Steady flow through collapsible tubes: measurements of flow and geometry.

Compliant tubes attain a complex three-dimensional geometry when the external pressure exceeds the internal pressure and the tube is partially collapsed. A new technique for remote measurement of dynamic surfaces was applied to classical experiments with collapsible tubes. This work presents measurements of the three-dimensional structure of the tube as well as pressure and flow measurements during static loading and during steady-state fluid flow. Results are shown for two tubes of the same material and internal diameter but with different wall thicknesses. The measured tube laws compare well with previously published data and suggest the possible existence of a similarity tube law. The steady flow measurements did not compare well with the one-dimensional theoretical predictions.

Wall shear stress distribution along the cusp of a tri-leaflet prosthetic valve.

High levels of wall shear stress on the surface of valvular cusps can cause mechanical damage to the blood cells and the cusp surfaces. The shear stresses are also responsible for mechanical failure of prosthetic heart valves. Qualitative measurements of wall shear stress in the vicinity of the leaflets are thus essential for diagnosis of suspected complications and provide important information for the design and fabrication of bioprosthetic heart valves. For this purpose we measured the velocity distribution along the inside wall of the cusps of a tri-leaflet heart valve with a two colour laser Doppler anemometer system. The wall shear stresses on the cusp surface were computed and found to range from 80 to 120 N/m2 during the ejection phase. Wall shear stresses of up to 180 N/m2 were measured in loci of cusp flexure and the accelerated boundary layer. The results of this study show a correlation between the high shear stress loci and the clinically (animal) observed regions of cusp calcification.

In-vitro evaluation of a valveless cardiac-assist pump.

Diseases of the cardiovascular system are a major health-care problem. Nearly 50% of all deaths are due to cardiovascular disease. A number of cardiovascular patients experience heart failure, mostly of the left ventricle, and require some form of mechanical support. Cardiac-assist devices are used widely in the treatment of patients with damaged heart muscle. Cardiac-assist devices have been introduced recently to maintain circulation in heart failure patients, until a suitable heart transplant donor is found. Earlier types of left ventricular assist devices were the copulsation and the counterpulsation pumps, which increased the efficiency of the vascular system and decreased the workload on the failed heart. In this study, the authors investigated the performance of a valveless cardiac assist device that can be operated in copulsation, counterpulsation, or any intermediate mode with the left ventricle. The device has only one connection to the ascending aorta, no valves, and a common inlet/outlet. The authors measured the hemodynamic parameters in all modes and have suggested an optimal mode of operation in a left ventricular heart failure mode.

Novel instantaneous laser Doppler velocimeter.

A laser Doppler velocimeter capable of directly measuring instantaneous velocities is described. The new LDV uses a novel detection technique based on the utilization of a static slightly defocused spherical Fabry-Perot interferometer used in conjunction with a special mask for the detection of instantaneous Doppler frequency shifts. The essential characteristics of this LDV are discussed, and such a system recently developed is described. Results of turbulent flow measurements show good agreement with data obtained using hot wire anemometry.