Biomedical subjects

A Tordesillas

Publications and source records attributed to A Tordesillas.

2 recordsLinked to original sources

PubMed2005

Characterization of force chains in granular material.

It has been observed that the majority of particles in a granular material carries less than the average load and that the number of particles carrying larger than the average load decreases exponentially with increasing contact force. The particles carrying above average load appear to form a strong network of forces while the majority of particles belong to a weak network. The strong network of forces appear to have a spatial characteristic whereby the stronger forces are carried though chainlike particle groups referred to as force chains. There is a strong case for a connection between force chains of the discrete medium and the trajectory of the most compressive principal stress in its continuous idealization. While such properties seem obvious from descriptive analysis of physical and numerical experiments in granular media, progress in quantification of the force chain statistics requires an objective description of what constitutes a force chain. A procedure to quantify the occurrence of force chains is built on a proposed definition having two parts: first, the chain is a quasilinear arrangement of three or more particles, and second, along the chain, stress concentration within each grain is characterized by the vector delineating the most compressive principal stress. The procedure is incorporated into an algorithm that can be applied to large particle assemblies to compile force chain statistics. The procedure is demonstrated on a discrete element simulation of a rigid punch into a half space. It was found that only approximately half of the particles within the group of so-called strong network particles are part of force chains. Throughout deformation, the average length of force chains varied slightly but the number of force chains decreased as the punch advanced. The force chain lengths follow an exponential distribution. The procedure provides a tool for objective analysis of force chains, although future work is required to incorporate branching of force chains into the analysis.

Journal Article↗

PubMed1998

The use of the dimensionless Womersley number to characterize the unsteady nature of internal flow.

Dimensionless numbers are very useful in characterizing mechanical behavior because their magnitude can often be interpreted as the relative importance of competing forces that will influence mechanical behavior in different ways. One dimensionless number, the Womersley number (Wo), is sometimes used to describe the unsteady nature of fluid flow in response to an unsteady pressure gradient; i. e., whether the resulting fluid flow is quasi-steady or not. Fluids surround organisms which themselves contain fluid compartments; the behaviors exhibited by these biologically-important fluids (e.g. air, water, or blood) are physiologically significant because they will determine to a large extent the rates of mass and heat exchange and the force production between an organism and its environment or between different parts of an organism. In the biological literature, the use of the Womersley number is usually confined to a single geometry: the case of flow inside a circular cylinder. We summarize the evidence for a broader role of the Womersley number in characterizing unsteady flow than indicated by this geometrical restriction. For the specific category of internal flow, we show that the exact analytical solution for unsteady flow between two parallel walls predicts the same pattern of fluid behavior identified earlier for flow inside cylinders; i.e., a dichotomy in fluid behavior for values of Wo < 1 and Wo > 1. When Wo < 1, the flow is predicted to faithfully track the oscillating pressure gradient, and the velocity profiles exhibit a parabolic shape such that the fluid oscillating with the greatest amplitude is farthest from the walls ("quasi-steady" behavior). When Wo > 1, the velocity profiles are no longer parabolic, and the flow is phase-shifted in time relative to the oscillating pressure gradient. The amplitude of the oscillating fluid may either increase or decrease as Wo > 1, as described in the text.

Animals↗