Turbulent blood flow in artificial hearts and ventricular assist devices (VAD) [1, 2] cause red blood cell (RBC) damage, which is a major concern when designing prosthetic heart devices. Although the effects of specific turbulent flow characteristics on the blood cells are uncertain,[3-5] it is generally known that turbulence affects RBCs causing trauma and hemolysis – an effect that increases when cells are exposed to turbulent stresses.
Blood damage is mostly represented using empirical power law models that assume that hemolysis would be a function of the magnitude of the shear stresses and exposure time to high stresses. While turbulent stresses, the well-known Reynolds stresses, might be responsible for hemolysis, having the same effect as viscous stresses in laminar flows, other researchers have examined the size of flow eddies relative to the size of the RBCs in order to identify the mechanism responsible for RBC trauma. If the Reynolds stresses are similar in effect as viscous stresses are for laminar flow, then one expects to see a large increase in the hemolysis at some threshold value of the Reynolds stress, as a result of the exponential feature of the power law relationship. However, the value of a threshold turbulent stresses for hemolysis has been controversial. The significance of Reynolds stresses and viscous stresses on hemolysis might be identified by performing a threshold analysis. In this study, we investigate the effect of time averaged, area averaged Reynolds stresses and viscous stresses to hemolysis by doing a threshold analysis for 5 Couette viscometer experiments and 4 capillary tube experiments. Moreover, we have also searched for relationships with hemolysis using extensive quantities, such as the total surface area from subpopulation distributions of eddies with different Kolmogorov Length Scales (KLS) for 13 jet experiments as an addition to Couette and capillary tube experiments. The relation between the hemolysis seen experimentally to the eddy surface area and the Reynolds and viscous stresses in the flow were investigated by using Reynolds-Averaged Navier-Stokes models of turbulence (k-ε and k-ω SST turbulence models, and finite volume techniques) to simulate different experimental conditions in a Couette viscometer, a capillary tube, and jet. Simulation results showed that for both capillary flow and flow in a Couette viscometer, there was no evidence of a threshold for hemolysis in terms of Reynolds and viscous stresses. Therefore, Reynolds and viscous stresses are not good predictors of hemolysis. Eddy analysis results of Couette, capillary, and jet experiments showed that hemolysis is related directly to the occurrence of eddies with diameters of up to about 10 µm. Thus, while it is easier to predict Reynolds stresses or total shear stresses in medical devices than to predict or measure KLS, the eddy size analysis presented can be applied in conjunction with various turbulence simulations, possibly across a wide range of conditions and devices. The prediction of the Kolmogorov eddy size distribution might then lead to an evaluation of whether a particular design of a medical device is more or less susceptible to hemolysis, and what changes need to be done in the design to increase the size of the Kolmogorov scales.
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6. Goubergrits L. Numerical modeling of blood damage: current status, challenges and future prospects. Expert Review of Medical Devices. 2006;3:527-31.
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