Measurements of steady flow velocity and turbulent stress downstream from jellyfish and St. Vincent aortic heart valves

Abstract

An experimental investigation was conducted to determine the velocity fields, magnitudes of shear stresses, and regions of stagnation of a jellyfish valve and a St. Vincent tilting-disk valve using laser Doppler anemometry (LDA). All experiments were performed in vitro and at steady volumetric flow rates of 15 and 261/min (representing peak systole). The St. Vincent valve flow field was very unsymmetrical in the measurement plane that spanned the major and minor outflow regions of the valve and persisted at least to 1D downstream. For both flow rates tested, a stagnation region was always observed just behind the occluder. The maximum axial velocity was in the major outflow side and reached 2.54 m/s for peak systole flow of 261/min. Moreover, in the immediate vicinity of the valve and for both flow rates tested, turbulence intensities and velocity gradients were higher in the minor outflow region than in the major outflow region. However, as the flow progressed downstream, the opposite was observed, with large peak velocity in the major outflow. The maximum shear stress across the St. Vincent valve occurred in the minor outflow region and increased from 30 to 60N/m2 as the flow rate increased from 15 to 261/min. In the core of the larger jet, the shear stresses were very small (0–3N/m2). The flow at the edges of the jellyfish valve membrane, 10 mm (0.5D) downstream from the ring valve, consisted of two nearly symmetric jets in the vicinity of the tube wall. The maximum axial velocity in these jet regions increased from 1.7 to 2.5 m/s as the flow rates increased from 15 to 261/min. Pressure effects due to the oscillation of the membrane of the jellyfish valve appear to generate high shear stress in the immediate vicinity of the jellyfish valve (0.5D downstream). The values of shear stress were 0–27 N/m2 for a flow of 151/min and 3–109 N/m2 for a flow of 261/min. However, as the flow progresses downstream, shear stresses decay rapidly and return to the upstream undisturbed level at about 4D downstream, but at a slower rate than the RMS axial velocities. In general, for all operating flow conditions tested here, the jellyfish valve performed better than the St. Vincent valve when velocity and shear stress distributions are compared at locations more than 0.5D downstream from the valve.