Influence of elastic tube deformation on flow behavior of a shear thinning fluid

The non-Newtonian fluid flow characteristics in collapsible elastic tubes are useful to understand biofluid mechanics in human body. We investigated the flow behavior of a shear thinning carboxymethyl-cellulose aqueous solution in a collapsible elastic tube immersed in water filled chamber under compressive transmural (internal minus external) pressure. The collapsed elastic tube shape is determined by computer tomography based image analysis and velocity profiles are monitored using pulsed ultrasound Doppler velocimetry technique. As external pressure increases at a steady volume flow rate, the elastic tube collapses from elliptical to line or area contacted two lobes shape. Since the outlet pressure in collapsed tube with reduced cross sectional area is lower, the compressive transmural pressure is more negative at downstream than that at upstream. When the downstream transmural pressure is −18 mbar, the collapsed tube cross sectional area A reduces to one-sixth of the undeformed tube. The corresponding maximum flow velocity in the collapsed tube is about twice than that in undeformed tube. The similarity tube law represents well the experimental variation in A with the transmural pressure. The tube shape and velocity profile are measured along the tube length, the velocity profiles being bi-modal in the two lobes (no line or area contact) shape over the tube length of about 8 cm starting from 5 cm near the downstream. The velocity profiles gradually turned into uni-modal as the tube deformed into elliptical shape after 13 cm length. Consequently, the average shear rate near the tube wall increased from 79 to 243 s−1 thereby reducing the viscosity from 0.11 to 0.087 Pa s at the most collapsed region along the tube length 5–9 cm from downstream. The shear thinning solution viscosity decreases as the cross sectional area reduces. The periodic flow characteristics during ramp-up and ramp-down are governed by the volume flow rate and applied external chamber pressure. The maximum velocity at the undeformed elastic tube center increases an order of magnitude as time elapses in a periodic flow cycle.

► Elastic tube collapses from elliptical to line or area contacted two lobes shape as the external pressure increased.
► The collapsed tube cross sectional area was found to be reduced to one-sixth of the undeformed tube.
► Similarity tube law represented well the experimental variation in tube cross sectional area with the transmural pressure.
► Velocity profiles being bi-modal in two lobes and gradually turned into uni-modal as the tube deformed into elliptical shape.
► The shear thinning solution viscosity decreases as the tube cross sectional area reduces.