Transient Shear Stress Research Articles

APPLICATION OF LASER-INDUCED BREAKDOWN CAVITATION BUBBLES FOR CELL LYSIS IN VITRO

Objective: Understanding the basic mechanism of the cavitation bubble action on living cells as a crucial step of development and application ofsophisticated methods based on controlled cavitation in cell behaviour manipulation. Optimisation of parameters in order to expand cell lysis regioncreated by a single bubble.Methods: The cavitation bubbles are generated by the laser-induced breakdown method. The impact of controlled cavitation bubble on thebiological system is synchronously monitored under a microscope and recorded. Visualization of the cavitation bubble course is monitored by a highspeedcamera. The impact of technology on the healthy confluent cell layer is verified. Evaluation of the cavitation bubbles´ effect on cells in real timeand by subsequent analysis of the cell lysis region and impact of the cavitation bubble on cell viability is carried out by optical visualization and life/dead fluorescence staining.Results: Cavitation bubble induced in distance of 1.5 mm from the cell surface overcomes properties of sessile bubble and enables to create cell lysisregion over 1000 μm in diameter due to transient shear stress produced by liquid displaced by the bubble expansion.Conclusion: Cell lysis region is strongly dependent on the spot laser energy (SLE) and the bubble induction distance from cells. This knowledge iscrucial for application in chemical free cell lysis in vitro, wound induction for experimental purposes and cell layers patterning in desired scale.

High-rate dislocation motion in stable nanocrystalline metals

Abstract

A numerical modeling of a profile velocity and Shear stress in transient flow applied in industrial technology

This research is devoted to theoretical and numerical models of transient shear stress in a transient laminar flow over the pipe wall. This work is an elementary method of the model Prado et al, which the base is on the expansion of velocity profiles ofthe flow in polynomial series of a radial space at acroass the right section of pipe. The set equations is derived from mass and momentum conservation laws. The system partial derivatives equations obtained is hyperbolique and suitable by the method of characteristics. This model is approved with the experimental results of Holmboe et al, and, Vardy et al. This digital code can be possible to join it in any tools, to simulateand to controle water hammer in pipe.

Thirty minutes of handgrip exercise potentiates flow-mediated dilatation in response to sustained and transient shear stress stimuli to a similar extent.

What is the central question of this study? This study sought to determine whether enhancement of brachial artery flow-mediated dilatation (FMD) after acute exposure to a sustained elevation in shear stress is greater when the shear stress stimulus for FMD is also sustained. What is the main finding and its importance? Brachial artery FMD in response to a sustained (handgrip exercise) and transient (reactive hyperaemia) shear stress stimulus was enhanced to a similar extent 10min after a 30min handgrip exercise intervention. This suggests that prior exposure to a sustained elevation in shear stress results in a similar acute augmentation of the ability of the endothelium to transduce sustained and transient shear stress stimuli. Brief (30min) exposure of the brachial artery (BA) to a sustained elevation in shear stress has been shown to potentiate subsequent BA flow-mediated dilatation (FMD) in response to a transient shear stress stimulus [reactive hyperaemia (RH) FMD]. It is unknown whether matching the sustained shear stress exposure to a subsequent sustained shear stress stimulus for FMD [via handgrip exercise (SS-FMD)] might enhance the potentiation of FMD. The purpose of the study, therefore, was to assess the impact of a 30min handgrip exercise intervention-induced elevation in shear stress on subsequent BA SS-FMD versus RH-FMD. Nineteen healthy men (22±3years) preformed a 30min rhythmic handgrip exercise intervention on two experimental days. BA-FMD was assessed using either an RH or a 6min sustained shear stress stimulus created via handgrip exercise (order of visits counterbalanced) at three time points: pre-intervention and 10 and 60min post-intervention. The FMD was assessed using duplex ultrasound. Shear stress was estimated as shear rate (SR=BA blood velocity/BA diameter). Data are mean±SD. Both SS and RH-FMD increased from pre-intervention to 10min post-intervention [SS-FMD (6min average), from 0.11±0.05 to 0.16±0.08mm; P=0.008; Cohen's d = 0.66; and RH-FMD, from 0.25±0.1 to 0.32±0.11mm; P=0.013; Cohen's d = 0.68]. The magnitude of enhancement in RH and SS-FMD did not differ (change in RH versus SS-FMD pre- versus 10min post-intervention, P=0.344). These findings suggest that exposure to elevated shear stress via 30min of handgrip exercise potentiates subsequent FMD in response to sustained and transient elevations in shear stress to a similar extent.

Numerical Simulation of the Effect of Transient Shear Stress and Rate of Heat Transfer Around Different Positions of Sphere in the Presence of Viscous Dissipation

The present study is devoted to the problem of oscillatory convective flow in the presence of viscous dissipation around different positions of a sphere. The system of differential equations governing the flow phenomenon is transformed into dimensionless form by using suitable group of variables and then transformed into convenient form for integration by using primitive variable formulation. Numerical simulation based on finite difference method is carried out to analyze the mixed convection flow mechanism. Special focus is given on the transient shear stress and the rate of heat transfer characteristics and their dependency on various dimensionless parameters that is mixed convection parameter λ, Prandtl number Pr, dissipation parameter N, and angular frequency parameter ω. The angles X=30deg,90 deg, and 360deg are the favorable positions around the sphere for different parameters, where the transient rate of shear stress and heat transfer is noted maximum. Later, the obtained results are presented graphically by using Tech Plot-360 and compared with the previous work given in the literature.

Experimental investigation on effect of alkali, salts on the rheological properties of polyacrylamide and mixed assembly of polymer-surfactant

Abstract An experimental study was conducted to evaluate the effect of addition of alkali, salts and surfactants on the rheological properties of polyacrylamide (PAM) solution by rheometry. The results show the addition of inorganic salts (NaCl, Na2SO4, MgCl2), alkali (NaOH) and surfactants (CTAB, Gemini-5 and Gemini-6) strongly influence the rheological behavior of polyacrylamide solution. The shear thinning behavior of the solution at low shear rate was observed. The scope of this experimental work also includes the measurements of viscoelastic behavior, transient shear stress response and yield stress. The results are quite interesting and the mixed systems can be used as ASP flooding in Enhance oil recovery.

Crack-like grain-boundary sliding wedges in nano-films

ABSTRACTTime-dependent grain-boundary (GB) sliding in a nano-film on a substrate is studied. The Gurtin–Murdoch surface elasticity is incorporated onto the GB by ignoring the residual surface tension, and the GB is allowed to slide by the diffusion-controlled mechanism. The sliding viscosity can be homogeneous or heterogeneous along the GB. By using the Green's function method, the original boundary value problem is reduced to a Cauchy singular integro-differential equation of the first order which can be further changed into state-space equations by means of an adapted collocation method. The state-space equations can be solved through a rigorous eigenmode analysis. We demonstrate that such a sliding process leads to the formation of a crack-like GB wedge which causes the shear stress along the GB to decay exponentially with time. Furthermore, the stress field exhibits the weak logarithmic singularity at the tip of the sliding wedge due to the incorporation of surface elasticity. Detailed numerical results are obtained for eigenvalues, eigenfunctions, the transient shear stress along the GB, the opening displacement of the wedge and the strength of the logarithmic singularity at the wedge tip.

Flow-mediated dilation stimulated by sustained increases in shear stress: a useful tool for assessing endothelial function in humans?

Investigations of human conduit artery endothelial function via flow-mediated vasodilation (FMD) have largely been restricted to the reactive hyperemia (RH) technique, wherein a transient increase in shear stress after the release of limb occlusion stimulates upstream conduit artery vasodilation (RH-FMD). FMD can also be assessed in response to sustained increases in shear stress [sustained stimulus (SS)-FMD], most often created with limb heating or exercise. Exercise in particular creates a physiologically relevant stimulus because shear stress increases, and FMD occurs, during typical day-to-day activity. Several studies have identified that various conditions and acute interventions have a disparate impact on RH-FMD versus SS-FMD, sometimes with only the latter demonstrating impairment. Indeed, evidence suggests that transient (RH) and sustained (SS) shear stress stimuli may be transduced via different signaling pathways, and, as such, SS-FMD and RH-FMD appear to offer unique insights regarding endothelial function. The present review describes the techniques used to assess SS-FMD and summarizes the evidence regarding 1) SS-FMD as an index of endothelial function in humans, highlighting comparisons with RH-FMD, and 2) potential differences in shear stress transduction and vasodilator production stimulated by transient versus sustained shear stress stimuli. The evidence suggests that SS-FMD is a useful tool to assess endothelial function and that further research is required to characterize the mechanisms involved and its association with long-term cardiovascular outcomes. NEW & NOTEWORTHY Sustained increases in peripheral conduit artery shear stress, created via distal skin heating or exercise, provide a physiologically relevant stimulus for flow-mediated dilation (FMD). Sustained stimulus FMD and FMD stimulated by transient, reactive hyperemia-induced increases in shear stress provide distinct assessments of conduit artery endothelial function.

Transient wall shear stress measurements and estimates at high Reynolds numbers

Transient wall shear stress measurements using hot-film anemometry have been performed in a large-scale laboratory setup at high Reynolds numbers. Starting from Reynolds numbers 1.7×106 and 0.7×106, the flow was brought to a complete rest by closing a knife gate thus replicating a pressure-time (also know as Gibson) flow rate measurement in a hydropower plant. Ensemble-averaged mean wall shear stresses obtained from 22 repeated runs have been compared with estimates obtained using the pressure-time method. The objective of the work has been to assess the accuracy of the frictional formulation entering the pressure-time integral. It is shown that both the standard method, a quasi-steady approach as well as the recently introduced unsteady method all reproduce the measured wall shear stresses quantitatively during most of the transient. The last phase, following the complete closure of the gate, which is characterized by a slow decay towards zero shear stress at the wall is, however, not captured by the available methods. In general, the unsteady formulation produces the smallest flow rate estimation error, which in turn, implies the best modeling of the frictional losses.

Modelling hydraulic jump using the bubbly two-phase flow method

Hydraulic jumps have complex flow structures, characterised by strong turbulence and large air contents. It is difficult to numerically predict the flows. It is necessary to bolster the existing computer models to emphasise the gas phase in hydraulic jumps, and avoid the pitfall of treating the phenomenon as a single-phase water flow. This paper aims to improve predictions of hydraulic jumps as bubbly two-phase flow. We allow for airflow above the free surface and air mass entrained across it. We use the Reynolds-averaged Navier–Stokes equations to describe fluid motion, the volume of fluid method to track the interface, and the k–e model for turbulence closure. A shear layer is shown to form between the bottom jet flow and the upper recirculation flow. The key to success in predicting the jet flow lies in formulating appropriate bottom boundary conditions. The majority of entrained air bubbles are advected downstream through the shear layer. Predictions of the recirculation region’s length and air volume fraction within the layer are validated by available measurements. The predictions show a linear growth of the shear layer. There is strong turbulence at the impingement, and the bulk of the turbulence kinetic energy is advected to the recirculation region via the shear layer. The predicted bottom-shear-stress distribution, with a peak value upstream of the toe of the jump and a decaying trend downstream, is realistic. This paper reveals a significant transient bottom shear stress associated with temporal fluctuations of mainly flow velocity in the jump. The prediction method discussed is useful for modelling hydraulic jumps and advancing the understanding of the complex flow phenomenon.

Time-frequency analyses of fluid–solid interaction under sinusoidal translational shear deformation of the viscoelastic rat cerebrum

During normal extracellular fluid (ECF) flow in the brain glymphatic system or during pathological flow induced by trauma resulting from impacts and blast waves, ECF–solid matter interactions result from sinusoidal shear waves in the brain and cranial arterial tissue, both heterogeneous biological tissues with high fluid content. The flow in the glymphatic system is known to be forced by pulsations of the cranial arteries at about 1 Hz. The experimental shear stress response to sinusoidal translational shear deformation at 1 Hz and 25% strain amplitude and either 0% or 33% compression is compared for rat cerebrum and bovine aortic tissue. Time-frequency analyses aim to correlate the shear stress signal frequency components over time with the behavior of brain tissue constituents to identify the physical source of the shear nonlinear viscoelastic response. Discrete fast Fourier transformation analysis and the novel application to the shear stress signal of harmonic wavelet decomposition both show significant 1 Hz and 3 Hz components. The 3 Hz component in brain tissue, whose magnitude is much larger than in aortic tissue, may result from interstitial fluid induced drag forces. The harmonic wavelet decomposition locates 3 Hz harmonics whose magnitudes decrease on subsequent cycles perhaps because of bond breaking that results in easier fluid movement. Both tissues exhibit transient shear stress softening similar to the Mullins effect in rubber. The form of a new mathematical model for the drag force produced by ECF–solid matter interactions captures the third harmonic seen experimentally.

Cellulose nanofibril‐reinforced polypropylene composites for material extrusion: Rheological properties

Polypropylene (PP) is not typically utilized in 3D printing material extrusion because PP shrinks and warps during the printing process. Cellulose nanofibrils (CNF) have the potential to make PP 3D printer processable and also enhance mechanical properties of PP printed parts. The rheological behavior of CNF‐PP composites during material extrusion requires study because it is different from injection molding and compression molding processes. This study revealed the effects of CNF contents (3 and 10 wt%) and maleic anhydride polypropylene (MAPP) coupling agent on the rheological properties of CNF–PP composites. Morphological analysis showed that CNF agglomerated during spray drying and a spherical structure was formed. Rheological tests showed that the elastic modulus, complex viscosity, viscosity, and transient flow shear stress of PP were increased by the addition of 10 wt% CNF, while the creep strain of PP was reduced. The damping factor and stress relaxation time remained the same when 10 wt% CNF was added to the PP. Incorporation of MAPP into the CNF–PP composites impacted the rheological properties of the CNF–PP composites. Flexural strength and modulus of PP were improved by 5.9% and 26.8% by adding 10 wt% CNF compared to the control. POLYM. ENG. SCI., 2017. © 2017 Society of Plastics Engineers

Numerical determination of shear stress relaxation modulus of polymer glasses.

Focusing on simulated polymer glasses well below the glass transition, we confirm the validity and the efficiency of the recently proposed simple-average expression [Formula: see text] for the computational determination of the shear stress relaxation modulus G(t). Here, [Formula: see text] characterizes the affine shear transformation of the system at t = 0 and h(t) the mean-square displacement of the instantaneous shear stress as a function of time t. This relation is seen to be particulary useful for systems with quenched or sluggish transient shear stresses which necessarily arise below the glass transition. The commonly accepted relation [Formula: see text] using the shear stress auto-correlation function c(t) becomes incorrect in this limit.

In Situ Measurements of Protein Forces and Intracellular Ca2+ under Fluid Shear Stimuli

During traumatic brain injury, brain cells experience transient shear stresses that alter the forces in structural proteins. An increase in intracellular Ca2+ is seen universally in cells subjected to shear forces, but how are mechanical forces coupled to the Ca2+ influx is not well understood. We used a microfluidic chamber driven with a high-speed pressure servo to generate controlled fluid shear stress to cultured astrocytes, and simultaneously measured the intracellular responses using FRET-based force sensors actinin-cpstFRET and Ca2+ probes jRCaMP1h. We found that fluid shear generated non-uniform stresses in actinin. The time dependent force distribution is highly sensitive to the feature of the stimulus such as amplitude, duration rise time, and the frequencies of stimulation. In cells with weaker stress fibers, a rapid shear pulse (23 dyn/cm2, 2 ms rise time, 400 ms duration) produced an immediate and long-lasting increase in actinin stress at the upstream end of the cell and minimal changes at the downstream end. In contrast, a slow ramp to the same amplitude caused a minimal, and a more uniform increase in actinin stresses. Simultaneous Ca2+ imaging showed that the initial Ca2+ rise began at the upstream end of the cell where there was high strain, and it propagated to the entire cell within ∼4 s. Moreover, the Ca2+ peaked much faster (∼4 s) at the front end and slower in the center of cell body (∼10 s). This behavior occurred in ∼40% cells. In cells with abundant strong actin bundles, there was minimal response to shear stress. The actinin stress increased transiently and uniformly over the entire cell during the period of stimulation, and the Ca2+ rise appeared starting from the cell body. These results suggest that the force distribution directly alter the initial Ca2+ dynamics in different cellular domains. This work was funded by NINDS.

On the quantification and visualization of transient periodic instabilities in pulsatile flows

Turbulent-like flows without cycle-to-cycle variations are more frequently being reported in studies of cardiovascular flows. The associated stimuli might be of mechanobiological relevance, but how to quantify them objectively is not obvious. Classical Reynolds decomposition, where the flow is separated into mean and fluctuating velocity components, is not applicable as the phase-average is zero. We therefore expanded on established techniques and present the idea, analogous to Reynolds decomposition, to decompose a flow with transient instabilities into low- versus high frequency components, respectively, to discriminate flow instabilities from the underlying cardiac pulsatility. Transient wall shear stress and velocity signals derived from computational fluid dynamic simulations were transferred to the frequency domain. A high-pass filter was applied to subtract the 99% most-energy-containing frequencies, which gave a cut-off frequency of 25Hz. We introduce here the spectral power index, and compute the fluctuating kinetic energy, based on the high-pass filtered velocity components, both being frequency-based operators. The efficacy was evaluated in an aneurysm model for multiple flow rates demonstrating transition to turbulent-like flows. The frequency-based operators were found to better correlate with the qualitatively observed flow instabilities compared to conventional descriptors, like time-averaged wall shear stress or oscillatory shear index. We demonstrate how the high frequencies beyond the physiological range could be analyzed and/or transferred back to the time domain for quantification and visualization purposes. We have introduced general frequency-based operators, easily extendable to other cardiovascular territories based on a posteriori heuristic filtering that allows for separation, isolation, and quantification of cycle-invariant turbulent-like flows.

Mixed Convection Flow Along a Horizontal Circular Cylinder with Small Amplitude Oscillation in Surface Temperature and Free Stream

The problem is considered on mixed convection flow due to the effect of small amplitude oscillations of a viscous incompressible fluid along a horizontal circular cylinder. Direct implicit finite-difference scheme is employed to solve the dimensionless system of partial differential equations. In case of steady flow, the solutions are presented as functions of the curvature parameter X on the entire surface of the cylinder and there is a visual comparison with the existing result. For fluctuating flow, considering Prandtl number, Pr=1.0, the results are shown graphically in terms of amplitude and phase of the Nusselt number for different values of buoyancy parameter Lambda. Due to the effect of Lambda and frequency parameter omega, streamlines and isotherms as well as transient shear stress and heat transfer are illustrated in the interplay of study.

The transient shear rheology of a thermotropic liquid crystalline polymer in the super-cooled state

Abstract To determine the processing temperature of a representative thermotropic liquid crystalline polymer (TLCP) in the super-cooled state, small amplitude oscillatory shear (SAOS), the startup of shear flow and differential scanning calorimetry (DSC) measurements were performed. Good agreement on the onset crystallization temperature was obtained from the SAOS and the startup of shear flow. Compared with DSC, the rheological measurements were more sensitive for detecting crystallization in the super-cooled state. The rheological study suggested that crystallization did not occur unless the TLCP was cooled 30 °C below its melting point, and its viscosity was significantly increased as the material was super cooled. Additionally, super cooling the TLCP did not significantly affect the relaxation of shear stress after preshearing. However, the recovery of transient shear stress in the interrupted shear measurements was suppressed to a great extent in the super-cooled state.

Simulations of Magnetohemodynamics in Stenosed Arteries in Diabetic or Anemic Models.

Pulsatile flow simulations of non-Newtonian blood flow in an axisymmetric multistenosed artery, subjected to a static magnetic field, are performed using FLUENT. The influence of artery size and magnetic field intensity on transient wall shear stress, mean shear stress, and pressure drop is investigated. Three different types of blood, namely, healthy, diabetic, and anemic are considered. It is found that using Newtonian viscosity model of blood in contrast to Carreau model underestimates the pressure drop and wall shear stress by nearly 34% and 40%, respectively. In addition, it is found that using a magnetic field increases the pressure drop by 15%. Generally, doubling the artery diameter reduces the wall shear stress approximately by 1.6 times. Also increasing the stenosis level from moderate to severe results in reduction of the shear stress by 1.6 times. Furthermore, doubling the diameter of moderately stenosed artery results in nearly 3-fold decrease in pressure drop. It is also found that diabetic blood results in higher shear stress and greater pressure drop in comparison to healthy blood, whereas anemic blood has a decreasing effect on both wall shear stress and pressure drop in comparison to healthy blood.

Obtaining repeatable initial fiber orientation for the transient rheology of fiber suspensions in simple shear flow

Many researchers reporting the transient rheology of fiber suspensions have not experimentally verified the initial fiber orientation. An assumption for the initial orientation is then required to simulate the fiber orientation evolution and stress response. Measurements of fiber orientation obtained prior to testing in a rheometer can confirm the homogeneity of fiber orientation throughout a sample and repeatability between multiple samples. In this work, the transient rheology of glass fiber suspensions above 0.5 mm in length was measured in a sliding plate rheometer with the initial fiber orientation generated through compression molding, flow reversal, and injection molding. Measurements of shear stress and fiber orientation were obtained to evaluate each sample preparation method and to gain insight into the stress-microstructure relationship. Preshearing and applying flow reversal was used in an effort to control the initial fiber orientation for transient shear stress measurements, but fiber orient...

CFD modeling of transient flow in pressurized pipes

The aim of this article is the analysis of the hydraulic transient flows in pressurized pipes using Computational Fluid Dynamics (CFD). The model uses a three-dimensional efficient mesh with a refined mesh in the viscous sublayer, corresponding to the best compromise between the maximum accuracy and the minimum computational effort. CFD results have been compared with collected data from an experimental pipe-rig and an excellent fitting was observed, providing that a hyperbolic time-domain function be used to describe the valve closure. Calculated velocity profiles have shown two regions with different behaviors: the wall region, dominated by the fluid viscosity, in which flow changes are faster and with sharp gradients; and the pipe core, strongly dependent on the fluid inertial forces, that tends to maintain its initial steady-state shape and to have memory of the past time history of the velocity distribution. Immediately after the valve closure, the flow is cancelled in the valve section, an invert flux is generated and a vortex sheet is formed (i.e., a cylindrical surface composed of vortices in the circumferential direction), propagating to the upstream end. The transient wall shear stress has shown a strong dependence on the time history of the local velocity variation.