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Efficiently Generating Mixing by Combining Differing Small Amplitude Helical Geometries

1
Department of Mechanical Engineering, University of Bath, Claverton Down, Bath BA2 7AY, UK
2
Department of Aeronautics, Imperial College London, South Kensington Campus, London SW7 2AZ, UK
*
Author to whom correspondence should be addressed.
Fluids 2019, 4(2), 59; https://doi.org/10.3390/fluids4020059
Received: 20 January 2019 / Revised: 12 March 2019 / Accepted: 14 March 2019 / Published: 27 March 2019
(This article belongs to the Special Issue Cardiovascular Flows)
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Abstract

Helical geometries have been used in recent years to form cardiovascular prostheses such as stents and shunts. The helical geometry has been found to induce swirling flow, promoting in-plane mixing. This is hypothesised to reduce the formation of thrombosis and neo-intimal hyperplasia, in turn improving device patency and reducing re-implantation rates. In this paper we investigate whether joining together two helical geometries, of differing helical radii, in a repeating sequence, can produce significant gains in mixing effectiveness, by embodying a ‘streamline crossing’ flow environment. Since the computational cost of calculating particle trajectories over extended domains is high, in this work we devised a procedure for efficiently exploring the large parameter space of possible geometry combinations. Velocity fields for the single geometries were first obtained using the spectral/hp element method. These were then discontinuously concatenated, in series, for the particle tracking based mixing analysis of the combined geometry. Full computations of the most promising combined geometries were then performed. Mixing efficiency was evaluated quantitatively using Poincaré sections, particle residence time data, and information entropy. Excellent agreement was found between the idealised (concatenated flow field) and the full simulations of mixing performance, revealing that a strict discontinuity between velocity fields is not required for mixing enhancement, via streamline crossing, to occur. Optimal mixing was found to occur for the combination R = 0.2 D and R = 0.5 D , producing a 70 % increase in mixing, compared with standard single helical designs. The findings of this work point to the benefits of swirl disruption and suggest concatenation as an efficient means to determine optimal configurations of repeating geometries for future designs of vascular prostheses. View Full-Text
Keywords: laminar flow; bypass graft; shunt; combined helical pipes; chaotic advection; streamline crossing; spectral/hp element method laminar flow; bypass graft; shunt; combined helical pipes; chaotic advection; streamline crossing; spectral/hp element method
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MDPI and ACS Style

Cookson, A.N.; Doorly, D.J.; Sherwin, S.J. Efficiently Generating Mixing by Combining Differing Small Amplitude Helical Geometries. Fluids 2019, 4, 59.

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