Assessment of wall shear rate measurements with ultrasound

An experimental investigation of steady flow of a red blood cell suspension downstream of a backward-racing step is presented. The velocity and shear rate distribution in this geometry can be regarded as representative for flow in large- and medium-sized curved and bifurcation arteries. The velocity distribution is measured by means of ultrasound velocimetry. In analogy to clinical procedures, the maximum value of the first derivative of measured velocity profile relative to the vessel diameter is used to estimate wall shear rate. A numerical investigation of the velocity field is presented to validate the procedure to determine the wall shear rate. Both an inelastic non-Newtonian shear thinning model, accounting for the shear-rate-dependent viscosityn of the red blood cell suspension, and a Newtonian model, based on a characteristic viscosity, are applied to compute the velocity field. In general, comparison of the numerical and experimental results reveals that the velocity distribution of the red blood cell suspension can be described quite accurately by the computational models. At near-wall sites, however, some deviations are found that can be attributed to the experimental technique. The clinical procedure to determine shear rates yields an accurate prediction of the shear rate in central part of the channel. At a certain distance from the wall, however, the procedure underestimates shear rates. This distance is related to the dimensions of the ultrasound sample volume. Thew actual wall shear rate in fully dveloped flow in the experimental setup is underestimated by approximately 25%