Applied Bionics and Biomechanics

Volume 2015, Article ID 406195, 12 pages

http://dx.doi.org/10.1155/2015/406195

## Biorheological Model on Flow of Herschel-Bulkley Fluid through a Tapered Arterial Stenosis with Dilatation

Department of Mathematics, National Institute of Technology, Tiruchirappalli, Tamilnadu 620015, India

Received 19 September 2014; Accepted 18 February 2015

Academic Editor: Cecilia Laschi

Copyright © 2015 S. Priyadharshini and R. Ponalagusamy. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

An analysis of blood flow through a tapered artery with stenosis and dilatation has been carried out where the blood is treated as incompressible Herschel-Bulkley fluid. A comparison between numerical values and analytical values of pressure gradient at the midpoint of stenotic region shows that the analytical expression for pressure gradient works well for the values of yield stress till 2.4. The wall shear stress and flow resistance increase significantly with axial distance and the increase is more in the case of converging tapered artery. A comparison study of velocity profiles, wall shear stress, and flow resistance for Newtonian, power law, Bingham-plastic, and Herschel-Bulkley fluids shows that the variation is greater for Herschel-Bulkley fluid than the other fluids. The obtained velocity profiles have been compared with the experimental data and it is observed that blood behaves like a Herschel-Bulkley fluid rather than power law, Bingham, and Newtonian fluids. It is observed that, in the case of a tapered stenosed tube, the streamline pattern follows a convex pattern when we move from to and it follows a concave pattern when we move from to . Further, it is of opposite behaviour in the case of a tapered dilatation tube which forms new information that is, for the first time, added to the literature.

#### 1. Introduction

Blood flow through a stenosed artery is one of the important areas of research because a stenosed artery affects the entire cardiovascular system. Aortic stenosis causes chest pain and decreased blood flow to the brain resulting in loss of consciousness and heart failure which increases the risk of death. It is well known that fluid dynamical factors play a pivotal role in the formation and development of stenosis. Young [1] and Young and Tsai [2] studied the effects of stenosis on blood flow through arteries. Several investigators [3–10] analyzed the blood flow through a stenosed artery and have shown that the physical parameters affect the blood flow. Pulsatile flow of blood through a stenosed porous medium under the influence of periodic body acceleration considering blood as a Newtonian fluid has been studied by El-Shahed [11]. El-Shehawey et al. [12] have examined the pulsatile flow of blood through a tube considering blood as a Newtonian fluid taking into account the body acceleration and porosity of the tube. Sharma et al. [13] investigated the effects of radial variation of hematocrit and magnetic field on the flow of blood as a Newtonian fluid through a porous medium in a stenosed artery.

Viscoplastic materials are concentrated suspensions of solid particles or macromolecules and are classified as generalized Newtonian fluids. They flow like liquids when subjected to a stress above a critical value but respond as elastic or inelastic solids below this critical stress. According to the von Mises yield criterion, flow is assumed to occur when the second invariant of the stress exceeds the so-called yield stress [4]. It is understood that the important time-independent non-Newtonian fluid possessing a fluid behavior index (power law index) and yield values is the Herschel-Bulkley fluid, which has pivotal applications in polymer processing industries [12], developing blood oxygenators, and biomechanics [4]. Further, Herschel-Bulkley fluids include both shear thinning and shear thickening materials. The practical examples of such materials are greases, colloidal suspensions, starch pastes, tooth pastes, paints, and blood flow in an artery. These fluids have been useful as lubricant in roller bearing [13].

The non-Newtonian behavior of blood has been considered and studied by [14–17]. Chaturani and Samy [18] investigated the effects of non-Newtonian nature of blood treating it as a Casson’s fluid and pulsatility on flow through a stenosed tube. The two-dimensional flow of power law fluid in stenosed arteries has been studied and the effect of power law index on the flow separation and reattachment point has been thoroughly investigated [19]. Nadeem et al. [20] and Ismail et al. [21] have investigated blood flow through a tapered artery with a stenosis assuming the blood as a non-Newtonian power law fluid model. They analyzed the influences of different parameters (power law index, flow rate, stenosis shape, and stenosis height) in different types of tapered arteries (converging tapered, diverging tapered, and nontaperted artery). Pincombe et al. [22] proposed a fully developed one-dimensional casson flow through a stenosed artery with multiple abnormal segments. They have studied the effects of multiple stenoses and poststenotic dilatation on non-Newtonian blood flow in small arteries. Scott Blair and Spanner [23] have suggested that blood obeys Casson’s model only for moderate shear rate flows and that there is no difference between Casson’s and Herschel-Bulkley plots over the range where Casson’s plot is valid (for blood). Furthermore, Sacks et al. [24] have experimentally pointed out that blood shows the behavior characteristic of a combination of Bingham-plastic and pseudoplastic fluid-Herschel-Bulkley fluid with the fluid behavior index greater than unity. In view of the experimental observation [24] and suggestion made in [23], it is pertinent to consider the behavior of blood as a Herschel-Bulkley fluid.

The non-Newtonian aspects of blood flow through stenosed arteries have been studied by [25] treating blood as a Herschel-Bulkley fluid. Biswas and Laskar [26] have investigated the steady flow of blood as a Herschel-Bulkley fluid through a stenosed artery. In these studies, the combined effects of the rheology of blood as Herschel-Bulkley fluid model, stenosis height, dilatation depth, and tapering on the flow of blood have not been investigated. Hence, the aim of the present paper is to analyze the flow of Herschel-Bulkley fluid in a tapered artery with stenosis and dilatation (Figure 2). The expressions for velocity, wall shear stress, and flow resistance have been derived. The effects of parameters such as power law index, shear dependent nonlinear viscosity, stenotic height, taper angle, dilatation depth, and the yield stress on physiologically important quantities, namely, wall shear stress and flow resistance, are presented graphically.

#### 2. Formulation of the Problem

Consider the steady and axially symmetric flow of an incompressible Herschel-Bulkley fluid lying in a tube having length (Figures 1 and 2). We take the cylindrical coordinate system in such a way that , , and are the velocity components in , , and directions, respectively. The equations governing the two-dimensional steady incompressible Herschel-Bulkley fluid are