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Research Papers: Fundamental Issues and Canonical Flows

A Numerical Investigation of the Onset of Flow Separation in Round-Edge Diverging Tees

[+] Author and Article Information
Foo Kok

Department of Aeronautics and Astronautics,
University of Southampton - Malaysia Campus,
Gelang Patah 79200, Malaysia
e-mail: f.kok@soton.ac.uk

Roy Myose

Department of Aerospace Engineering,
Wichita State University,
Wichita, KS 67260-0044
e-mail: roy.myose@wichita.edu

Klaus A. Hoffmann

Department of Aerospace Engineering,
Wichita State University,
Wichita, KS 67260-0044
e-mail: klaus.hoffmann@wichita.edu

Contributed by the Fluids Engineering Division of ASME for publication in the JOURNAL OF FLUIDS ENGINEERING. Manuscript received December 12, 2016; final manuscript received September 14, 2017; published online October 24, 2017. Assoc. Editor: Samuel Paolucci.

J. Fluids Eng 140(3), 031206 (Oct 24, 2017) (9 pages) Paper No: FE-16-1816; doi: 10.1115/1.4038087 History: Received December 12, 2016; Revised September 14, 2017

The onset condition of flow separation in diverging tee junctions was investigated numerically. Flow separation and recirculation at the proximal region of a bypass graft can contribute to early phase graft failure in aortocoronary bypass (ACB) surgery. Rounding the inlet edge of the branch reduces the likelihood of flow separation and recirculation. The recirculating zone at the upstream end of the branch is fully eliminated when a threshold value of mass flow rate ratio is reached. The corresponding flow characteristics obtained from diverging tees with a diameter ratio ≤0.2 and a radius of curvature ≤ 0.25 for a Reynolds number ≤ 1817 indicate that an increasing flow rate ratio induces an exponential decrease in the recirculation length. An increase in the diameter ratio and Reynolds number increases both the onset condition of the flow separation and the recirculation length at the upstream end of the branch. However, a decrease in the diameter ratio reduces the onset condition of separation more effectively than a decrease in the radius of curvature at the junction.

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Figures

Grahic Jump Location
Fig. 1

Geometry and parameters of the diverging tee model

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Fig. 2

Computational grid for the flow field in the diverging tee models

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Fig. 3

(a) Comparison of the separation and reattachment points of the recirculation zones at the inner wall of the branch with different edge radii and (b) comparison of the velocity profiles between the numerical results and the experimental results for a diverging tee with R = 0 at Re = 469

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Fig. 4

Variation in the recirculation length with respect to the flow rate ratio for diverging tees with R = 0 and R = 0.2 at Re = 1817

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Fig. 5

The evolution of the separation and reattachment points of the recirculation zone with respect to the flow rate ratio for models with d2/d1 = 0.2 and R = 0.2 at 227 < Re < 1817. The minimum β corresponding to the separation-free condition is denoted by the critical flow rate ratio βcrit.

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Fig. 6

Distribution of the critical flow rate ratio βcrit plotted as a function of the normalized edge radius for the round-edge tee with d2/d1 = 0.167 at Re = 1817

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Fig. 7

Variation of the critical flow rate ratio βcrit as a function of the radius of curvature for models with different diameter ratios at Re = 1817. The dashed line shows the local minimum of each curve, which is denoted by the minimum critical flow rate ratio βmin,crit.

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Fig. 8

Variation in the critical flow rate ratio βcrit as a function of the normalized edge radius R for the round-edge tees with a diameter ratio d2/d1 = 0.133 at different Reynolds numbers

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