Hw Hoogstraten

Numerical simulation of blood flow in an artery with two successive bends

Blood flow in an artery with two successive bends is simulated by a finite-element computation of steady flow of a Newtonian viscous fluid through a rigid tube having the same shape as a specific part of the femoral artery. Notwithstanding the fact that the bends in the model geometry are rather gentle, the axial and secondary flow patterns, computed for a range of values of the Reynolds number Re, show strong and complicated three-dimensional flow effects. In particular, the flow pattern in the second bend for relatively small values of Re (Re < 240) turns out to be drastically different from that for larger Re-values.

The influence of the angle of confluence on the flow in a vertebro-basilar junction model

In earlier work, it was demonstrated that the flow in models of the vertebro-basilar junction is highly three-dimensional and the geometry exerts a strong influence on the hemodynamics. The morphology of the vertebro-basilar junction is very variable amongst individuals. In a study of 85 human vertebro-basilar junctions, the angle between the vertebral arteries varied between 10 and 160 degrees. To determine how the flow is influenced by this geometrical parameter, the flow is studied both experimentally, with laser Doppler velocimetry, and numerically, with a finite element package. A series of junction models is used with a range of confluence angles (45, 85 and 125 degrees). It appears that the angle of confluence has a strong influence on the structure and strength of the secondary flow field. The secondary velocities persist far downstream. Furthermore, near the apex, a region with low velocities is present. The larger the confluence angle is, the larger this region is, and even backflow may occur. In addition, the occurrence of atherosclerotic plaques in 85 human vertebro-basilar junctions is studied. Only one preferential location was found: the apex, the other plaques seem to be randomly distributed. The magnitude of the confluence angle of junctions with sharp-edged apices has a significant influence (p = 0.006) on the occurrence of a plaque at the apex. Apparently, a large confluence angle is a geometrical risk factor for atherosclerosis.

COMPUTATION OF STEADY THREE-DIMENSIONAL FLOW IN A MODEL OF THE BASILAR ARTERY

The flow in the basilar artery arises from the merging of the flows from the two vertebral arteries. To study the flow phenomena in the basilar artery, computations have been performed using a finite element (FE) method. We consider steady flow in a geometrically symmetric confluence. For simplicity, channels with a rectangular cross-section have been used. Both symmetric and asymmetric flow cases have been considered. The results show that for the Reynolds number of interest the flow downstream of the junction is highly three-dimensional, and that the flow at the end of the basilar artery, where it splits again, will not be fully developed. The computed phenomena have been confirmed by laser Doppler velocity measurements.

A study of transverse and longitudinal size effects in high-strength polyethylene fibres

The results of an experimental study of the effects of fibre diameter and testing length on the strength of polyethylene fibres prepared by the gel-spinning-hot-drawing process are reported. These size effects have been studied in fibres covering a range of mechanical properties in order to establish how size effects are related to fibre structure. It was found that both types of size effect studied are present in polyethylene fibres, and that the magnitude of both changes with increasing modulus of elasticity of the fibre; length effects become weaker and disappear completely, whereas diameter effects become more pronounced as the fibre modulus increases. This behaviour shows that transverse and longitudinal size effects cannot be described simultaneously within the framework of statistical strength theory. A model is proposed, based on the well known Griffith approach, which shows that a strength-diameter dependence may arise owing to geometrical effects rather than from the presence of flaws and imperfections.

Pulsating entry-flow in a plane channel

To obtain results for the title problem, the time-dependent Navier-Stokes equations have been solved numerically. Axial-velocity profiles at various distances from the entrance of the channel are shown for a number of points in time during one period of oscillation. Further some results for the time-dependent inlet length are presented.

Marangoni instability in a liquid layer bounded by two coaxial cylinder surfaces

A liquid layer, confined between two coaxial cylinder surfaces, has either a gasliquid interface on the inside and is heated from the outer (solid) boundary, or it has a gas-liquid interface on the outside and is heated from the inner (solid) boundary. Neglecting gravity and using a standard normal-mode approach, we analyse surface-tension driven instability (Marangoni instability) of the motionless steady state in which the temperature depends on the radial coordinate only. Numerical results for the critical Marangoni number and corresponding wave-number pair are presented for various values of the curvature of the interface. This curvature turns out to exert a significant influence on the onset of Marangoni convection flows. Further, the stability behaviour of the system is found to be quite variable, depending on whether the interface is on the inside or on the outside of the layer and whether it is well-conducting or nearly-isolated.

Marangoni instability in a liquid layer confined between two concentric spherical surfaces under zero-gravity conditions

A liquid layer containing a single solute is bounded on the outside by a rigid spherical surface and on the inside by a concentric gas/liquid interface. The solute evaporates from the liquid to the gas phase and, if the surface tension depends on the solute concentration, surface-tension driven convective flows may arise (Marangoni instability). Assuming zero-gravity conditions and using a normal-mode approach, we study the linear stability of the time-dependent, spherically-symmetric concentration profiles in a motionless liquid. Numerical results are presented for Marangoni numbers and perturbation wave numbers in the case of neutral stability. It turns out that the systems stability properties are strongly dependent on the curvature of the interface and on the mass-transfer Biot number.

Steady two-dimensional merging flow from two channels into a single channel

Two-dimensional steady symmetric merging flow from two channels into a single one is investigated. The geometry of the configuration has been chosen such that it can be mapped conformally onto a rectangular geometry, thus facilitating the numerical solution procedure for the governing Navier-Stokes equations. Computed velocity profiles and streamline patterns are presented in graphical form. Furthermore, results concerning the inlet length are given.

Marangoni convection in V-shaped containers

This paper presents a numerical study of the time evolution of Marangoni convection in two V-shaped containers involved in the microgravity experiments reported in Hoefsloot et al. [7]. First the case of the triangular container with a plane gas/liquid interface is considered, next the container having the shape of a circular sector with a curved interface is dealt with. The numerical results show the same behaviour as observed experimentally: convection caused by macroscale effects in the former, and microconvection in the latter case.

Growth factors for Marangoni instability in a spherical liquid layer under zero-gravity conditions

The neutral-stability analysis presented by Hoefsloot et al. [3] is completed by computing the growth factors β for the normal modes and by showing that the neutral states (Re(β)=0) are stationary (Im(β)=0) rather than oscillatory (Im(β)≠0).