Artificial heart valves, flow acceleration and shear stresses

In the reviewed literature, it was found that estimated turbulent shear stresses under pulsatile flow were higher than those found under steady flow conditions for the same valve. To find the possible cause of the flow acceleration on elevating turbulent shear stresses, mechanical energy (sum of kinetic and pressure energies) differences across the St. Vincent valve were calculated under both pulsatile and steady flow conditions at the same flow rate and compared. An Eulerian approach, which is preferred in fluid mechanics, was used to find temporal flow acceleration in the St. Vincent valve. Two sample windows of 10 ms were selected during the acceleration and deceleration phases of the St. Vincent valve where instantaneous flow rates were 15 or 26 l/m in (at cardiac outputs of 4, 5.5 and 7 l/min). These two instantaneous flow rates were identical to the flow rates established in steady flow investigation. Using the energy equations, kinetic and pressure energy differences across the valve in the pulsatile flow and steady flow and the difference were calculated. It was found that the energy of the flow due to the flow acceleration was: H(acc-dec(viscous-dissipation)-at 26l/min) = H(Pulsatile-acc-dec)-H(steady) = 3.655 Watts Due to the fact that size and shape of the valve, blood analogue fluid, shape and size of valve chambers and Reynolds numbers were the same in both steady and pulsatile flow investigations (for the instantaneous flow rate identical to that of steady flow), the differences between mechanical energies of the pulsatile flow and of the steady flow, H(acc-dec(viscous-dissipation)), into thermal as viscous dissipation.