Pulsatile flow in a planar channel with a one-sided semicircular constriction has been simulated using direct numerical simulation and large-eddy simulation. This configuration is intended as a simple model for studying blood flow in a constricted artery. Simulations have been carried out over a range of Reynolds numbers (based on channel height and peak bulk velocity) from 750 to 2000 and a fixed non-dimensional pulsation frequency of 0.024. The results indicate that despite the simplicity of the chosen geometry, the simulated flow exhibits a number of features that have been observed in previous experiments carried out in more realistic configurations. It is found that over the entire Reynolds number range studied here, the flow downstream of the constriction is dominated by the complex dynamics associated with two shear-layers, one of which separates from the lip of the constriction and other from the opposite wall. Computed statistics indicate that for Reynolds numbers higher than about 1000, the flow transitions to turbulence downstream of the region where the separated shear layers first reattach to the channel walls. Large fluctuations in wall pressure and shear stress have also been associated with this reattachment phenomenon. Frequency spectra corresponding to velocity and pressure fluctuations have been analysed in detail and these indicate the presence of a characteristic shear-layer frequency which increases monotonically with Reynolds number. For Reynolds numbers greater than 1000, this frequency is found to be associated with the periodic formation of vortex structures in the shear-layers and the impact of this characteristic shear-layer frequency on the dynamics of the flow is described in detail.

Numerical study of pulsatile flow in a constricted channel

Disorder distal to modeled stenoses in steady and pulsatile flow

The effect of posterior tibial slope was evaluated in a rotationally unconstrained anterior cruciate ligament-sacrificing total knee replacement (Ortholoc, Dow Corning Wright, Arlington, TN). Sloping the surface 7 degree posteriorly loosened the knee to varus-valgus and anterior-posterior stress in flexion. Eliminating posterior slope improved varus-valgus stability in flexion but slightly increased anterior-posterior laxity in extension. Values for the 3 degree posterior slope knee were between the values for 0 degree and 7 degree sloped surfaces, but not significantly different statistically from either. Total knee replacement designs with a posterior tibial slope are likely to promote anterior-posterior and varus-valgus laxity in flexion, and those with no posterior slope or 3 degree posterior slope are most likely to achieve normal stability in flexion and extension.

The effect of posterior tibial slope on knee stability after Ortholoc total knee arthroplasty.

Initiation of turbulence in models of arterial stenoses

Acoustic measurements obtained from sensitive microphones placed on the chest are used in a procedure to noninvasively diagnose coronary artery disease. Utilizing specially developed signal processing techniques, the spectral content of isolated diastolic heart sounds has been estimated, and these sounds usually show an increase in high-frequency components in patients with occlusive coronary arteries. In order to establish a theory for the origin of these spectral features, a sound source model has been developed which combines an incremental network model of the left coronary artery tree with a transfer function model describing arterial chamber resonant characteristics. The network model predicts flow in both normal and stenosed coronary arteries. From this flow information, the arterial chamber transfer function model predicts the development of acoustic signals from the chamber resonant characteristics. The transfer function of a segment of coronary artery demonstrates two resonance frequencies. These resonance frequencies depend on the length and diameter of the chamber segment, as well as on the distal hydraulic impedance loading the segment. >

Robert J. Tobin

Papers

Wall pressure spectra scaling downstream of stenoses in steady tube flow