Clifton R. Johnston

On the characterization of a non-Newtonian blood analog and its response to pulsatile flow downstream of a simplified stenosis.

Particle image velocimetry (PIV) was used to investigate the influence of a non-Newtonian blood analog of aqueous xanthan gum on flow separation in laminar and transitional environments and in both steady and pulsatile flow. Initial steady pressure drop measurements in laminar and transitional flow for a Newtonian analog showed an extension of laminar behavior to Reynolds number (Re)xa0~xa02900 for the non-Newtonian case. On a macroscale level, this showed good agreement with porcine blood. Subsequently, PIV was used to measure flow patterns and turbulent statistics downstream of an axisymmetric stenosis in the aqueous xanthan gum solution and for a Newtonian analog at Rexa0~xa0520 and Rexa0~xa01250. The recirculation length for the non-Newtonian case was reduced at Rexa0~xa0520 resultant from increased viscosity at low shear strain rates. At Rexa0~xa01250, peak turbulent intensities and turbulent shear stresses were dampened by the non-Newtonian fluid in close proximity to the blockage outlet. Although the non-Newtonian case’s recirculation length was increased at peak pulsatile flow, turbulent shear stress was found to be elevated for the Newtonian case downstream from the blockage, suggesting shear layer fragmentation and radial transport. Our findings conclude that the xanthan gum elastic polymer prolongs flow stabilization, which in turn emphasizes the importance of non-Newtonian blood characteristics on the resulting flow patterns in such cardiovascular environments.

The viscous behaviour of HES 130/0.4 (Voluven®) and HES 260/0.45 (Pentaspan®)

PurposeSeveral fluids are available for volume therapy to address hypovolemia. We focus on two hydroxyethyl starches (HES) available for volume expansion in Canada, HES 130/0.4 (Voluven®) and HES 260/0.45 (Pentaspan®). Although information is available regarding their pharmacokinetic and risk/benefit profiles, this paper examines their viscous properties.MethodsDynamic viscosities of HES 130/0.4 and HES 260/0.45 were measured through capillary viscometry at 21°C and 37°C. The viscosities of the solutions were then measured through a closed flow loop at room temperature across physiologically relevant flow rates that maintained a laminar flow regime.ResultsMeasured dynamic viscosity through capillary viscometry for HES 130/0.4 and HES 260/0.45 was 2.76 centipoises (cP) and 7.62 cP, respectively, at 21°C decreasing to 1.74 cP and 4.25 cP, respectively, at 37°C. Pipe flow analysis found that HES 130/0.4 (expiry 02/13) and HES 260/0.45 (expiry 10/10) displayed marginal variation in viscosity suggesting Newtonian behaviour. However, a sample of HES 130/0.4 (expiry 10/10) displayed an appreciable increase in viscosity (13%) at higher flow rates suggesting shear thickening behaviour.ConclusionThis study represents an innovative characterization of not only the viscosity of two commonly utilized HES solutions but also their viscous behaviour across physiologically relevant flow rates. The shear thickening behaviour of a sample of HES 130/0.40 (expiry 10/10) at high flow rates was not expected, and the effect this result may have on endothelial cell function is unknown.RésuméObjectifxa0Plusieurs liquides de remplissage sont disponibles pour le traitement d’une hypovolémie. Nous nous concentrerons sur deux produits d’expansion volumique à base d’amidon hydroxyéthylé (HES)xa0disponibles au Canada: le HES 130/0.40 (Voluven®) et le HES 260/0.45 (Pentaspan®). Bien qu’une information soit disponible sur leurs profils pharmacocinétiques et risques/bénéfices, cet article s’intéresse à leurs propriétés visqueuses.Méthodesxa0Les viscosités dynamiques du HES 130/0.4 et du HES 260/0.45 ont été mesurées par viscométrie capillaire à 21xa0°C et à 37xa0°C. La viscosité de ces solutions a ensuite été mesurée dans une boucle fermée à température ambiante avec des débits physiologiquement pertinents qui maintenaient un régime de flux laminaire.Résultatsxa0La viscosité dynamique mesurée par viscométrie capillaire pour le HES 130/0,4 et le HES 260/0,45 ont été, respectivement, de 2,76 centipoises (cP) et de 7,62 cP, à 21xa0°C, baissant respectivement à 1,74 cP et 4,25 cP à 37xa0°C. L’analyse du débit en conduite a montré que le HES 130/0,4 (date d’expiration: 02/13) et le HES 260/0,45 (date d’expiration: 10/10) présentaient une variation marginale de viscosité, suggérant un comportement newtonien. Cependant, un échantillon de HES 130/0,4 (date d’expiration: 10/10) a présenté une augmentation appréciable de la viscosité (13xa0%) pour des débits supérieurs, suggérant un comportement d’épaississement par cisaillement.ConclusionCette étude présente une caractérisation innovante, non seulement de la viscosité de deux solutions fréquemment utilisées d’HES mais encore de leur comportement pour des débits physiologiquement pertinents. Le comportement d’épaississement par cisaillement d’un échantillon de HES 130/0.40 (date d’expiration: 10/10) à des débits élevés était inattendu et les conséquences potentielles de ce résultat sur la fonction des cellules endothéliales est inconnu.

The Experimental Investigation of Flow Separation Downstream of an Axisymmetric Stenosis in Newtonian and Non-Newtonian Flow Environments

The assumption of Newtonian behavior to characterize circulating blood is applicable to large, healthy, arteries of the vasculature. However, at low shear strain rates, blood displays marked non-Newtonian behavior resulting from red blood cell adhesion. Consequently, the consideration of non-linear viscous behavior may be of critical importance to accurately quantify critical hemodynamic parameters in separated flow regimes associated with low shear strain rates (1). Walker et al. (1) measured noteworthy reductions in the propagation of negative axial velocities and oscillatory shear patterns downstream of a stent wire using a non-Newtonian blood analog while Choi and Barakat (2) used CFD to predict non-Newtonian reductions in recirculation length downstream of a backward facing step.Copyright

The Investigation of Flow Separation Downstream of a Slotted Tube Stent in the Presence of a Pulsatile Flow Environment

Stent strut configuration has been found to alter local fluid flow and wall shear stress (WSS) patterns that can promote intimal hyperplasia. To quantify these alterations, we used particle image velocimetry (PIV) to investigate local fluid dynamics distal to a deployed stent within an acrylic tube. A blood analogue mixture of glycerol and water was pumped through a flow loop in both steady and pulsatile conditions. Steady and pulsatile velocity profiles and near WSS (NWSS) predicted by PIV were in good agreement with the Poiseuille and Womersley estimations. Introduction of a Palmaz balloon expanded slotted tube stent increased centerline velocities between 6.9% and 9% and decreased NWSS distal from the stent outlet by > 40%. Peak normalized vorticity was similar between non-stented and stented flows, although spatial coverage of higher vorticity was increased upon stent introduction. Future work will incorporate Echo PIV to complement PIV findings on how strut configuration affects flow dynamics.Copyright