An investigation into the flow fields around bluff bodies and artificial heart valves

• Main Author: Hind, Andrew Kenneth
• Published: University of Edinburgh 1997
• Subjects: 617
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.652456

A milk analogue for clotting blood was developed by Lewis. This was then used by Christy, and subsequently Marosek, to investigate the clot deposited in the region of a series of bluff bodies and heart valves in both steady and pulsatile flows. Bodies of revolution, such as a teardrop, were used to simplify the flow structures present in the flow, making the relation of flow features to the deposition of clot more amenable to analysis. Christy concluded that stasis was a necessary but not sufficient condition for thrombus deposition. A quantitative flow visualisation technique known as particle image velocimetry (PIV) has been developed and applied to the flows investigated by Christy but with water as the test fluid. Steady flow PIV investigations around a Björk-Shiley valve showed significant three dimensional structure which varied markedly over time. In pulsatile flows, both at and below physiologically relevant flowrates, significant variation was observed between recordings made at the same point in successive pulse cycles. This may indicate that the assumptions underlying the sampling methods used in LDA investigations to estimate the Reynolds stresses downstream of heart valve prostheses in pulsatile flows are flawed. In pulsatile flows of a period of 1 Hz and a mean volumetric flowrate of 6.7x10<SUP>-5</SUP> m<SUP>3</SUP>/s, the maximum bulk viscous shear stresses determined using PIV were of the order of 0.05 N/m<SUP>2</SUP> with maximum values of 0.5 N/m<SUP>2</SUP> near the wall, well below the value expected to cause lysis of red blood cells even for extended exposure. In the same physiologically relevant flow conditions, the maximum viscous shear stress at the trailing tip of the valve designed by Dr Norman Macleod was again of the order of 0.5 N/m<SUP>2</SUP>. This level of wall shear is at the lower end of the range over which behavioural and shape changes are induced in endothelial cells and the adhesion of platelets to endothelial cells may be promoted. Particle image velocimetry is a powerful augmentation to the range of techniques available for assessing the performance of heart valve prostheses <I>in vitro. </I>