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Biomedical subjects

L W Ehrlich

Publications and source records attributed to L W Ehrlich.

7 recordsLinked to original sources
PubMed1985

Simple pulsatile flow in an artery with a constriction.

A smooth isolated, axisymmetric occlusion in a straight vascular tube is a tractable problem for pulsatile flow calculations via finite-difference approximations to the Navier-Stokes equation. Steady flow depends on the Reynolds number and two geometric parameters which describe the stenosis. The mere addition of a simple harmonic to the mean flow adds two more parameters. One is the reduced frequency, or Strokes number, and the other epsilon, the ratio of unsteady to steady flux. After describing steady stenosis flow examples, the dynamic patterns of pulsatile flow are illustrated indicating the inadequacy of basing hypotheses of atherosclerosis on mean (steady) flow.

Arterial Occlusive Diseases↗
PubMed1984

Numerical simulation of aortic bifurcation flows: the effect of flow divider curvature.

Two dimensional steady flow calculations in computational regions obtained from radiographs of human aortic bifurcations correlate well with unsteady measurements of wall shear in flow-through casts of the same vessels. The results suggest that wall slope may be an important factor affecting the variability of shear along the medial walls of this arterial segment. If extremes of shear stress promote atherogenesis, then variations in the curvature of the proximal iliac arteries may affect the susceptibility of these vessels to vascular disease on their medial aspect.

Aorta↗
PubMed1975

Calculations of pulsatile flow through a branch: implications for the hemodynamics of atherogenesis.

Numerical simulations of pulsatile blood flow through a symmetrical branch modeling the aortic bifurcation were carried out to assess several hemodynamic theories of atherogenesis by comparing the distribution of hemodynamic variables with that of early lesions in arterial branches. Considerable spatial and temporal variations in wall shear were found when the flow was pulsatile; the highest values occurred at the convex corner on the outer wall of the branch and in the neighborhood of the flow divider tip, and the lowest shears were experienced by the outer wall of the daughter vessel a short distance distal to the corner. Transient flow reversal occurred almost everywhere in the branch, and a transient separated region was found corresponding to the low-shear region in the daughter vessel. The shear profiles and the calculated separated region were influenced to some degree by the extent of flow development at the branch inlet and markedly by the branch area ratio. All of the proposed hemodynamic promoters of atherosclerosis that were examined--high shear, low shear, and separation--were found at sites in the branch where lesions commonly develop. Comparisons with a steady-flow calculation at the same mean flow rate showed that the severity of all of these proposed hemodynamic determinants was increased by pulsatility.

Arteries↗
PubMed1975

Effect of spatial variations in shear on diffusion at the wall of an arterial branch.

The effect of spatially varying shear on transport to the wall of a two-dimensional branch was examined, using oxygen as the test solute and the results of earlier fluid mechanical calculations to provide the shear profiles in a region characterizing the aortic bifurcation. The numerical technique employed allowed both blood-phase and mural resistances to solute uptake to be treated simultaneously and self-consistently. The calculated profiles of wall concentration and mural flux were significantly different from those which would have obtained if the shear had been uniform. The calculations suggest that, even when solute is rapidly taken up from the blood, the occasional high-shear and flow-development sites encountered along the arterial tree prevent the diffusion boundary layer adjacent to the wall from thickening to the point at which nutrition is compromised. The indirect effect of arterial geometry on transport, consequent to its direct effect on the magnitude and the distribution of the relevant hemodynamic variables, was illustrated using the branch area ratio as the geometric parameter. The shapes of the flux and interfacial concentration profiles along the branch wall were markedly dependent on the extent to which wall shear affected intimal permeability.

Animals↗