Добірка наукової літератури з теми "Cylindrical Couette device"

The orientations of fibres in a semi-dilute, index-of-refraction-matched suspension in a Newtonian fluid were observed in a cylindrical Couette device. Even at the highest concentration (nL3 = 45), the particles rotated around the vorticity axis, spending most of their time nearly aligned in the flow direction as they would do in a Jeffery orbit. The measured orbit-constant distributions were quite different from the dilute orbit-constant distributions measured by Anczurowski & Mason (1967b) and were described well by an anisotropic, weak rotary diffusion. The measured ϕ-distributions were found to be similar to Jeffery's solution. Here, ϕ is the meridian angle in the flow-gradient plane. The shear viscosities measured by Bibbo (1987) compared well with the values predicted by Shaqfeh & Fredrickson's theory (1990) using moments of the orientation distribution measured here.

SummaryWe recently reported that washed platelets (WP) activated with ADP and expressing surface-bound vWF aggregated in flow through small tubes or in a cylindrical couette device at physiological shear rates of G = 300 s–1-1000 s–1 in the absence of exogenous ligands, with GPIb-vWF partially, and activated GPIIb-IIIa totally required for the aggregation. We have now extended these studies to aggregation of platelets “activated” with ristocetin or thrombin. Washed platelet suspensions with added soluble vWF and ristocetin (0.3-0.75 mg/ml), or activated with thrombin (0.01-0.5 U/ml) but no added ligand, were sheared in a coaxial cylinder device at uniform shear rate, G = 1000 s–1. The collision capture efficiency (αG) with which small aggregates form (= experimental/calculated initial rates of aggregation) was correlated with vWF platelet binding assessed by flow cytometry. The vWF-GPIb interaction was exclusively able to support ristocetin-mediated shear aggregation of metabolically active platelets, with very few vWF monomer equivalents bound per platelet (representing ≤10 molecules of 10 million Da) required to yield high capture efficiencies (αG = 0.38 ± .02; n = 11), suggesting rapid and stable bond formations between vWF and GPIb. However, platelet surface-expressed vWF, generated by addition of thrombin to washed platelets, was found to mediate platelet aggregation with αG = 0.08 ± .01 (n = 6), surprisingly comparable to that previously reported for WP and ADP activation. Blocking the GPIIb-IIIa receptor decreased αG by 95 ± 3% (n = 3), while a monoclonal antibody to the vWF site on GPIb caused a 49 ± 7% (n = 8) decrease in αG. The partial role for GPIb thus appears to reflect a facilitative function for increasing contact time between flowing platelets, and allowing engagement of the GPIIb-IIIa receptor to yield stable attachment.

AbstractUndesirable thrombotic reactions count among the most frequent and serious complications for patients who rely on the use of medical devices. To improve the design of medical devices, it is essential to develop a more precise understanding of platelet reactions. Clinical studies and experiments have shown a strong dependence of platelet deposition behavior on the flow. However, today the influence of hemodynamic parameters such as the shear rate on thrombotic reactions is not well understood. For the study of the shear-dependent mechanisms leading to the activation, adhesion and aggregation of platelets, a Couette flow system was used to investigate thrombocyte behavior with regard to well-defined flow conditions at shear-rate values between

Extending previous linear stability analyses of the instabilities developing in permeable Taylor–Couette–Poiseuille flows where axial and radial throughflows are superimposed on the usual Taylor–Couette flow, we further examine the linear behaviour and expand the analysis to consider the weakly nonlinear behaviour of convective-type instabilities by means of the derivation of the fifth-order amplitude equation together with direct numerical simulations. Special attention is paid to the influence of the radius ratio $\unicode[STIX]{x1D702}=r_{in}/r_{out}$, and particularly to wide gaps (small $\unicode[STIX]{x1D702}$) and how they magnify the effects of the radial flow. The instabilities take the form of pairs of counter-rotating toroidal vortices superseded by helical ones as the axial flow is increased. Increasing the radial inflow draws these vortices near the inner cylinder, where they shrink relative to the annular gap, when this gap is wide. Strong axial and radial flows in a narrow annular gap lead to a very large azimuthal wavenumber with steeply sloped helical vortices. Strong radial outflow in a wide annular gap results in very large helical vortices. The analytical and numerical saturated vortices match quite well. In addition, radial inflows or outflows can turn the usually supercritical bifurcation from laminar to vortical flow into a subcritical one. The radial flow above which this change occurs decreases as the radius ratio $\unicode[STIX]{x1D702}$ decreases. A practical motivation for this weakly nonlinear analysis is found in modelling dynamic filtration devices, which rely on vortical instabilities to reduce the processes of accumulation on their membranes.

ABSTRACTThe slow flow of granular materials is often marked by the existence of narrow shear layers, adjacent to large regions that suffer little or no deformation. This behaviour, in the regime where shear stress is generated primarily by the frictional interactions between grains, has so far eluded theoretical description. In this paper, we present a rigid-plastic frictional Cosserat model that captures thin shear layers by incorporating a microscopic length scale. We treat the granular medium as a Cosserat continuum, which allows the existence of localised couple stresses and, therefore, the possibility of an asymmetric stress tensor. In addition, the local rotation is an independent field variable and is not necessarily equal to the vorticity. The angular momentum balance, which is implicitly satisfied for a classical continuum, must now be solved in conjunction with the linear momentum balances. We extend the critical state model, used in soil plasticity, for a Cosserat continuum and obtain predictions for flow in plane and cylindrical Couette devices. The velocity profile predicted by our model is in qualitative agreement with available experimental data. In addition, our model can predict scaling laws for the shear layer thickness as a function of the Couette gap, which must be verified in future experiments. Most significantly, our model can determine the velocity field in viscometric flows, which classical plasticity-based model cannot.

Granular materials are frequently encountered in our daily lives and are widely processed forms of matter in various industries. Despite decades of research, the mechanical behavior of granular materials is not well understood. This thesis focuses on two problems in granular materials, flow in a cylindrical Couette geometry, and emergence of the force network. Recent experiments on granular materials sheared in a cylindrical Couette device revealed a puzzling anomaly, wherein all components of the stress rise nearly exponentially with depth. In this thesis, using particle dynamics simulations and imaging experiments, we show that the stress anomaly arises from a remarkable vortex flow. For the entire range of fill heights explored, we find a single toroidal vortex that spans the entire Couette cell, and whose sense is opposite to the uppermost Taylor vortex in a fluid. In addition, we show that the vortex is driven by a combination of shear-induced dilation, a phenomenon that has no analogue in fluids and gravity flow. We also find that the secondary flow exhibits interesting features like dual vortices in flow conditions where the inertia of grains is relevant. Dilation is a well-known characteristic of a flowing granular medium, but not adequately represented in existing models. This thesis makes a case for properly incorporating cross-streamline dilation in constitutive models. In the second part of this thesis, we focus on force transmission in amorphous materials. Force transmission in amorphous materials like grains, suspensions, emulsions, and foams is primarily characterised by a complex network of inter-particle contact forces called the force network. Important transport and mechanical properties of these forms of matter have been experimentally shown to be associated with the underlying force network. The origin and features of the force network has remained elusive. By defining connectivity in particle packings based on linearity of particle contacts, we show the existence of a criticality. The paths with critical linearity are shown to be mechanically important and constitute the strong force network observed in experiments. The origin of this criticality is shown to be a feature that emerges out of geometric constraints inherent to particle packings. We explain how this critical feature helps us understand particulate matter better, like the stress dip in granular piles and Janssen effect in silos. With a simple path linearity dependent random walk model, we provide insights about force transmission in amorphous materials.

Abstract In this research, we consider the relationship between roughness of the wall and frictional resistance in the range of high-Reynolds number regime which is important for practical use, and its goal is to build a more accurate and highly versatile formula for predicting the frictional resistance acting on the complex surface with irregular roughness. In addition to the parameter corresponding to the distribution of the roughness used in a conventional and empirical formula, we aim to construct an empirical formula including the parameter representing the wavelength of the rough surface. In this study, we conduct laboratory experiments of Taylor-Couette flow, using the cylindrical test specimens roughly sprayed with an actual ship paint, and investigate the influence of irregular roughness on flow field and the surface frictional stress based PIV (Particle Image Velocimetry) measurements and torque measurements in high Reynolds numbers. The azimuthal mean velocity for rough surfaces increased in the entire flow field in comparing to the flow for a smooth surface, and this tendency is remarkable in a bulk region. Also, we measure the rough surfaces of the specimens using a laser type one-shot three-dimensional measurement device. Based on the results of above measurements, we propose the direct relationship between the parameter of a rough surface and frictional resistance.