John Watterson

Numerical investigation of the haemodynamics at a patched arterial bypass anastomosis

Intimal hyperplasia at arterial bypass graft anastomoses is a major factor responsible for graft failure. A revised surgical technique, incorporating a Taylor vein patch into the distal anastomosis of PTFE grafts, results in a decrease in intimal hyperplasia and improved patency rates. Numerical simulations of pulsatile, non-Newtonian blood flow through life-like femorodistal bypass models have been performed to determine whether haemodynamic benefits arise from the modified geometry of the Taylor anastomosis. In a conventional bypass, the distal anastomotic flow exhibited considerable spatial and temporal variations. Steep spatial gradients in the shearing force acted along the floor during systole. The effect of the Taylor geometry was to reduce gradually the momentum of the blood approaching the junction. Thus, flow disturbances were abated, undesirable flow separation at the toe was diminished, and a less adverse floor shear stress distribution prevailed in that case. Intimal thickening should be alleviated at the toe in the Taylor model where separation is reduced, and where the thrombogenic graft surface is replaced with a vein patch. Intimal hyperplasia on the floor may be inhibited in the Taylor model due to more favourable shear stresses. The improved flow through the patched anastomosis should contribute to its enhanced performance.

A Comparison of the Flow Structures and Losses Within Vaned and Vaneless Stators for Radial Turbines

This paper details the numerical analysis of different vaned and vaneless radial inflow turbine stators. Selected results are presented from a test program carried out to determine performance differences between the radial turbines with vaned stators and vaneless volutes under the same operating conditions. A commercial computational fluid dynamics code was used to develop numerical models of each of the turbine configurations, which were validated using the experimental results. From the numerical models, areas of loss generation in the different stators were identified and compared, and the stator losses were quantifaed. Predictions showed the vaneless turbine stators to incur lower losses than the corresponding vaned stator at matching operating conditions, in line with the trends in measured performance. Flow conditions at rotor inlet were studied and validated with internal static pressure measurements so as to judge the levels of circumferential nonuniformity for each stator design. In each case, the vaneless volutes were found to deliver a higher level of uniformity in the rotor inlet pressure field.

Predicting turbulent flow in a staggered tube bundle

Abstract This paper presents the results of calculations performed for the turbulent, incompressible flow around a staggered array of tubes for which carefully obtained experimental results are available as part of an established ERCOFTAC-IAHR test case. The Reynolds-averaged Navier–Stokes equations are solved using a pressure-based finite volume algorithm, using collocated cell vertex store on an unstructured and adaptive mesh of tetrahedra. Turbulence closure is obtained with a truncated form of a low-Reynolds number k – e model developed by Yang and Shih. The computational domain covers all seven rows of tubes used in the experimental study and periodic flow is allowed to develop naturally. The results of the computations are surprisingly good and compare favourably with results obtained by others using a wide range of alternative k – e models for a single cylinder with periodic inflow and outflow boundaries on structured meshes.

Is there a haemodynamic advantage associated with cuffed arterial anastomoses

The development of intimal hyperplasia at arterial bypass graft anastomoses is a major factor responsible for graft failure. A revised surgical technique, involving the incorporation of a small section of vein (vein cuff) into the distal anastomosis of PTFE grafts, results in an altered distribution of intimal hyperplasia and improved graft patency rates, especially for below-knee grafts. Numerical simulations have been conducted under physiological conditions to identify the flow behaviour in a typical cuffed bypass model and to determine whether the improved performance of the cuffed system can be accounted for by haemodynamic factors. The flow patterns at the cuffed anastomosis are significantly different to those at the conventional end-to-side anastomosis. In the former case, the flow is characterised by an expansive, low momentum recirculation within the cuff. Separation occurs at the graft heel, and at the cuff toe as the blood enters the recipient artery. Wall shear stresses in the vicinity of the cuff heel are low, but high shear stresses and large spatial gradients in the shearing force act on the artery floor during systole. In contrast, a less disturbed flow prevails and the floor shear stress distribution is less adverse in the conventional model. In conclusion, aspects of the anastomotic haemodynamics are worsened when the cuff is employed. The benefits associated with the cuffed grafts may be related primarily to the presence of venous material at the anastomosis. Therefore, caution is advised with regard to the use of PTFE grafts, pre-shaped to resemble a cuffed geometry.

Computational and Experimental Simulations of the Haemodynamics at Cuffed Arterial Bypass Graft Anastomoses

Abstract The development of intimal hyperplasia at arterial bypass graft anastomoses is a major factor responsible for graft failure. A revised surgical technique, involving the incorporation of a small section of vein (vein cuff) into the distal anastomosis of polytetrafluoroethylene (PTFE) grafts, alters the distribution of intimal hyperplasia and improves graft performance. Numerical and in vitro flow visualization experiments have been conducted to identify the flow behaviour in the cuffed bypass model and to determine whether the improved performance of the cuffed system can be accounted for by haemodynamic factors. The flowfield at the cuffed anastomosis is characterized by an expansive recirculation. Separation occurs at the graft heel, and at the cuff toe as the blood enters the recipient artery. Wall shear stresses in the vicinity of the cuff heel are low, but high shear stresses and large spatial gradients in the shearing force act for a time on the artery floor. In the conventional model, a less disturbed flow prevails while the gradients of shear stress on the floor are smaller. Aspects of the anastomotic haemodynamics are worsened when the cuff is employed. The superior patency rates of cuffed bypasses may not be explained purely on the basis of local haemodynamic factors.

Two-phase co-current flow in inclined pipe

Abstract Data are presented on horizontal and slightly inclined flows at +5 and −5° for a 0.058 m inner diameter (i.d.) pipe with the co-current air–water system. The prediction capabilities of existing flow regime maps were shown to be inadequate. However, the transitions for stratified ripple to role wave, for slug to blow-through-slug, for film plus droplet to stratified, and the modified maps for stratified type to slug flows all gave good prediction performance with horizontal and slightly inclined flows. The largest liquid hold-up occurred in upward flow except at high gas rates and low liquid rates where the downflow condition gave the highest liquid hold-up. The lowest liquid hold-up occurred in downward flow at low gas flow rates and horizontal flow at high gas flow rates. Hold-up prediction proved to be flow regime dependent. The inclined total average pressure drop data crossed over the horizontal data from higher to lower values with increasing gas rate at a gas rate of just under VSG = 10 m s−1. Below this gas rate the horizontal pipe gave the lowest pressure drop while above this gas rate the upwardly inclined pipe gave the lowest pressure drop. A pressure loss minimum occurred at VSG = 10 m s−1 for upward flows. Below VSG = 10 m s−1 the pressure loss for downward flow was virtually independent of gas rate being mainly due to hydrostatic head. As the gas flow approached VSG = 50 m s−1 there was very little effect of inclination on the pressure loss. Pressure drop was successfully predicted although the accompanying hold-up prediction was not always reliable.

A Solution Adaptive Mesh Procedure for Predicting Confined Explosions

SUMMARY Explosion hazards constitute a significant practical problem for industry. In response to the need for better-resolved predictions for confined explosions, and particularly with a view to advancing safety cases for offshore oil and gas rigs, an existing unstructured, adaptive mesh, finite volume Reynolds-averaged Navier‐Stokes computational fluid dynamics code (originally developed to handle non-combusting turbomachinery flows) has been modified to include a one-equation, eddy break-up combustion model. Two benefits accrue from the use of unstructured, solution-adaptive meshes: first, great geometrical flexibility is possible; second, automatic mesh adaptation allows computational effort to be focused on important or interesting areas of the flow by enhancing mesh resolution only where it is required. In the work reported here, the mesh was adaptively refined to achieve flame front capture, and it is shown that this results in a 10%‐33% CPU saving for two-dimensional calculations and a saving of between 57% and 70% for three-dimensional calculations. The geometry of the three-dimensional calculations was relatively simple, and it may be expected that the use of unstructured meshes for truly complex geometries will result in CPU savings sufficient to allow an order-of-magnitude increase in either complexity or resolution.

A model of airflow in the nasal cavities: Implications for nasal air conditioning and epistaxis.

Background A friction force is generated when moving air contacts the nasal walls, referred to as wall shear stress. This interaction facilitates heat and mass transfer between the mucosa and air, i.e., air-conditioning. The objective of this research was to study the distribution of wall shear stress within the nasal cavity to identify areas that contribute significantly to air-conditioning within the nasal cavity. Methods Three-dimensional computational models of the nasal airways of five healthy subjects (three male and two female subjects) were constructed from nasal CT scans. Numerical simulations of nasal airflow were conducted using the commercial computational fluid dynamics code Fluent 6 (Ansys, Inc., Canonsburg, PA). Wall shear stress was derived from the numerical simulation. Air-conditioning was simulated to confirm the relationship with wall shear stress. Results Nasal airflow simulations predicted high wall shear stress along the anterior aspect of the inferior turbinate, the anteroinferior aspect of the middle turbinate, and within Littles area. Conclusion The airflow simulations indicate that the inferior and middle turbinates and Littles area on the anterior nasal septum contribute significantly to nasal air-conditioning. The concentration of wall shear stress within Littles area indicates a desiccating and potentially traumatic effect of inhaled air that may explain the predilection for spontaneous epistaxis at this site.

Steady-State Rimming Flow of the Generalized Newtonian Fluid

Rimming flow of a liquid polymer on the inner surface of a horizontal rotating cylinder is investigated. Using a scale analysis, a theoretical description for steady-state non-Newtonian flow is obtained. Simple lubrication theory is applied since the Reynolds number is small and the liquid film thin. Since a steady-state viscometric flow is considered, the general constitutive law requires only a single function relating shear stress and shear rate that corresponds to a generalized Newtonian liquid. For this case the existence of a continuous steady-state solution is proved. The properties of the solution for the different flow regimes are discussed. Numerical results are carried out for the Carreau–Yasuda model, which exhibits the Newtonian behavior at low shear rates with transition to power-law shear thinning at moderate shear rates.

Sub-Boundary Layer Vortex Generator Control of a Separated Diffuser Flow

*† ‡ § The application of sub-boundary layer vortex generators to control flow separation in a diffuser with an opening angle of 10 degrees has been studied using the computational fluid dynamics (CFD) code Fluent 6™. Experimental data is available for the uncontrolled flow in the diffuser. The section of the duct upstream of the diffuser has a height H equal to 15 mm; its length and breadth are 101H and 41H respectively; the diffuser has an expansion ratio of 4.7:1. Fully developed flow is achieved upstream of the diffuser. Sub-boundary layer vortex generators with a trailing edge span of 2 mm (13.3% of the duct height) have been considered. A parametric study was performed in which generator spacing, angle of incidence, configuration (counter and co-rotating) and streamwise position were varied. The best results were obtained with a counter-rotating array of generators at 18° incidence with a spacing equal to 2.5 times the generator trailing edge span (5H/16) and placed just upstream of the entrance to the diffuser.