Viscoelastic Processes Research Articles

SimHydraulics approach to water hammer problems with the development of new formulas for static pressure diameter coefficient

Water hammer analysis and prediction is of overriding importance for operation safety of hydraulic systems. Therefore, the verification of computer codes capability to model and simulate water hammer is a very interesting topic for the analysis of different fluid-filled pipeline systems. Until now, transient approach of SimHydraulics is not developed, and the validity of this approach against benchmark examples, has never been investigated in scientific literature. In this paper, a numerical model of a reservoir-pipelines-valve system is created using Matlab SimHydraulics software. The main component of this model is the Segmented Pipeline with wall compliance parameters: the viscoelastic process time constant τ and the static pressure diameter coefficient K P for which new formulas were derived and validated. The numerical model is performed to solve transient flow problems caused by closure of a valve and the obtained results are presented and compared with those using the method of characteristic. Also, the effectiveness of the transient model of SimHydraulics is evaluated by comparing the obtained numerical results with the most pertinent experimental results found in the literature. The proposed transient model was adapted for the investigation of more complex piping systems. The results of this study show the ability of the SimHydraulics model to predict the pressure oscillation behavior accurately in all cases considered. This study provides useful information for future research in pump transient simulation using SimHydraulics.

Dynamic Electromechanical Response of a Viscoelastic Dielectric Elastomer under Cycle Electric Loads

Dielectric elastomer (DE) is able to produce large electromechanical deformation which is time-dependent due to the viscoelasticity. In the current study, a thermodynamic model is set up to characterize the influence of viscoelasticity on the electromechanical and dynamic response of a viscoelastic DE. The time-dependent dynamic deformation, the hysteresis, and the dynamic stability undergoing viscoelastic dissipative processes are investigated. The results show that the electromechanical stability has strong frequency dependence; the viscoelastic DE can attain a larger stretch in the dynamic response than the quasistatic actuation. Furthermore, with the decreasing frequency of the applied electric load, the viscoelastic DE system will present dynamic stability evolution from an aperiodic motion to the quasiperiodic motion. The DE system may also experience a stability evolution from a single cycle motion to multicycle motion with the increasing relaxation times. The value and variation trend of the amplitude of the stretch are highly dependent on the excitation frequency and the relaxation time.

EVALUATING FRONTAL AUTOMOTIVE BUMPER SYSTEM TO IMPROVE SAFETY

Automotive Frontal bumper systems address two goals. First, is to lessen the inward intrusion during crash scenario and the second is to guarantee pedestrian safety. A good bumper design would try to fulfill these two goals while maintaining its weight as low as feasible to improve energy consumption. In this paper, we try to add to these two functions, a new condition, that is the ability to maintain the minimum possible lasting plastic damage during low speed crashes. This would develop both the crashworthiness of the bumper and the pedestrian safety. We address this condition by assuring that the bumper deformations stay close to the elastic zone of the material. Due to the limited range of the material elastic deformations of the bumper, even at low speeds, we try to complement it with a visco-elastic process. This is achieved by alternating the brackets connecting the bumper to the car structure elements with a spring-dashpot system that provides the desired visco-elastic response, allowing the bumper beam to deform nearly elastically. Finite element analysis is used in the study of the spring-dashpot system parameters to reach the optimum configuration that ensure minimization of plastic deformations in the bumper structure at low speeds of 5 miles/hour crash. The consequence of varying the thickness of the bumper beam is also investigated in this regard. A basic parameterized finite element model of the Ford Crown Victoria bumper form is used in several crash simulations carried out with the explicit dynamics system LS-DYNA3D to test the validity of this bumper system.

A three-dimensional thermal model for viscoelastic polymer melt packing process in injection molding

In this study, a 3D weakly compressible viscoelastic flow model with the thermal effect is developed for simulations of the non-isothermal XPP melt packing process. The 3D multi-field coupled model is solved by the collocated finite volume CLEAR algorithm with the AVLSMART scheme. The numerical model is validated against the benchmark problem. The XPP melt packing process in a rectangular cavity is simulated, and the predicted flow-induced birefringence is in agreement with the experimental and numerical results reported in the literature. For a case with a rather complex cavity, the XPP melt packing process in a 3D hemispherical shell cavity is further simulated. The distributions of melt temperatures, flow-induced stresses as well as the corresponding rheological characteristics of XPP melt in 3D cavity are vividly presented, which are difficult to be captured by previous simulations and experiments. Especially, the present model allows the effects of melt temperature and holding pressure on the first normal stress difference and density to be studied. It is shown that with an increase in melt temperature, the absolute values of the first normal stress difference increases slightly and the density decreases gradually, while the high holding pressure improves significantly the first normal stress difference and density. Numerical results demonstrate that the present 3D thermal model is an effective tool for simulating real-world viscoelastic polymer melt packing processes.

Numerical Simulation on Creep Process of Surrounding Rock in Soft Rock Tunnel

It is obvious that creep is one of important characteristics of surrounding rock in soft rock tunnel. There was less study on creep process of surrounding rock in the past which affected the engineer's judgment to long term stability of tunnel seriously. In this paper, the Cvsic model is selected and its parameters are determined based on soft rock creep test results, furthermore, the software FLAC3D was used to analyze the viscoelastic plastic creep process of surrounding rock in soft rock roadway. The results show that: the maximum creep rate occurs at the moment when the soft rock tunnel is excavated , then the creep rate decreases and finally evolves into steady creep; it will initially form a creep zone in the tunnel surface and then the deep surrounding rock will also form a damage zone by stress adjustment, forming a larger creep area; hoop stress decreases while the radial stress increases gradually from the surface to the deep rock mass of roadway, and eventually both will approach to the initial in-situ stress ; The plastic zone will gradually expand after the excavation of viscoelastic-plastic tunnel and tend to be stable when reaches to the steady creep stage. The hoop stress on the border between elastic and plastic zone of the surrounding rock reaches to peak stress; the deformation of surrounding rock changes suddenly and the deformation rate, the area of plastic zone increase with the increase of moisture content in the process of creep.

Influence of Nanodisperse Metal Fillers on the Viscoelastic Properties and Processes of Mechanical Relaxation of Polymer Systems

The results of research into the viscoelastic properties and processes of mechanical relaxation of polyvinylchloride (PVC) containing Cu nanoparticles obtained by means of electroerosion crushing and electrohydraulic destruction of agglomerates of disperse Cu in the presence of an ultrasonic field are presented. It is shown that, in the case of longitudinal shear deformation at a frequency of 0.4 × 106 s–1 over a wide range of temperatures and content of ingredients, viscoelastic phenomena depending on structural changes in the PVC system occur. An analysis of quantitative results of the elastic and viscoelastic deformation of a body is carried out taking into account the energy and entropy components of interaction of the polymer and filler at their interface.

Numerical Models for Viscoelastic Liquid Atomization Spray

Atomization spray of non-Newtonian liquid plays a pivotal role in various engineering applications, especially for the energy utilization. To operate spray systems efficiently and well understand the effects of liquid rheological properties on the whole spray process, a comprehensive model using Euler-Lagrangian approaches was established to simulate the evolution of the atomization spray for viscoelastic liquid. Based on the Oldroyd model, the viscoelastic linear dispersion relation was introduced into the primary atomization; an extended viscoelastic version of Taylor analogy breakup (TAB) model was proposed; and the coalescence criteria was modified by rheological parameters, such as the relaxation time, the retardation time and the zero shear viscosity. The predicted results are validated with experimental data varying air-liquid mass flow ratio (ALR). Then, numerical calculations are conducted to investigate the characteristics of viscoelastic liquid atomization process. Results showed that the evolutionary trend of droplet mean diameter, Weber number and Ohnesorge number of viscoelastic liquids along with axial direction were qualitatively similar to that of Newtonian liquid. However, the mean size of polymer solution increased more gently than that of water at the downstream of the spray, which was beneficial to stable control of the desirable size in the applications. As concerned the effects of liquid physical properties, the surface tension played an important role in the primary atomization, which indicated the benefit of selecting the solvents with lower surface tension for finer atomization effects, while, for the evolution of atomization spray, larger relaxation time and zero shear viscosity increased droplet Sauter mean diameter (SMD) significantly. The zero shear viscosity was effective throughout the jet region, while the effect of relaxation time became weaken at the downstream of the spray field.

Innovative viscoelastic material selection strategy based on dma and mini-shaker tests for spacecraft applications

With the increase of payload sensitivity (such as high precision optics for sub-metric imager), micro-vibration disturbances generated by spinning actuators, if not controlled, may affect on-board instruments and may worsen the quality of pictures taken by an Earth observation imager. For the last two decades, viscoelastic materials have been gradually used in isolators designed for space applications. Their attractiveness comes from their ability to act as a second order low pass filter to minimise micro-vibration forces. In this study, an innovative viscoelastic material pre-selection process has been developed to assess the mechanical and thermal properties of viscoelastic isolators during early design stages. In order to characterise the viscoelastic isolators, tests have been performed at viscoelastic material level (material characterisation) and at viscoelastic isolator level (isolator characterisation). A qualitative correlation has been established between the master curves (material characterisation) and the transmissibility curves (isolator characterisation) which leads to a possible prediction of expected isolation performances of a viscoelastic material during early design stages.

Microscopic flow simulation of viscoelastic fluid based on interface tracking method

Microscopic residual oil in reservoirs after water flooding can be classified into four types: oil films, oil in the dead end, oil in pores throat, and oil clusters. The oil is trapped in pores throat when the viscous force is not sufficiently large to overcome the capillary forces with water flooding. The oil in the dead end is constrained by the rock configuration. The oil clusters mainly exist in the lower permeability portion of the porous media, which is constrained by the capillary force and rock configuration. In particular, the efficiency of water flooding is extremely low at the high water cut period. Recently, viscoelastic fluid flooding (polymer flooding) was widely applied to improve the recovery after water flooding in the petroleum industry, and the displacement efficiency was greatly improved. However, the poor understanding of viscoelastic fluid flooding restricts its further progress. The investigation of how the viscoelastic fluid mobilizes the residual oil is warranted. In this study, a direct numerical simulation method is employed to simulate immiscible two-phase flows in a micro channel. Viscoelastic effects are simulated using the Oldroyd-B rheological model. The position of the interface between two immiscible fluids is determined by using the phase field method. The model incorporates the wetting condition of the porous media. The dynamics of oil film and residual oil in the dead end are explored under the displacement of water and viscoelastic fluid. The forces exerting on the oil film in the micro channel are analyzed, which contribute to the difference of oil film movement and deformation between the water flooding process and viscoelastic fluid process. Moreover, the sweep efficiencies of water and viscoelastic fluid in the dead end are observed and compared. The concept of dimensionless velocity is introduced as the sweep boundary in the dead end to explain the discrepancy of recovery between water and viscoelastic fluid. The numerical solution used for the simulation is performed using a finite element method. Additionally, a benchmark of the flow of Oldroyd-B fluid past a cylinder between two parallel plates is given to validate the correction of the model and solution. Compared with water, viscoelastic fluid can more easily mobilize the residual oil which can greatly improve the displacement efficiency. The viscosity is not the only one mechanism of the viscoelastic fluid’s mobilization of the oil film; ‘elasticity’ of the fluid is another key driving mechanism which is reflected by the Weissenberg number. The horizontal stress difference of viscoelastic fluid acting on the residual oil film is considerably larger than that of water. The uneven distribution of normal stress caused by the viscoelastic fluid leads the residual oil film to deform significantly. Viscoelastic fluid can greatly improve the sweep efficiency in the dead end and this phenomenon is more obvious in the water-wet reservoir. The sweep boundary of viscoelastic fluid in the dead end is deeper than that of water which explain the reason why less residual oil trapped in the dead end with viscoelastic fluid flooding. The study provides effective guidance for tertiary oil recovery.

Ligament Mediated Fragmentation of Viscoelastic Liquids.

The breakup and atomization of complex fluids can be markedly different than the analogous processes in a simple Newtonian fluid. Atomization of paint, combustion of fuels containing antimisting agents, as well as physiological processes such as sneezing are common examples in which the atomized liquid contains synthetic or biological macromolecules that result in viscoelastic fluid characteristics. Here, we investigate the ligament-mediated fragmentation dynamics of viscoelastic fluids in three different canonical flows. The size distributions measured in each viscoelastic fragmentation process show a systematic broadening from the Newtonian solvent. In each case, the droplet sizes are well described by Gamma distributions which correspond to a fragmentation-coalescence scenario. We use a prototypical axial step strain experiment together with high-speed video imaging to show that this broadening results from the pronounced change in the corrugated shape of viscoelastic ligaments as they separate from the liquid core. These corrugations saturate in amplitude and the measured distributions for viscoelastic liquids in each process are given by a universal probability density function, corresponding to a Gamma distribution with n_{min}=4. The breadth of this size distribution for viscoelastic filaments is shown to be constrained by a geometrical limit which can not be exceeded in ligament-mediated fragmentation phenomena.

Transient deformation in the Asal‐Ghoubbet Rift (Djibouti) since the 1978 diking event: Is deformation controlled by magma supply rates?

AbstractThe Asal‐Ghoubbet Rift (AG Rift) in Djibouti lies in the subaerial continuation of the Aden ridge system, thereby constituting a unique location to study rifting processes and mechanisms involved in continental breakup and oceanic spreading. Continually upgraded and expanded geodetic technology has been used to record the 1978 Asal rifting event and postdiking deformation. In light of recent results obtained for the Manda Hararo‐Dabbahu rifting event (2005–2010), we propose that the horizontal and vertical geodetic data can be modeled with a double source, involving a dike‐like inflation component aligned along the rift axis and a spherical pressure source located at midsegment below the Fieale caldera. By revisiting the codiking data, we propose that the reservoir below Fieale could have fed, at least partially, the 1978 injection and the contemporaneous Ardoukôba eruption and potentially induced local subsidence due to magma draining out of the central reservoir. As an alternative to previously proposed viscoelastic relaxation models, we reinterpret postdiking observations using a purely elastic rheology. We determine the relative contribution of a midsegment reservoir inflation and a dike‐like opening component, together with their respective time evolutions. Our results suggest that interactions between steadily accumulating tectonic strain and temporal variations in melt supply to the shallow magma plumbing system below the AG Rift may entirely explain the geodetic observations and that viscoelastic deformation processes played a minor role in the 30 years following the 1978 rifting event.

Mapping Viscoelastic and Plastic Properties of Polymers and Polymer-Nanotube Composites using Instrumented Indentation.

An instrumented indentation method is developed for generating maps of time-dependent viscoelastic and time-independent plastic properties of polymeric materials. The method is based on a pyramidal indentation model consisting of two quadratic viscoelastic Kelvin-like elements and a quadratic plastic element in series. Closed-form solutions for indentation displacement under constant load and constant loading-rate are developed and used to determine and validate material properties. Model parameters are determined by point measurements on common monolithic polymers. Mapping is demonstrated on an epoxy-ceramic interface and on two composite materials consisting of epoxy matrices containing multi-wall carbon nanotubes. A fast viscoelastic deformation process in the epoxy was unaffected by the inclusion of the nanotubes, whereas a slow viscoelastic process was significantly impeded, as was the plastic deformation. Mapping revealed considerable spatial heterogeneity in the slow viscoelastic and plastic responses in the composites, particularly in the material with a greater fraction of nanotubes.

Effect of Mechanical Constraint on Tearing Energy of Polymer Membranes

The role of mechanical constraint on the tearing behavior of perfluorosulfonic acid and polyethylene membranes is examined. A constrained tearing test is employed to measure the tearing energy when the formation of a plastic zone ahead of the tear tip is constrained. It is shown that the tearing energy decreases with increasing constraint associated with a smaller plastic zone and is much lower than the tearing energy with no constraint. Furthermore, it is demonstrated that the tearing rate has a strong effect on the tearing energy when the membrane is unconstrained, but a much smaller effect when the membrane is constrained as the viscoelastic relaxation processes related to the plastic zone are suppressed. The results highlight the significant role of mechanical constraint on the tearing behavior of membranes during both testing and in application. image

Viscoelastic granular dampers under low-amplitude vibration

This paper considers dampers comprising collections of viscoelastic particles that are subjected to vibrations whose amplitude is such that slip between particles is negligible. Energy dissipation occurs primarily by viscoelastic processes within each particle and is maximized when standing waves are set up in the granular medium. In this work, the medium is represented as an equivalent viscoelastic solid and predictions of performance employ models constructed using standard finite element software. Two numerical approaches are considered: one uses the Direct Frequency Response and the other uses standard modal analysis in conjunction with analytical expressions for energy dissipation based on the wave equation. The performance of these prediction techniques is compared with measured behavior from experiments on a box-shaped structure and a hollow composite tube assembly. The computational efficiency of the modal technique allowed a brief investigation of the effects of uncertainties in the actual nature of the granular arrangement. Results show that both prediction methods give a reasonable level of accuracy. Differences between predicted and measured behavior are shown to be of the same order as the uncertainty in the prediction itself. For the systems considered, it is shown that the methods are appropriate for acceleration amplitudes up to almost that of gravity.

Evidence of mechano-sorptive effect during moisture adsorption process under hygrothermal conditions: Characterized by static and dynamic loadings

The viscoelasticity of Chinese fir (Cunninghamia lanceolata [Lamb.] Hook.) was examined during moisture adsorption at a relative humidity step (RHstep) mode (made up of RHramp and RHisohume periods) from 0 to 90% RH and temperatures ranging from 30 to 80°C. The viscoelastic tests were carried out by static and dynamic loadings. The moisture adsorption resulted in an increase in compliance or damping. The phenomena that more pronounced mechano-sorptive (MS) effect during RHramp periods than RHisohume periods were observed, indicating that the MS effect could be detected by both static and dynamic loading tests. At 70 or 80°C dynamic loading tests, the lignin transition was observed. Before the occurrence of the lignin transition, or when temperature at or below 60°C, a smooth master curve of E″ vs. E′ was formed and indicated that during the adsorption process the temperature not only accelerated the viscoelastic processes, but also increased their intensity.

Determination of draw resonance onsets in tension-controlled viscoelastic spinning process using transient frequency response method

Draw resonance onsets of actual viscoelastic spinning processes with secondary forces operated by constant draw ratio are determined via transient frequency response simulations under a constant tension or force boundary condition, which causes the system to be always stable. Transfer function data between the spinline velocity and the tension at the take-up position in the frequency domain, which are transformed from the transient responses of the take-up velocity with respect to a step change of the take-up force, play a key role in the determination of the onset points for various spinning cases. It is confirmed that the critical draw ratios established in this study are almost the same as those decided by linear stability or direct transient simulation methods for velocity-controlled spinning systems. The transfer function data for the stability analysis can be readily and beneficially applied to an analysis of sensitivity of a spinning process with respect to a sinusoidal disturbance.

Rheological benchmark of silicone oils used for analog modeling of short- and long-term lithospheric deformation

Analog models of tectonic processes at various scales commonly use silicone polymers to simulate viscous flow in the lower crust and mantle. To achieve dynamic similarity with the natural prototype and to improve comparability between analog models, better knowledge of the rheology of commonly used silicones is required. In this study, we present a rheological benchmark of silicones used in various laboratories. Rheometric tests, including rotational and oscillatory tests, were performed and the viscoelastic behavior of silicone is quantitatively described. We found that silicone oils show a transition from Newtonian viscous to power-law, shear thinning behavior around shear rates of 10−2 to 10−1s−1. The viscosity of chemically similar silicones varied between 2 and 3×104Pas. Maxwell relaxation times are about 0.1–0.2s. Such a behavior is able to mimic slow to fast deformation mechanisms in the ductile regime, such as diffusion and dislocation creep as well as viscoelastic relaxation processes. Temperature and aging effects are verified, but can be considered minor with respect to the uncertainty in rheological properties in the natural prototype. Nevertheless, to assure comparability between models and proper scaling the exact properties and conditions should be reported.

Viscoelastic relaxation in fluids

The shear elasticity of different fluids are studied experimentally at relatively low frequencies of 40 and 74 kHz. The real and imaginary shear moduli of several liquids are measured using the acoustic resonance method. A low-frequency viscoelastic relaxation process is assumed to occur in a fluid with a period of relaxation much longer than the lifetime of the individual fluid particles.

Numerical simulation of flow-induced stresses in mold filling process using a three-dimensional two-phase flow model

The flow-induced stresses which arise during the filling stage have significant influences on the mechanical and optical properties of injection-molded products. The prediction of flow-induced stresses in mold-filling process is a challenging problem, which includes viscoelastic polymer melt free surface flow. Thus, this paper presents a three-dimensional (3D) non-isothermal two-phase flow model to quantitative predict the flow-induced stresses in polymer mold-filling process based on the XPP model. In this model, the governing equations for gas and viscoelastic melt are united into a system namely the generalized Navier–Stokes equations. In order to improve the stability of the numerical method in 3D, an enhanced treatment of stress solid boundaries is proposed. The stresses on solid boundaries are calculated using an effective second-order Runge–Kutta method. The model is successfully solved by using the finite volume method, and the melt front is accurately captured by level set method. The validity of the method is verified by two benchmark problems. Then the mold-filling process of a rectangular plate is studied numerically, obtaining the evolutions of flow-induced stresses at different levels along the gap-wise direction, which can not be simulated by 2.5D model. The predicted flow-induced birefringence is in agreement with the experimental result reported in the literature. Furthermore, the effects of injection velocity and some model parameters of XPP model on flow-induced stresses are numerically predicted. The numerical results show that the 3D simulation technology can effectively predict the flow-induced stresses and birefringence in non-isothermal viscoelastic polymer mold-filling process. The results can provide the theory foundation for improving the properties of products in actual production processes.

Adhesion and debonding kinetics of photovoltaic encapsulation in moist environments

Debonding of photovoltaic (PV) encapsulation in moist environments is frequently reported but presently not well understood or quantified. Temperature cycling, moisture, and mechanical loads often cause loss of encapsulation adhesion and interfacial debonding, initially facilitating back-reflectance and reduced electrical current, but ultimately leading to internal corrosion and loss of module functionality. To investigate the effects of temperature (T) and relative humidity (RH) on the kinetics of encapsulation debonding, we developed a mechanics-based technique to measure encapsulation debond energy and debond growth rates in a chamber of controlled environment. The debond energy decreased from 2.15 to 1.75 kJ m−2 in poly(ethylene-co-vinyl acetate) (EVA) and from 0.67 to 0.52 kJ m−2 in polyvinyl butyral when T increased from 25 to 50°C and 20 to 40°C, respectively. The debond growth rates of EVA increased up to 1000-fold with small increases of T (10°C) and RH (15%). To elucidate the mechanisms of environmental debonding, we developed a fracture-kinetics model, where the viscoelastic relaxation processes at the debonding-tip are used to predict debond growth. The model and techniques constitute the fundamental basis for developing accelerated aging tests and long-term reliability predictions for PV encapsulation. Copyright © 2015 John Wiley & Sons, Ltd.