scholarly journals Thixotropic Behavior of Reconstituted Debris-Flow Mixture

Water ◽  
2021 ◽  
Vol 13 (2) ◽  
pp. 153
Author(s):  
Leonardo Schippa ◽  
Ferruccio Doghieri ◽  
Anna Maria Pellegrino ◽  
Elisa Pavesi
Keyword(s):  
Steady State ◽  
Shear Rate ◽  
Yield Stress ◽  
Initial Conditions ◽  
Sediment Concentration ◽  
Time Dependent ◽  
Initial State ◽  
Creep Tests ◽  
Lower Shear ◽  
Short Period

Time-dependent rheological properties and thixotropy of reconstituted debris-flows samples taken from channel bank deposits are examined using a commercial rheometer equipped with a vane rotor geometric system. Sweep tests and creep tests were carried out involving mixtures having different grain concentrations ranging between 50% and 58%. Different initial conditions of the mixtures were considered in order to analyze the effects of aging and rejuvenation (thixotropy) over a short period of time and long period of time. Tested slurries show viscosity bifurcation, yield stress and time-dependent behavior. According to the experimental results, three different regimes were identified: a lower shear rate regime, corresponding to a shear rate lower than the critical value; an intermediate banding shear rate regime characterized by static and dynamic yield stress level; and a higher shear rate regime where the flowing debris behaves as a non-Newtonian fluid characterized by a constant steady state ultimate apparent viscosity. In any case, the initial state of the mixture and the sediment concentration affects the ultimate steady state rheology and the time-dependent (thixotropy) slurries’ behavior.

Time-dependent shear rate inhomogeneities and shear bands in a thixotropic yield-stress fluid under transient shear

Soft Matter ◽  
2019 ◽  
Vol 15 (39) ◽  
pp. 7956-7967 ◽  
Author(s):  
Yufei Wei ◽  
Michael J. Solomon ◽  
Ronald G. Larson
Keyword(s):  
Shear Rate ◽  
Yield Stress ◽  
Fumed Silica ◽  
Shear Banding ◽  
Shear Bands ◽  
Flow Reversal ◽  
Time Dependent ◽  
Yield Stress Fluid ◽  
Silica Suspension

We study the rheological responses and shear-rate inhomogeneities and shear banding behaviors of a thixotropic fumed silica suspension in shear startup tests and flow reversal tests.

Time-dependent process of M/G/1 vacation models with exhaustive service

1992 ◽  
Vol 29 (02) ◽  
pp. 418-429 ◽  
Author(s):  
Hideaki Takagi
Keyword(s):  
Steady State ◽  
Initial Conditions ◽  
Setup Time ◽  
Time Dependent ◽  
Time Model ◽  
Vacation Models ◽  
Single Vacation ◽  
Multiple Vacation ◽  
Virtual Waiting Time ◽  
Vacation Model

Generalized M/G/1 vacation systems with exhaustive service include multiple and single vacation models and a setup time model possibly combined with an N-policy. In these models with given initial conditions, the time-dependent joint distribution of the server's state, the queue size, and the remaining vacation or service time is known (Takagi (1990)). In this paper, capitalizing on the above results, we obtain the Laplace transforms (with respect to time) for the distributions of the virtual waiting time, the unfinished work (backlog), and the depletion time. The steady-state limits of those transforms are also derived. An erroneous expression for the steady-state distribution of the depletion time in a multiple vacation model given by Keilson and Ramaswamy (1988) is corrected.

Time-dependent process of M/G/1 vacation models with exhaustive service

1992 ◽  
Vol 29 (2) ◽  
pp. 418-429 ◽  
Author(s):  
Hideaki Takagi
Keyword(s):  
Steady State ◽  
Initial Conditions ◽  
Setup Time ◽  
Time Dependent ◽  
Time Model ◽  
Vacation Models ◽  
Single Vacation ◽  
Multiple Vacation ◽  
Virtual Waiting Time ◽  
Vacation Model

Generalized M/G/1 vacation systems with exhaustive service include multiple and single vacation models and a setup time model possibly combined with an N-policy. In these models with given initial conditions, the time-dependent joint distribution of the server's state, the queue size, and the remaining vacation or service time is known (Takagi (1990)). In this paper, capitalizing on the above results, we obtain the Laplace transforms (with respect to time) for the distributions of the virtual waiting time, the unfinished work (backlog), and the depletion time. The steady-state limits of those transforms are also derived. An erroneous expression for the steady-state distribution of the depletion time in a multiple vacation model given by Keilson and Ramaswamy (1988) is corrected.

scholarly journals Correction: Time-dependent shear rate inhomogeneities and shear bands in a thixotropic yield-stress fluid under transient shear

Soft Matter ◽  
2020 ◽  
Vol 16 (40) ◽  
pp. 9393-9393
Author(s):  
Yufei Wei ◽  
Michael J. Solomon ◽  
Ronald G. Larson
Keyword(s):  
Shear Rate ◽  
Yield Stress ◽  
Soft Matter ◽  
Shear Bands ◽  
Time Dependent ◽  
Yield Stress Fluid

Correction for ‘Time-dependent shear rate inhomogeneities and shear bands in a thixotropic yield-stress fluid under transient shear’ by Yufei Wei et al., Soft Matter, 2019, 15, 7956–7967, DOI: 10.1039/C9SM00902G.

scholarly journals Stability and solution of the time-dependent Bondi–Parker flow

2020 ◽  
Vol 493 (2) ◽  
pp. 2834-2840
Author(s):  
Eric Keto
Keyword(s):  
Steady State ◽  
Transonic Flow ◽  
Initial Conditions ◽  
Steady State Solution ◽  
State Solution ◽  
Time Dependent ◽  
Hamiltonian Description ◽  
Bernoulli’S Equation ◽  
Wide Range ◽  
Isothermal And Adiabatic Flows

ABSTRACT Bondi and Parker derived a steady-state solution for Bernoulli’s equation in spherical symmetry around a point mass for two cases, respectively, an inward accretion flow and an outward wind. Left unanswered were the stability of the steady-state solution, the solution itself of time-dependent flows, whether the time-dependent flows would evolve to the steady state, and under what conditions a transonic flow would develop. In a Hamiltonian description, we find that the steady-state solution is equivalent to the Lagrangian implying that time-dependent flows evolve to the steady state. We find that the second variation is definite in sign for isothermal and adiabatic flows, implying at least linear stability. We solve the partial differential equation for the time-dependent flow as an initial-value problem and find that a transonic flow develops under a wide range of realistic initial conditions. We present some examples of time-dependent solutions.

New Description of Steady-State Creep Rate, Yield Stress, Stress Relaxation and Their Interrelation

2005 ◽  
Vol 482 ◽  
pp. 319-322
Author(s):  
Jan Kohout
Keyword(s):  
Activation Energy ◽  
Steady State ◽  
Creep Rate ◽  
Yield Stress ◽  
Applied Stress ◽  
Stress Sensitivity ◽  
Steady State Creep ◽  
Steady State Creep Rate ◽  
Creep Tests ◽  
Sensitivity Parameter

The steady-state creep rate increases with temperature according to the Arrhenius equation and its increase with applied stress is mostly described by the power law. Combining both these laws, equation ) exp( RT Qa n − µ s e& is obtained, where apparent activation energy a Q and stress sensitivity parameter n are considered to be constants. But most measurements show some dependence of activation energy on applied stress and of stress sensitivity parameter on temperature. An equation respecting these facts is derived in the paper and verified using some published results of creep tests. Based on this derived equation, the dependence of yield stress on temperature and strain rate and an equation describing the relaxation curves are deduced.

Calculation of Complex Viscosities from Slow-Speed Transient Torque Measurements with the Mooney Rheometer

1982 ◽  
Vol 55 (5) ◽  
pp. 1426-1436 ◽  
Author(s):  
N. Nakajima ◽  
E. R. Harrell
Keyword(s):  
Steady State ◽  
Shear Rate ◽  
Practical Interest ◽  
Actual Process ◽  
Steady State Flow ◽  
Shear Rates ◽  
State Flow ◽  
Lower Shear ◽  
Rate Limit ◽  
Low Shear

Abstract It has been customary to assign certain time scales to given polymer processes. For example, the extrusion process is said to occur at a shear rate in the order of some hundred reciprocal seconds and injection molding at some thousand reciprocal seconds or higher. These statements are usually accompanied by instructions that the viscosity of a material is to be measured at the respective shear rate in order to characterize its processability. However, the above argument is only partially valid, and a single-point viscosity measurement is only a part of the processability evaluation. Inadequacy of the above rationale has been recognized by industry for a long time. With the rapid growth of plastics production in the 1960's, plastic processing went through a technological evolution. In the early stage of evolution of various fabrication techniques, development of suitable grades of material for the respective processes was the major effort of the plastic producers. Soon it became clear that resins which had the same viscosity at the so-called processing shear rate often behaved differently in the actual process. This led to the measurement of the steady-state flow properties at lower shear rates than the so-called processing shear rate, which was representative of the highest shear rate involved in the process. The significant observation was that the viscosity differences of resins often were magnified at the lower shear rate. Sometimes, a subtle difference in processability corresponded to a viscosity difference observable only at very low shear rates. Thus, acquisition of the steady-state flow curve from the low-shear-rate limit (i.e., the Newtonian viscosity) to the high shear rate limit (i.e., the limiting power-law region) became a subject of practical interest. The characterization of such flow curves and their relation to molecular weight distribution (MWD) became a subject of intense study for commercial plastics having a large variation in MWD.

scholarly journals Preparing to move: Setting initial conditions to simplify interactions with complex objects

2021 ◽  
Vol 17 (12) ◽  
pp. e1009597
Author(s):  
Rashida Nayeem ◽  
Salah Bazzi ◽  
Mohsen Sadeghi ◽  
Neville Hogan ◽  
Dagmar Sternad
Keyword(s):  
Steady State ◽  
Force Feedback ◽  
Inverse Dynamics ◽  
Chaotic Behavior ◽  
Initial Conditions ◽  
Human Subjects ◽  
Virtual Object ◽  
Complex Objects ◽  
Initial State ◽  
Alternative Hypotheses

Humans dexterously interact with a variety of objects, including those with complex internal dynamics. Even in the simple action of carrying a cup of coffee, the hand not only applies a force to the cup, but also indirectly to the liquid, which elicits complex reaction forces back on the hand. Due to underactuation and nonlinearity, the object’s dynamic response to an action sensitively depends on its initial state and can display unpredictable, even chaotic behavior. With the overarching hypothesis that subjects strive for predictable object-hand interactions, this study examined how subjects explored and prepared the dynamics of an object for subsequent execution of the target task. We specifically hypothesized that subjects find initial conditions that shorten the transients prior to reaching a stable and predictable steady state. Reaching a predictable steady state is desirable as it may reduce the need for online error corrections and facilitate feed forward control. Alternative hypotheses were that subjects seek to reduce effort, increase smoothness, and reduce risk of failure. Motivated by the task of ‘carrying a cup of coffee’, a simplified cup-and-ball model was implemented in a virtual environment. Human subjects interacted with this virtual object via a robotic manipulandum that provided force feedback. Subjects were encouraged to first explore and prepare the cup-and-ball before initiating a rhythmic movement at a specified frequency between two targets without losing the ball. Consistent with the hypotheses, subjects increased the predictability of interaction forces between hand and object and converged to a set of initial conditions followed by significantly decreased transients. The three alternative hypotheses were not supported. Surprisingly, the subjects’ strategy was more effortful and less smooth, unlike the observed behavior in simple reaching movements. Inverse dynamics of the cup-and-ball system and forward simulations with an impedance controller successfully described subjects’ behavior. The initial conditions chosen by the subjects in the experiment matched those that produced the most predictable interactions in simulation. These results present first support for the hypothesis that humans prepare the object to minimize transients and increase stability and, overall, the predictability of hand-object interactions.

Photoinduced Charge Transfer Dynamics in Carotenoid-Porphyrin-C60 Triad via the Linearized Semiclassical Nonequilibrium Fermi's Golden Rule

2020 ◽  
Author(s):  
Zhengqing Tong ◽  
Margaret S. Cheung ◽  
Barry D. Dunietz ◽  
Eitan Geva ◽  
Xiang Sun
Keyword(s):  
Charge Transfer ◽  
Golden Rule ◽  
Time Dependent ◽  
Rate Coefficients ◽  
Initial State ◽  
Photoinduced Charge Transfer ◽  
Transfer Rates ◽  
Fermi's Golden Rule ◽  
Photoinduced Charge ◽  
Fermi’S Golden Rule

The nonequilibrium Fermi’s golden rule (NE-FGR) describes the time-dependent rate coefficient for electronic transitions, when the nuclear degrees of freedom start out in a <i>nonequilibrium</i> state. In this letter, the linearized semiclassical (LSC) approximation of the NE-FGR is used to calculate the photoinduced charge transfer rates in the carotenoid-porphyrin-C<sub>60</sub> molecular triad dissolved in explicit tetrahydrofuran. The initial nonequilibrium state corresponds to impulsive photoexcitation from the equilibrated ground-state to the ππ* state, and the porphyrin-to-C<sub>60</sub> and the carotenoid-to-C<sub>60</sub> charge transfer rates are calculated. Our results show that accounting for the nonequilibrium nature of the initial state significantly enhances the transition rate of the porphyrin-to-C<sub>60</sub> CT process. We also derive the instantaneous Marcus theory (IMT) from LSC NE-FGR, which casts the CT rate coefficients in terms of a Marcus-like expression, with explicitly time-dependent reorganization energy and reaction free energy. IMT is found to reproduce the CT rates in the system under consideration remarkably well.

scholarly journals Effect of an oscillating time-dependent pressure gradient on Dean flow: transient solution

2020 ◽  
Vol 9 (1) ◽  
Author(s):  
Basant K. Jha ◽  
Dauda Gambo
Keyword(s):  
Pressure Gradient ◽  
Initial Conditions ◽  
Fluid Velocity ◽  
Outer Cylinder ◽  
Time Dependent ◽  
Navier Stokes ◽  
Dean Flow ◽  
Continuity Equations ◽  
Riemann Sum ◽  
Flow Formation

Abstract Background Navier-Stokes and continuity equations are utilized to simulate fully developed laminar Dean flow with an oscillating time-dependent pressure gradient. These equations are solved analytically with the appropriate boundary and initial conditions in terms of Laplace domain and inverted to time domain using a numerical inversion technique known as Riemann-Sum Approximation (RSA). The flow is assumed to be triggered by the applied circumferential pressure gradient (azimuthal pressure gradient) and the oscillating time-dependent pressure gradient. The influence of the various flow parameters on the flow formation are depicted graphically. Comparisons with previously established result has been made as a limit case when the frequency of the oscillation is taken as 0 (ω = 0). Results It was revealed that maintaining the frequency of oscillation, the velocity and skin frictions can be made increasing functions of time. An increasing frequency of the oscillating time-dependent pressure gradient and relatively a small amount of time is desirable for a decreasing velocity and skin frictions. The fluid vorticity decreases with further distance towards the outer cylinder as time passes. Conclusion Findings confirm that increasing the frequency of oscillation weakens the fluid velocity and the drag on both walls of the cylinders.

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