Contraction Flow Research Articles

A shear and elongational decomposition approach of the rate-of-deformation tensor for non-Newtonian flows with mixed kinematics

We propose a novel approach for non-Newtonian viscoelastic steady flows based on a decomposition of the rate-of-deformation tensor which here, in a simplified version, leads to an anisotropic generalised Newtonian fluid-like model with separated treatment of kinematics pertaining to shear and extensional flows. Care is taken to assure that the approach is objective and does not introduce spurious effects due to dependence on a specific reference frame. This is done by separating the rate-of-deformation tensor into shear and elongational components, with the local scalar shear and elongation rates being the second invariant of each tensor separately, and by modifying the method used to separate the rate-of-deformation tensor so that it becomes independent of superimposed rigid rotations, thus satisfying the principle of material indifference. We assess the model with two test cases: planar contraction flow, often employed to evaluate numerical methods or constitutive equations and for which experimentally observed corner vortex enhancement and pressure drop increase are seldom found in numerical simulations; and flow past confined or unconfined cylinders, for which experiments indicate a drag increase due to elasticity and most predictions give a drag decrease. With our anisotropic model, incorporating additional elongational-flow-related terms, large vortices and accentuated pressure drop coefficients can be predicted for the contraction problem and enhanced drag coefficient for the cylinder problems. This is the first work where problems of non-Newtonian fluid mechanics are solved numerically with separation of strain rate into shear and elongational components.

Flow structure and characteristics of fluid undergoing a sudden contraction to an annular gap under dynamic boundary

Pipeline transport serves as an effective means to alleviate traffic congestion and reduce carbon emissions from transportation. The hydraulic delivery system, which employs pipeline cars as carriers, addresses the limitations of existing systems. However, its transportation efficiency is affected by variations in the flow structure within the pipelines. During the acceleration of the pipeline car, the sudden contraction flow field from circular to annular gap formed in the vicinity of the end face under dynamic boundary conditions. This study utilized particle image velocimetry (PIV) to visualize and measure the sudden contraction flow field. Based on the obtained experimental results, it investigated the impact of dynamic boundary velocity on the flow structure, velocity characteristics, and energy dissipation of the annular gap. The acceleration process of the dynamic boundary is the conversion of flow energy into the kinetic energy of the annular gap flow and the kinetic energy of the pipeline car. This process is accompanied by phenomena of velocity slip and velocity overshoot. As the velocity of the pipeline car increases, the recirculating vortex within the annular gap dissipates and eventually disappears. The velocity slip gradually decreases, the location of the overshoot point shifts radially, and the magnitude of the overshoot diminishes before ultimately vanishing. From static to steady, the probability density distribution of the slipstream face transitions from a distribution with high skewness and low peak value to a normal distribution with high peak value and low skewness. The irreversible losses that arise in a sudden contraction flow field can be quantified by the increase in entropy. Due to the similarity of the solving processes of large Eddy simulation and PIV, a combined sub-grid stress model is used to solve the flow losses in the flow field. The turbulent dissipation occurs mainly in the recirculation region, shear layer, and high-speed shear regions near the wall.

A lattice Boltzmann flux solver with log-conformation representation for the simulations of viscoelastic flows at high Weissenberg numbers

In this work, a viscoelastic lattice Boltzmann flux solver (VLBFS) with log-conformation representation is proposed for simulating the incompressible flows of a viscoelastic fluid at high Weissenberg number conditions. Compared with the original lattice Boltzmann flux solver (LBFS), the present method has two main new features. First, the method solves the polymer constitutive equations with log-conformation representation. Second, an upwind-biased scheme is incorporated in the interpolation when performing flux reconstructions at the cell interface. With the aid of these two treatments, the numerical stability of VLBFS is significantly improved, making it capable of solving high Weissenberg number problems (HWNP). Compared with using the lattice Boltzmann method (LBM) to solve the viscoelastic fluid flow, VLBFS inherits the advantages of LBFS, such as flexible mesh generation, decoupling of the grid spacing and time interval, and low memory requirement. VLBFS can also precisely recover the macroscopic constitutive equation. The present method has been critically validated using three benchmark cases, namely, the plane Poiseuille flow, lid-driven cavity flow, and 4:1 abrupt planar contraction flow. The numerical results fully demonstrate the solver’s powerful ability in simulating HWNP.

Molecular dynamics simulation of slit rheometer for predicting shear thinning of short polyethylene chains

Understanding the molecular basis of rheological properties is crucial from both experimental and theoretical perspectives. Slit rheometry is commonly employed to measure the viscosity of fluids. This study utilized molecular dynamics simulations to investigate isothermal contraction flow at the nanoscale. Short linear polyethylene chains were uniformly extruded by a constant-speed piston from a reservoir through an abrupt contraction slit into the surrounding vacuum. Overall, die swelling and die wetting phenomena were observed. Molecular chains were stretched within the slit, while those outside the slit shrunk. Notably, the velocity profile within the slit varied with wall slip at different extrusion velocities. The relationship between the apparent shear viscosity and shear rate exhibited two primary characteristics: the first-Newtonian plateau and the shear thinning slope. Therefore, this molecular simulation method effectively demonstrates the general non-Newtonian behavior of macroscopic polymer fluids.

Abstract We112: Volumetric imaging to recapitulate cardiac injury and arrhythmias

Advanced understanding of cardiac structure and function is crucial for uncovering the underlying mechanism of injury and arrhythmias. To explore cues to structural and functional abnormalities in extensively established animal models, we have developed optical imaging and computational methods tailored to investigate intact hearts of zebrafish and rodents at cellular resolution. Our cardiac light-field microscopy allows us to capture instantaneous dynamics such as calcium transients, irregular contraction, and blood flow at 200 volumes per second in zebrafish larvae 1 . To improve the spatial resolution and enable the study of rodent models, we have customized two light-sheet imaging systems. One system is empowered by the retrospective synchronization and machine learning for the study of contractile function and focal myocardial mechanics from end-systole to end-diastole 2 , while the other integrates tissue clearing and axially scanning approaches for the exploration of ventricular trabeculation and myocardial compaction 3 . These methods aim to overcome the trade-off in spatial resolution, imaging speed, field of view across the atria and ventricles with minimal photo-damage and maximal penetration depth, enabling long-term investigation of cardiac development and regenerative processes. To further interpret the large volume of datasets, we have customized a successive subspace learning and virtual reality-based platform for interactive image analysis 4 . Collectively, our multi-scale strategy has opened up new landscapes for exploring the cardiac micro-structure and contractile function, both in health and disease.

On more insightful dimensionless numbers for computational viscoelastic rheology

Abrupt contraction flows involving viscoelastic fluids represent a longstanding computational challenge within the field of non-Newtonian fluid mechanics. Despite the apparent simplicity of the geometry, these flows have given rise to intricate discussions in the study of viscoelastic phenomena. This study aims to re-examine the numerical solutions for flows through abrupt contractions, offering a fresh interpretation through the lens of reformulated dimensionless numbers. These numbers are designed to consider the characteristic shear rate of the problem, providing a more comprehensive understanding of the underlying dynamics.When investigating models with intermediate levels of complexity, such as the Giesekus and Phan-Thien-Tanner constitutive equations, the usual comparison with the corresponding Oldroyd-B model becomes inadequate because it tends to rely on the nominal relaxation time (λ) and the nominal total viscosity (η) instead of their effective counterparts when defining the Reynolds number (Re), the Weissenberg number (Wi) and the ratio of solvent to total viscosities (β) (β plays a role only in rheological models involving a solvent contribution). If these dimensionless numbers are tailored to account for the characteristic shear rate specific to the problem under investigation, the choice of the corresponding Oldroyd-B flow, at the adequate values of Re, Wi, and β allows for significantly better quantification of the correct effects of nonlinear viscoelasticity of the original model.We show the conventional approach tends to overemphasize the role of the nonlinear parameter in nonlinear constitutive equations, like the Giesekus and PTT models, when examining standard abrupt contraction flow outputs such as the Couette correction and vortex size. This overestimation occurs because the conventional method does not allow the Reynolds and Weissenberg numbers (and possibly β) to carry the portion of the nonlinear effect that can potentially be captured by the linear Oldroyd-B model through the use of characteristic shear rate-based values. We believe the present approach provides a better perspective of the role played by the nonlinear parameter and its extension to more general flows is also discussed.

Elastic and shear-thinning effects in contraction flows: a comparison

The flow through a 4:1 planar contraction has been investigated using different rheological models having the same shear viscosity, namely, the inelastic Carreau-Yasuda model (CY), the enhanced Bautista-Manero-Puig (eBMP), and the exponential version of the Phan-Thien/Tanner (PTT). Noticeable discrepancies were observed with the CY model and the eBMP in terms of the velocity profiles along the centerline and in the exit channel (near the end of the geometry) normal to the flow direction. Transient planar extensional viscosity shows a large effect on vortex dynamics although the effect of transient and steady elongation on pressure drop seems negligible. Simulation results allowed gathering that pressure drop is largely influenced by the shear-thinning behavior of the fluid, noticeably affected by elasticity, and less by extensional viscosity.Graphical

Continuous Solvothermal Synthesis of Metal-Organic Framework (Cu-BTC) Nanoparticles Using a Microstructured Mixer Equipped with Double-Tube, Swirl, and Contraction Flow Channels

Continuous Solvothermal Synthesis of Metal-Organic Framework (Cu-BTC) Nanoparticles Using a Microstructured Mixer Equipped with Double-Tube, Swirl, and Contraction Flow Channels

Gastric Insufflation with High Flow Nasal Oxygen Therapy in Adult Patients Admitted to Intensive Care Unit: An Observational Study.

With the provision of a small positive end-expiratory pressure (PEEP) effect, high-flow nasal oxygen (HFNO) therapy carries a risk of stomach distension. The present study was conducted to find out the air leak in the gastric antrum leading to gastric distension in adult patients with acute respiratory failure receiving HFNO therapy. Adult patients with early hypoxemic respiratory failure requiring HFNO therapy were enrolled in this trial. Before initiation of HFNO therapy, baseline gastric volume (GV) and the average number of peristaltic contractions over one minute were measured using ultrasound. Once the patient was stabilized on HFNO therapy, a 2nd, 3rd, and 4th ultrasound scans were acquired at 10, 20, and 30 minutes respectively. Vitals and blood gas values were recorded at the baseline and after 30 min of initiation of HFNO therapy. Patient comfort, duration of HFNO therapy, and outcome were also recorded. The GV at 10, 20, and 30 minutes were significantly larger (p < 0.001) compared to baseline. This increase in GV was associated with a significantly increased number of peristaltic contractions and had a significant positive correlation with the HFNO flow (r = 0.541; p < 0.001). The HFNO therapy was well tolerated by most of the patients and led to a significant improvement in the vitals and blood gas parameters at 30 minutes after initiation of HFNO therapy. In adult patients with hypoxemic respiratory failure, the use of HFNO therapy produces gas leaks into the stomach leading to increased gastric volume. The gastric distension increases the peristaltic contraction and higher flows result in more distension. Ramachandran A, Bhatia P, Mohammed S, Kamal M, Chhabra S, Paliwal B. Gastric Insufflation with High Flow Nasal Oxygen Therapy in Adult Patients Admitted to Intensive Care Unit: An Observational Study. Indian J Crit Care Med 2024;28(4):393-398.

Mechanical regulation of substrate adhesion and de-adhesion drives a cell-contractile wave during Drosophila tissue morphogenesis

During morphogenesis, mechanical forces induce large-scale deformations; yet, how forces emerge from cellular contractility and adhesion is unclear. In Drosophila embryos, a tissue-scale wave of actomyosin contractility coupled with adhesion to the surrounding vitelline membrane drives polarized tissue invagination. We show that this process emerges subcellularly from the mechanical coupling between myosin II activation and sequential adhesion/de-adhesion to the vitelline membrane. At the wavefront, integrin clusters anchor the actin cortex to the vitelline membrane and promote activation of myosin II, which in turn enhances adhesion in a positive feedback. Following cell detachment, cortex contraction and advective flow amplify myosin II. Prolonged contact with the vitelline membrane prolongs the integrin-myosin II feedback, increases integrin adhesion, and thus slows down cell detachment and wave propagation. The angle of cell detachment depends on adhesion strength and sets the tensile forces required for detachment. Thus, we document how the interplay between subcellular mechanochemical feedback and geometry drives tissue morphogenesis.

Inelastic Solution for Power Law Fluid with Taylor Galerkin-Pressure Correction Finite Element Method: Axisymmetric Contraction Flows

In this study we examine the flow of inelastic fluids with various shear properties in axisymmetric contractions with various contraction ratios are selected as 4:1, 6:1 and 8:1 with both rounded-corner and sharp. Particular attention is paid to the effect of shear thickening and shear thinning upon the solution behavior. Power-law inelastic model is employed coupling with the conservation of momentum equation and continuity equation. The numerical simulation of such fluid is performed by using the Taylor Galerkin pressure correction (T-G/P-C) finite element algorithm. The effects of geometry structure and many factors such as Reynolds number (Re) and the parameters of power law model are presented in this study. Particularly, in this study we are focused on the influence of these factors on the solution components and the level of convergence. This research was a comparative study between sharp and rounded-corner contraction geometries with a ratio of 4:1, and to another comparative study among sharp contraction geometries with ratios of 4:1, 6:1, and 8:1. The practical implications of this study focused on vortex length and the impact of varying the parameters of the power law model and the Reynolds number (Re) on it for 4:1 contraction flow. The study dealt with the effect of different geometries on the rates of convergence of velocity and pressure as well as the characteristics of axial velocity and pressure on the axis of symmetry.

Simulation of blood flow in a stenosed and bifurcating artery using Finite Volume Methods and OpenFOAM

The article focuses on the shear-thinning and viscoelastic constitutive modelling and numerical simulation of blood flow in a stenosed and bifurcating artery. Specifically, the shear-thinning and viscoelastic behaviour of blood are modelled and implemented via the Oldroyd-B and Generalized Oldroyd-B constitutive models. A robust and efficient general purpose numerical (and computational) methodology for the simulation of blood flow in a stenosed and bifurcating artery is also developed and implemented. The numerical algorithm is developed more generally to resolve the mathematical model equations arising out of the all-encompassing Generalized Giesekus constitutive model. This model reduces to the Generalized Oldroyd-B model and subsequently also to the standard Oldroyd-B model simply by switching off certain material parameters. The inclusion of the Generalized Giesekus model must therefore be viewed in this context, to facilitate the development of an all encompassing general purpose numerical code. The blood flow modelling is otherwise done via the Oldroyd-B and Generalized Oldroyd-B constitutive models. The shear-thinning effects are implemented via the Cross model for shear-viscosity. The Generalized Oldroyd-B model results all illustrate that the velocity is directly proportional to the constriction caused by the stenosis. The higher the blockage from the constriction, the higher would the velocity spurt through the constriction. This velocity behaviour correspondingly enhances the wall shear-stresses as the constriction increases, caused by the presence of the stenosis. High wall shear-stresses greatly increase the possibility of rupture of the stenosis. This can lead to catastrophic consequences in the usual case where the stenosis is caused, say, by tumor growth. As demonstrated near the contraction of a standard 4:1 contraction flow geometry, dramatic fluid flow effects, which are attributable to the polymeric-stresses, specifically to the first normal stress difference, are observed in the vicinity of the constrictions resulting from the presence of the stenosis. Such effects, include, flow recirculation and reversal, vortex formation, and spurt phenomena.

Embryonic exposure of polystyrene nanoplastics affects cardiac development

Micro- and nanoplastics have recently been detected in human blood and placentas, indicating inevitable embryonic exposure to these particles. However, their influence on human embryogenesis and the underlying mechanisms are still unknown. In this study, the effects of polystyrene nanoplastics (PS-NPs) exposure on cardiac differentiation of human embryonic stem cells (hESCs) were evaluated. Uptake of PS-NPs not only caused cellular injury, but also regulated cardiac-related pathways as revealed by RNA-sequencing. Consequently, the efficiency of cardiomyocyte differentiation from hESCs was compromised, leading to immature of cardiomyocytes and smaller cardiac organoids with impaired contractility. Mechanistically, PS-NPs promoted mitochondrial oxidative stress, activated P38/Erk MAPK signaling pathway, blocked autophagy flux, and eventually reduced the pluripotency of hESCs. Consistently, in vivo exposure of PS-NPs from cleavage to gastrula period of zebrafish embryo led to reduced cardiac contraction and blood flow. Collectively, this study suggests that PS-NPs is a risk factor for fetal health, especially for heart development.

Numerical Study on the Influence of Variable Section-Enhanced Mass Transfer Flow Field Design on PEMFC Performance Based on Underrib Convection Flow

The optimal design of variable section is a flow field optimization method that changes the pressure of the reacting gas in the flow channel by setting up a contraction surface in the flow channel, which is effective in improving many aspects of the proton exchange membrane fuel cell (PEMFC) performance. However, there are also problems such as uneven oxygen distribution in some of the flow channels, and the current density and net output power are not significantly improved. To find a balance among gas transfer efficiency, drainage capacity, uniformity of current density distribution, and output performance, this paper investigates the design of a variable cross-section-enhanced mass transfer flow field for PEMFC flow channel contraction by establishing a three-dimensional numerical model, starting from the sidewall width direction and the bottom height direction of the flow channel cross-section dimensions, in order to assess the effects of variable cross-section contraction at the bottom and sidewalls of the flow channel on the cell performance. The polarization curves, mass transfer capacity, current density distribution, and net output power of the variable cross-section contraction flow field are analyzed and compared with the straight flow field. On this basis, the optimum size parameters and operating parameter combinations are determined. Variable cross-sectional contraction flow field improves PEMFC output performance. The contraction of the variable cross-section contraction flow field at the sidewalls effectively improves reactive gas transfer efficiency and drainage capacity, improves current density distribution, and increases the net output power of the PEMFC. In particular, the sidewall contraction flow field has a 9.0% increase in power density, a 6.45% increase in the effective mass transfer coefficient (EMTC), a 6.52% increase in uniformity of current density distribution, and a 6.05% net power increment compared to the straight flow field, respectively. The best output performance, the highest oxygen concentration, and the lowest water concentration are achieved with a combination of parameters: number of sidewall contractions N = 6 , length L f = 6 mm , and width W f = 1.2 mm . And a wide range of air stoichiometry ratios are applied when the contraction of the variable cross-section contraction flow field is located at the sidewalls. Increasing the cathode stoichiometry ratio from 10 to 50 increased the cell power density by 25.5% and from 50 to 90 increased the cell power density by 9.5%. The simulation results can provide theoretical basis for the application of variable section flow field design for practical PEMFC.

The importance of extensional viscosity on a peculiar ear-flow front for injection molding simulations of fiber-reinforced composites

An ear-flow front is discovered in the industrial practice of injection molding, especially for fiber-reinforced composites; the advance of the flow front in the center of the cavity is obviously slower than at the edges. To date, such a peculiar flow has not yet been predicted theoretically or computationally and is beyond the predictive capabilities of existing commercial computer aid design (CAE) packages for the injection molding process. Recently, the GNF-X (Generalized Newtonian Fluid eXtended) model of weighted shear/extensional viscosity has been incorporated availably in state-of-the-art predictive engineering software of the 3D-CFD (three-dimensional computational fluid dynamics) framework to show the extension-induced vortex growth in contraction flows of polymer melts. Through 3D-CFD coupled with GNF-X in injection molding simulations, it is significant to demonstrate the importance of extensional viscosity on the ear-flow formation in relation to the weighted viscosity and extension fraction for long-branched and fiber-filled polymer materials. In addition, one can point out that the ear-flow front probably occurs in fast filling conditions for anisotropic fluids.

α2C-Adrenergic Receptor Blockade Inhibits Langendorff-Isolated Rat Heart Work.

We studied the effect of selective α2C-adrenergic receptor antagonist JP-1302 in concentrations of 10-9-10-6 M on inotropy, chronotropy, and coronary flow in the Langendorff-isolated rat heart. JP-1302 in all studied concentrations decreased the left-ventricular myocardium force contraction, HR, and coronary flow. The maximum inotropic, chronotropic, and vascular effects were observed when the antagonist was applied to the perfused solution in a concentration of 10-7 M. The least pronounced decrease in the studied parameters was observed at JP-1302 concentrations of 10-8 and 10-9 M. The obtained data indicate the participation of this subtype of α2-adrenergic receptors in the regulation of activity of isolated adult rats heart.

Analysis of the flow phenomenon of fluid undergoing a sudden contraction to an annular gap

The paper presents the visualization results of the sudden contraction flow field with an annular gap downstream of the contraction cross section. The contraction ratio directly influences the flow phenomena. No stationary vortices were observed upstream of the contraction cross section, and flow separation divided the flow field within the annular gap into a recirculation zone, a shear layer, and a mainstream zone. Vena-contracta point and reattachment point occurred within the flow field of the annular gap. The production of Reynolds stress varied with the contraction ratio, and the momentum exchange mainly occurred in the recirculation zone and the shear layer, while the flow state in the mainstream zone tended to become isotropic. Rapid changes in turbulent kinetic energy primarily occurred in the shear layer, and the production term of turbulent kinetic energy contributed the most. Applying computational fluid dynamics and experimental results, this study has formulated an expression for the contraction coefficient of the proposed model. The axial positions of the vena-contracta point and reattachment point have a strong correlation with the contraction ratio, which can be represented by an exponential function.

Comparing different stabilization strategies for reduced order modeling of viscoelastic fluid flow problems

This paper presents a computational framework for model order reduction of viscoelastic fluid flows, with a particular focus on the stabilization of the reduced order model (ROM) and the efficient approximation of non-linear terms appearing in the governing equations. We compare three different stabilization approaches, two of which are based on offline stabilization only and one that uses both offline and online stabilization. Two main techniques for the treatment of non-linear terms are examined and compared: the discrete empirical interpolation method, and a reduced quadrature method based on a sparse recovery technique. Numerical experiments are conducted on two benchmark flows such as the flow past a sphere and the 4:1 contraction flow problem. We show that significant speedups can be achieved using offline streamline upwind Petrov–Galerkin stabilization together with hyper-reduction. We also discuss the potential accuracy losses of the ROM that could result from mesh-related issues of the full-order model (FOM) and propose possible remedies.

THE RELATIONSHIP BETWEEN THE DIFFERENT TYPES OF TERMS IMPLIED INTO CONTRACTS (PART II)

In the first part of this contribution, I showed that the different categories of implied terms of a contract flow into each other in three respects. This raises the question whether the present approach of our courts to implied terms is satisfactory, especially given that it results in a blurring of borders between tacit and ex lege terms.Two alternative approaches to the present understanding of the borderlines between different types of implied terms will be considered in this final part of this article, before conclusions are drawn.

Solidification Microstructure and Segregation in the Medium-carbon Steel Cast with a Laboratory-scale Local-chilled Mold

In the industrial steel manufacturing process, such as continuous casting and ingot casting, macrosegregation occurs due to the effect of bridging and contraction flow during the middle and end periods of solidification. Since the macrosegregation results in a nonuniform structure and eventually cracks, there has been a demand for technological development that does not cause macrosegregation in the casting process. In this study, model experiments using the medium-carbon steel cast with a laboratory-scale local-chilled mold at different superheating have been carried out to investigate the relationship between solidified structure and macrosegregation occurred by local bridging during casting. The morphology of shrinkage porosities and dendrite structures was observed. The concentrations of the alloying elements were analyzed for macrosegregation by an electron probe micro-analyzer. Chill plates successfully formed the columnar dendrite bridging area and the columnar dendrite shell in the sample with high superheating. During solidification, the negative pressure of the region below the bridging increased, and the concentrated contraction flow flowed into the bottom of the large shrinkage porosity. Finally, V segregation was formed in the bridging area, large shrinkage porosities remained below the bridging area, and point-like or band-like positive macrosegregation occurred in the interdendritic region between columnar dendrites and equiaxed dendrites below the bridging. In comparison, a lower casting temperature increased the grain density and formed shrinkage porosities that were smaller in size but larger in number and more dispersed.