F. S. Henry

Low Reynolds Number Viscous Flow in an Alveolated Duct

Flow visualization studies and supplementary numerical simulations are carried out on slow flow through a model alveolated duct. The results reveal that the type of flow that develops in the alveoli, or cavities, is controlled by the ratio of the depth to the width of the cavity and by the ratio of cavity volume to duct volume. While weak, the slowly rotating flow in the cavity is thought to be important to the convective transport of heat and mass transfer to, or from, the walls of the cavity. The relevance of these finding to particle transport and deposition deep in the lung is discussed.

Gas and aerosol mixing in the acinus

This review is concerned with mixing and transport in the human pulmonary acinus. We first examine the current understanding of the anatomy of the acinus and introduce elements of fluid mechanics used to characterize the transport of momentum, gas and aerosol particles. We then review gas transport in more detail and highlight some areas of current research. Next we turn our attention to aerosol transport and in particular to mixing within the alveoli. We examine the factors influencing the level of mixing, review the concept of chaotic convective mixing, and make some brief comments on how mixing affects particle deposition. We end with a few comments on some issues unique to the neonatal and developing lung.

Numerical model of boundary-layer control using air-jet generated vortices

Numerical calculations of the three-dimensional flowfield generated by pitched and skewed air jets issuing into an otherwise undisturbed turbulent boundary layer are presented. It is demonstrated that each such jet produces a single strong longitudinal vortex. The strength of the vortex, as inferred from its effect on the development of skin friction, is shown to be influenced by pitch and skew angles, exit velocity, and downstream distance in ways which accord with published experimental results. The calculated beneficial effect that the longitudinal vortices have on the development of skin friction in an adverse pressure gradient demonstrates the mechanism by which vortex generators delay boundary-layer separation. It follows that the numerical model could be used to optimize arrays of air-jet vortex generators. Furthermore, the facility to quantify the interaction between the vortex and the boundary layer should also be valuable in the application of vane vortex generators, and possible even more generally. 18 refs.

Particle Transport and Deposition: Basic Physics of Particle Kinetics

The human body interacts with the environment in many different ways. The lungs interact with the external environment through breathing. The enormously large surface area of the lung with its extremely thin air-blood barrier is exposed to particles suspended in the inhaled air. The particle-lung interaction may cause deleterious effects on health if the inhaled pollutant aerosols are toxic. Conversely, this interaction can be beneficial for disease treatment if the inhaled particles are therapeutic aerosolized drugs. In either case, an accurate estimation of dose and sites of deposition in the respiratory tract is fundamental to understanding subsequent biological response, and the basic physics of particle motion and engineering knowledge needed to understand these subjects is the topic of this article. A large portion of this article deals with three fundamental areas necessary to the understanding of particle transport and deposition in the respiratory tract. These are: (i) the physical characteristics of particles, (ii) particle behavior in gas flow, and (iii) gas-flow patterns in the respiratory tract. Other areas, such as particle transport in the developing lung and in the diseased lung are also considered. The article concludes with a summary and a brief discussion of areas of future research.

Nanoparticle delivery in infant lungs

The lung surface is an ideal pathway to the bloodstream for nanoparticle-based drug delivery. Thus far, research has focused on the lungs of adults, and little is known about nanoparticle behavior in the immature lungs of infants. Here, using nonlinear dynamical systems analysis and in vivo experimentation in developing animals, we show that nanoparticle deposition in postnatally developing lungs peaks at the end of bulk alveolation. This finding suggests a unique paradigm, consistent with the emerging theory that as alveoli form through secondary septation, alveolar flow becomes chaotic and chaotic mixing kicks in, significantly enhancing particle deposition. This finding has significant implications for the application of nanoparticle-based inhalation therapeutics in young children with immature lungs from birth to ˜2 y of age.

Formation and travel of vortices in model ventricles: application to the design of skeletal muscle ventricles

Vortex-ring production was studied in axisymmetric elastomeric ventricles designed to stimulate flow in a cardiovascular assist device. A flow visualization technique was used to investigate the effects of reducing the inlet diameter and predilating the ventricle on vortex travel in two ventricles of different shape and size. In most cases, vortex rings formed during the filling phase. They were bounded by the incoming jet of fluid and the ventricular wall. The velocity of their centres during the filling period was proportional to the inflow velocity. During filling, vortex velocity was substantially independent of the shape and diameter of the two ventricles studied. It was dependent mainly on orifice diameter: a narrower inlet led to greater inflow velocities and proportionately greater vortex velocities. At the end of the filling phase, each vortex increased in size to occupy the full radial extent of the ventricle. This process was associated with a decrease in the axial velocity and strength of the vortex. At low flow rates, these losses resulted in the arrest of the vortex at end filling. Vortex motion in ventricles is particularly important in the design of a cardiovascular device such as the skeletal muscle ventricle (SMV), where small ejection fractions may leave blood at the apex of the ventricle relatively undisturbed. It is suggested that inlet diameter could be selected to favour the formation and travel of vortices, with a resultant reduction in apical residence time and hence a reduced risk of thrombus formation.

Cell model of aerosol collection by fibrous filters in an electrostatic field

Abstract The difficulty in collecting fine particles (0.1 to 1 μm range) may require the use of fibrous filters with gas velocity of only a few centimeters per second. Kuwabaras cell model has been quite successful in the determination of the viscous flow field in aerosol filtration by fibrous filters where the Reynolds number is less than one. In this paper, Kuwabaras cell model is extended to the electrostatic filtration in a viscous flow field for both charged and dielectric spherical particles. The particles are assumed to be large enough to allow diffusion to be neglected. The utilized model takes into account the interference effects, namely the influence of neighbouring fibers on the flow field, in a more effective way than the single and three fiber models. A formula relating theoretical filter efficiency to a deposition coefficient is proposed. The filter efficiency obtained using this formula is compared to that given by the single fiber theory.

The effect of neighboring fibers on the electric field in a fibrous filter

Abstract The effect of neighboring fibres on the electric field of an electrically enhanced fibrous filter is considered. The electric field around a cylinder, to represent a fiber, surrounded by six other cylinders of equal radius is analyzed using the method of images. It was found that taking account of the neighboring fiber effects on the electrostatic field (ENFE) reduces the cell deposition coefficient and hence the efficiency of the filter. Therefore, the investigations without ENFE overestimate the efficiency of electrostatic fibrous filters. Furthermore, it would appear that the neighboring fiber effect is more important to situations where electrophoresis dominates. However, it is also shown that neighboring fiber effect is less important to the electric field than it is to the flow field.

Simulation of flow through a Miller cuff bypass graft.

Unnatural temporal and spatial distributions of wall shear stress in the anastomosis of distal bypass grafts have been identified as possible factors in the development of anastomotic intimal hyperplasia in these grafts. Distal bypass graft anastomoses with an autologus vein cuff (a Miller cuff) interposed between the graft and artery have been shown to alleviate the effects of intimal hyperplasia. In this study, pulsatile flow through models of a standard end-to-side anastomosis and a Miller cuff anastomosis are computed and the resulting wall shear stress and pressure distributions analysed. The results are inconclusive, and could be taken to suggest that the unnatural distributions of shear stress that do occur along the anastomosis floor may not be particularly important in the development of intimal hyperplasia. However, it seems more likely that the positive effects of the biological and material properties of the vein cuff, which are not considered in this study, somehow outweigh the negative effects of the shear stress distributions predicted to occur on the floor of the Miller-cuff graft.

Flow in a Simple Model Skeletal Muscle Ventricle: Comparison Between Numerical and Physical Simulations

Flow patterns generated during ventricular filling have been investigated for three different combinations of flow rate and injection volume. The numerical solutions from a commercially available computational fluid dynamics package were compared with observations made under identical flow conditions in a physical model for the purpose of code validation. Particle pathlines were generated from the numerical velocity data and compared with corresponding flow-visualization pictures. A vortex formed at the inlet to the ventricle in both cases: During the filling phase, the vortex expanded and traveled toward the apex of the ventricle until, at the end of filling, the vortex occupied the full radial extent of the ventricle; the vortex continued to travel once the filling process had ended. The vortices in vitro were more circular in shape and occupied a smaller volume than those generated by the numerical model. Nevertheless, comparison of the trajectories of the vortex centres showed that there was good agreement for the three conditions studied. Postprocessing of velocity data from the numerical solution yielded wall shear-stress measurements and particle pathlines that clearly illustrate the mass-transport qualities of the traveling vortex structure. For the cases considered here, the vortex transit produced a time-dependent shear stress distribution that had a peak value of 20 dynes cm-2, with substantially lower levels of shear stress in those regions not reached by the traveling vortex. We suggest that vortex formation and travel could reduce the residence time of fluid within a skeletal muscle ventricle, provided that the vortex travels the complete length of the ventricle before fluid is ejected at the start of the next cycle.