses, surfactants also induce surface rheological effects when they deform against themselves at fluid interfaces. Because surface viscosities are functions of surfactant concentration, surface rheological stresses can compete with capillary, Marangoni, and bulk stresses in surfactant-laden free surface flows with breakup. To elucidate the effects of surface rheology, we examine the breakup of a Stokes thread covered with a monolayer of insoluble surfactant when either surfactants are convected away from the space-time singularity or diffusion is dominant. Surprisingly, in both limits, surface rheological effects always enter the dominant balance of forces and alter the thread's thinning rate. Moreover, if surfactants are convected away from the singularity, we provide an analytical expression for thinning rate that explicitly depends on surface rheological parameters, providing a simple route for measuring surface viscosity.

Research on the Concrete Mixture Stability and Sliding on the Inclined Plane

The stability of conventional concrete mixture was investigated using an inclined plane method. The experiment consisted of two steps: first, the research was made to determine the influence of coarse aggregate content in aggregates mixture on the stability and sliding of concrete mixture, when the specimens of mixture are on the inclined plane without additional roughening of surface; second, the research was made to determine the influence of inclined plane surface roughening on the stability and sliding of concrete mixture. During the research the condition was checked: the concrete mixture on the sloping plane will be stable, if the yield stress τ0 is higher than the shear stress τ in. (τ 0 ≥ τ). The shear stresses and rheological property yield stresses of conventional concrete mixtures were calculated analytically by using an empirical formula. Test results showed that the increase of coarse aggregate (4/16 fraction gravel) content from about 417 to 1175 kg in concrete mixture is enough to achieve the stability of concrete mixture, when plane inclination angles are 25°, 35° and 45°, but not enough to stop sliding process. In this case, additional implements are needed to increase the adhesion of fresh concrete to the base. By using the inclined planes, which were coated with a special dimpled membrane and geotextile, the fresh concrete does not slide downwards by inclined plane.

Numerical Investigation of Turbulence Modulation by Sediment-Induced Stratification and Enhanced Viscosity in Oscillatory Flows

Recent turbulence-resolving simulations of fine sediment transport in the oscillatory bottom boundary layer (OBBL) revealed the existence of a diverse range of flow regimes over muddy seabeds. Transitions between these flow regimes are caused by different degrees of sediment-induced stable density stratification in the OBBL. These transitions have critical implications for the role of wave resuspension in the delivery of fine sediment and hydrodynamic dissipation over muddy seabeds. This study further investigates the effect of Newtonian rheology, parameterized as a concentration-dependent effective viscosity, on turbulence modulation and the transition from turbulent to laminar states. Assuming small particle Stokes number, the equilibrium approximation is adopted to simplify the Eulerian two-phase flow governing equations. The resulting simplified equations are solved with a high-accuracy hybrid scheme in an idealized OBBL. A sixth-order centered compact finite difference is implemented in the vertical direction to solve the governing equations with a flow-dependent viscosity while the pseudospectral method is retained for two horizontal directions. At Stokes Reynolds number RΔ=1,000, simulations reveal that when rheology is incorporated, the enhanced effective viscosity can further attenuate the flow turbulence in addition to the well-known sediment-induced stable density stratification. Through the enhanced viscosity, the velocity gradient very near the bed is significantly reduced, which leads to much weaker turbulent production and the onset of laminarization. Although the sediment-induced stable density stratification is known to cause laminarization of bottom boundary layer flows, our preliminary finding that the enhanced viscosity via rheological stresses encourages the flow laminarization may be useful in explaining the field-observed large wave dissipation rate over muddy seabeds during the waning stage of a storm.

On the dynamics of wave‐mud interaction: A numerical study

Many previous studies consider wave attenuation over muddy seabeds and bottom boundary layer fluid‐mud transport as two distinct research topics. Hence, various processes related to the physics of wave‐mud interaction, such as turbulence‐sediment interactions, rheological stresses, and nonlinear wave‐wave interactions are incorporated rather artificially. The aim of this work is to present a new modeling approach which allows for the resolution of nonlinear wave propagation and bottom boundary layer mud transport with a single set of governing equations and closures. By adopting a fluid‐mud modeling framework, a well‐validated depth/phase‐resolving wave propagation model, based on the Reynolds‐Averaged Navier‐Stokes (RANS) equations, is extended to model cohesive sediment transport. The numerical model consists of a set of governing equations based on the equilibrium Eulerian approach accurate for the fine sediment limit. The numerical model reduces to the clear fluid RANS equations when the sediment concentration approaches zero. Hence, the model is able to calculate continuously and consistently the nonlinear wave propagation, wave boundary layer processes, and fluid‐mud transport without the need to prescribe the mud layer characteristics. Numerical simulations reveal several important physical processes that are critical for understanding the water‐wave dynamics over muddy seabeds: (i) an enhancement of the wave boundary layer thickness due to the presence of the fluid‐mud and rheological stress, which leads to a scaling relation between the enhanced wave boundary‐layer and the fluid‐mud layer and (ii) a direct wave amplitude dissipation due to rheological effects and clear evidences of low‐ and high‐frequency wave attenuation via nonlinear energy transfer.

High‐resolution numerical modeling of wave‐supported gravity‐driven mudflows

Wave‐supported gravity‐driven mudflow has been identified as a major offshore fine sediment transport mechanism of terrestrial sediment into the coastal ocean. This transport process essentially occurs within the wave boundary layer. In this study, wave‐supported gravity‐driven mudflow is investigated via a wave‐phase‐resolving high‐resolution numerical model for fluid mud transport. The model results are verified with field observation of sediment concentration and near‐bed flow velocities at Po prodelta. The characteristics of wave‐supported gravity‐driven mudflows are diagnosed by varying the bed erodibility, floc properties (fractal dimension), and rheological stresses in the numerical simulations. Model results for moderate concentration suggest that using an appropriately specified fractal dimension, the dynamics of wave‐supported gravity‐driven mudflow can be predicted without explicitly incorporating rheological stress. However, incorporating rheological stress makes the results less sensitive to prescribed fractal dimension. For high‐concentration conditions, it is necessary to incorporate rheological stress in order to match observed intensity of downslope gravity‐driven current. Model results are further analyzed to evaluate and calibrate simple parameterizations. Analysis suggests that when neglecting rheological stress, the drag coefficient decreases with increasing wave intensity and seems to follow a power law. However, when rheological stress is incorporated, the resulting drag coefficient is more or less constant (around 0.0013) for different wave intensities. Model results further suggest the bulk Richardson number has a magnitude smaller than 0.25 and is essentially determined by the amount of available soft mud (i.e., the erodibility), suggesting a supply limited condition for unconsolidated mud.

Effectiveness of nonequilibrium closures for the many body forces in concentrated colloidal dispersions

Here we present a theory for predicting the effect of interparticle interactions on the nonequilibrium dynamics of concentrated colloidal dispersions. A configuration-space conservation equation for the pair density P2 provides a fundamental basis for calculating the nonequilibrium microstructure; however, it includes pairwise additive three-body couplings. The resulting forces depend on the three-particle distribution function, necessitating an additional equation to completely specify P2. In this paper nonequilibrium Percus–Yevick and hypernetted chain closures complete the formulation by relating these forces to the interparticle force and pair distribution function. A computational algorithm exploiting Fast Fourier Transforms solves the resulting integro-differential equations for weak perturbations from equilibrium, yielding the perturbed pair density as a function of the volume fraction φ and the interparticle potential. The advantage of a fundamental approach is that clearly defined approximations lead from the characteristics of the individual colloidal particles to the nonequilibrium structure and macroscopic properties. The calculation of all dynamic properties, both rheological stresses and diffusion coefficients, is accomplished with the same approximations. Detailed predictions of the structure provide an additional comparison with simulation and experiment lacking in theories that calculate only bulk properties. The numerical methods demonstrated here allow efficient solution of a class of models more sophisticated than previously attempted. To test the merits of nonequilibrium closures we present predictions of the low-shear viscosity and long-time self-diffusion coefficient as a function of volume fraction for various repulsive potentials without hydrodynamic interactions. Comparison with results available from computer simulations demonstrates that the closures capture the trends in the transport properties with volume fraction and interparticle potential and yield realistic predictions for the nonequilibrium structure. The hypernetted chain closure yields the best agreement with the available data for bulk properties at moderate volume fractions (φ<0.4), but significant quantitative deviations appear at φ⩾0.45.

Rheological stresses and stress relaxation at underground excavations in yielding rock (In German) : Borm, G FelsbauV5, N4, Nov 1987, P175–181

Rheological stresses and stress relaxation at underground excavations in yielding rock (In German) : Borm, G FelsbauV5, N4, Nov 1987, P175–181

Intensification of rheological destruction of rat fibrosarcoma KMT 17 cells by elimination of divalent cations.

In an effort to find a method to intensify the rheological destruction of tumor cells, rat fibrosarcoma KMT 17 cells in ascitic form were exposed to rheological stresses in test solutions; modified Eagle medium with EDTA, and phosphate buffer solution without divalent cations or with verapamil. KMT 17 cells were exposed to a uniform shear stress produced by the rotation of a rotating cone plate viscometer for 1 to 2 hours and to the strong deformation by the passage through Nuclepores of 10, 8 and 5 micrometer. KMT 17 cells suspended in the test solutions were more effectively destroyed by the stresses than those suspended in normal solutions containing divalent cations without any other calcium-suppressing agents. These results suggest that the elimination of divalent cations and/or the block of calcium ion channels of cell membranes intensify the rheological destruction of tumor cells migrating in the circulatory system.

De la rhéologie des systèmes aqueux à base de gommes

AbstractOn the Rheology of Aqueous Systems on the Basis of Gums. The water „solutions”︁ of macromolecular carbohydrates (arabic gum, alginate, guaran, starches) often show a non‐Newtonian behaviour. Therefore it is impossible to give only one viscosity value for the definition of the rheological behaviour; viscosity changes with the different rheological conditions. But it is possible to speak of an „apparent viscosity”︁ for a given shear rate Di or stress ri. To characterize the rheological behaviour it is necessary to establish the relationship between shear rate and shear stress. Sometimes, for limited decades of shear rates, the behaviour may be described by a power law = τ = K Da: sometimes the Cross relation \documentclass{article}\pagestyle{empty}\begin{document}$ \overline \eta = \eta _\infty + \frac{{\eta _0 - \eta _\infty}}{{1 + \alpha D^{2/3}}} $\end{document} has to be applied; but it may also occur, that one can only give the curve log τ = f (log D). To obtain absolute values it is necessary to apply a correcting factor at the actual values given by Newtonian approximation. Thus it is possible to study the influence of temperature, concentration and ageing on the rheological parameters K, a, η or the graph log τ = f (log D).From the results it seems that the behaviour of this kind of systems is rather like the behaviour of emulsions, than like that of true solutions.Perhaps that is the reason why, the process for the preparation of these systems has to be well defined and rigorously followed in order to obtain reproducible results.To give a sense to rheological studies in the laboratory, it is important to simulate the exact industrial conditions of preparations and of the rheological stresses of these systems. This calls for an exact knowledge of the industrial rheological stress; up to now this knowledge is very rare in most cases.