An in vitro study on the effect of branch points on the stability of coronary artery flow.

Empirical correlations for the onset of turbulence in pulsatile flow through a straight tube and a 45 degrees T-bifurcation are presented. We pumped three different test fluids of kinematic viscosity 0.008-0.035 cm2 s-1 through four straight tubes 0.4-3.0 cm in diameter and three 45 degree T-bifurcations 0.45-2.2 cm in diameter. A Scotch yoke mechanism provided an oscillatory sine wave flow component of known stroke volume and frequency. To determine transition to turbulence, we adjusted the mean flow independently until we detected signal instabilities from hot film or electrochemical wall shear stress probes. The critical peak Reynolds number was found to correlate with two independent dimensionless groups: the Womersley parameter and the Strouhal number. We derived power low functions of these groups to provide an accurate and convenient method of predicting transition in both straight and bifurcating tubes. When compared to pulsatile flow through the straight tube, the presence of flow separation within the 45 degrees T-bifurcation induced flow instabilities at lower values of the peak Reynolds number. The correlation for the 45 degrees T-bifurcation is also a suitable model for predicting transition at coronary branch points, which we previously studied in an in vitro pulse duplicator. Flow instabilities at coronary branch points may play an important role in atherogenesis.