Richard S. Meyer

Cavitation inception in quiescent and co-flow nozzle jets

The prediction and scaling of cavitation inception in jets remains a difficult task. This paper presents findings of an experimental investigation to study the cavitation inception of quiescent and co-flow submerged jets. Experimental data were collected in the ARL/PSU 12-inch and 48-inch Diameter Water Tunnels. A submerged nozzle was mounted axially along the centerline of the test section with jet mass flow supplied using an external pump. The setup allowed for independent control of both jet and freestream velocities. Observations of cavitation patterns, inception locations and cavitation inception numbers for quiescent (VR=V∞/Vjet=0), near-quiescent (0<VR<0.1) and co-flow (VR>0.1) operating conditions were recorded. Data were measured using two- 25.4mm and one- 101.6mm diameter axisymmetric nozzles. Visual observations of cavitation indicated that the cavitation occurs in different location for co-flow jets compared to quiescent jets. PIV measurements show that different flow mechanisms are responsible for this cavitation inception.

Mean Velocity and Reynolds Stress Measurements in the Regurgitant Jets of Tilting Disk Heart Valves in an Artificial Heart Environment

AbstractLaser Doppler velocimetry, with a high temporal resolution (1 ms time windows), was used to measure the flow field in two regions (major and minor orifices) near the aortic and mitral valves (Bjork Shiley™ monostrut Nos. 25 and 27, respectively) of the Penn State artificial heart. The motion of each valve was also investigated using a 1000 frame/s video camera in order to estimate the valves closing velocity. Fluid velocities in excess of and opposite to valve closing velocity were detected near the valve, providing evidence of “squeeze flow.” Maximum Reynolds shear stresses of approximately 20,000 dyn/cm2 and time-averaged Reynolds shear stresses of approximately 2000 dyn/cm2 were observed during the regurgitant flow phase. These elevated Reynolds shear stresses suggest that regurgitant jets play a role in the hemolysis and thrombosis associated with tilting disk heart valves in an artificial heart environment.

Three-component laser doppler velocimetry measurements in the regurgitant flow region of a björk-shiley monostrut mitral valve

Three-dimensional laser Doppler velocimetry measurements were acquired in a mock-circulatory loop proximal to a Björk-Shiley monostrut valve in the mitral position, and synchronous ensemble-averaging was applied to form an “average” beat. Two axial locations in the regurgitant flow region of the valve (in the minor orifice) were mapped, and maximum Reynolds shear stresses were calculated. A large spike in regurgitant flow was noted at the beginning of systole, which may be thesqueeze flow phenomenon computed by other researchers. A region of sustained regurgitant flow 50 msec later was the focus of this study. Maximum velocities of ∼3.7 mps were noted, and maximum Reynolds shear stresses of ∼10,000 dyne/cm2 were calculated. Comparisons were made of two-dimensional (ignoring tangential component)versus three-dimensional shear stresses, and, in this case, in regions of high stress, the differences were insignificant. This suggests that the tangential component of velocity can probably be ignored in similar measurements where the tangential velocity is likely to be small.

In Vitro Quantification of Time Dependent Thrombus Size Using Magnetic Resonance Imaging and Computational Simulations of Thrombus Surface Shear Stresses

Thrombosis and thromboembolization remain large obstacles in the design of cardiovascular devices. In this study, the temporal behavior of thrombus size within a backward-facing step (BFS) model is investigated, as this geometry can mimic the flow separation which has been found to contribute to thrombosis in cardiac devices. Magnetic resonance imaging (MRI) is used to quantify thrombus size and collect topographic data of thrombi formed by circulating bovine blood through a BFS model for times ranging between 10 and 90 min at a constant upstream Reynolds number of 490. Thrombus height, length, exposed surface area, and volume are measured, and asymptotic behavior is observed for each as the blood circulation time is increased. Velocity patterns near, and wall shear stress (WSS) distributions on, the exposed thrombus surfaces are calculated using computational fluid dynamics (CFD). Both the mean and maximum WSS on the exposed thrombus surfaces are much more dependent on thrombus topography than thrombus size, and the best predictors for asymptotic thrombus length and volume are the reattachment length and volume of reversed flow, respectively, from the region of separated flow downstream of the BFS.

Experimental and Computational Studies of a Formed Thrombus Within a Backward-Facing Step Geometry

Heart disease is one of the leading causes of death in the United States. This condition affects roughly 5.7 million Americans, with approximately 670,000 new cases and 300,000 deaths each year [1]. Heart failure, resulting from heart disease, is primarily treated with the implantation of a ventricular assist device (VAD) [2]. Along with VADs, arterial stents (primarily for treatment of atherosclerosis) and prosthetic heart valves (for defects in or other failures of the native heart valves) are other devices that are regularly used by clinicians to treat conditions within the circulatory system. While complications relating to cardiovascular devices have seen a decrease over the years, thrombosis and thromboembolization still remain obstacles. These phenomena are dependent upon the blood/material interface, surface topography, and fluid mechanics within the device [3].Copyright