Advective mixing due to longitudinal and segmental contractions in the ileum of the rabbit

Abstract

In this work four types of local intestinal wall contractions were simulated using ANSYS Polyflow to numerically investigate how these non-propagating contractions, and their combined activity, affect the dynamics of intestinal flow and their ability to promote mixing. These contractions are categorised as a longitudinal contraction in the oral direction, a single and two adjacent segmental contractions of the wall, and the combined activity of a pendular displacement and a single segmental contraction. The fluid rheology in the lumen of the organ was Newtonian with viscosity μ=0.001Pas and μ=1Pas. The mixing properties of the flow were characterised by tracking the level of stretching experienced by differential material elements throughout the intestinal domain. The ability of the flow motions to advect (transport) a neutrally buoyant species, initially located at the wall, was also investigated. It was found that the longitudinal contraction, and in particular, its combination with a segmental contraction produced the greatest levels of mixing and transport in the case of low viscous fluids. In agreement with expectations, longitudinal contractions promote mixing and species transport in the vicinity of the wall, whereas its combined activity with a segmental contraction enhance these flow properties throughout the intestinal domain. For a fluid of high viscosity, the segmental contractions were found to enhance mixing above the level of longitudinal and even combined contractions, although it is noted that the advection of the tracer was greatest locally in the location of the segmental contractions.