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Research Papers: Flows in Complex Systems

Analysis of Aerothermal Characteristics of Surface Microstructures

[+] Author and Article Information
M. Kapsis

Department of Engineering Science,
University of Oxford,
Oxford OX2 0ES, UK
e-mail: marios.kapsis@eng.ox.ac.uk

L. He

Department of Engineering Science,
University of Oxford,
Oxford OX2 0ES, UK
e-mail: li.he@eng.ox.ac.uk

Contributed by the Fluids Engineering Division of ASME for publication in the JOURNAL OF FLUIDS ENGINEERING. Manuscript received September 15, 2017; final manuscript received November 9, 2017; published online January 9, 2018. Assoc. Editor: Kwang-Yong Kim.

J. Fluids Eng 140(5), 051104 (Jan 09, 2018) (11 pages) Paper No: FE-17-1585; doi: 10.1115/1.4038667 History: Received September 15, 2017; Revised November 09, 2017

Recent advances in manufacturing technologies, such as additive manufacturing (AM), have raised the potential of choosing surface finish pattern as a design parameter. Hence, understanding and prediction of aerothermal effects of machined microstructures (machined roughness) would be of great interest. So far, however, roughness has been largely considered as a stochastic attribute and empirically modeled. A relevant question is: if and how would shape of the machined roughness elements matter at such fine scales? In this paper, a systematic computational study has been carried out on the aerothermal impact of some discrete microstructures. Two shapes of configurations are considered: hemispherical and rectangular elements for a Reynolds number range typical for such structures (Re < 5000). Several validation cases are studied as well as the turbulence modeling and grid sensitivities are examined to ensure the consistency of the results. Furthermore, large eddy simulation (LES) analyses are performed to contrast the behavior in a well-established turbulent to a transitional flow regime. The results reveal a distinctive common flow pattern change (from an “open separation” to a “reattached separation”) associated with a drastic change of drag correlation from a low to a high loss regime. The results indicate a clear dependence of drag and heat transfer characteristics on the element pattern and orientation relative to the flow. The distinctive performance correlations with Reynolds number can be affected considerably by the element shape, for both a transitional and a turbulent flow regime. The results also consistently illustrate that conventional empirical stochastic roughness parameters would be unable to predict these trends.

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Figures

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Fig. 1

Case 4: domain boundaries

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Fig. 2

Case 4: example of inlet velocity profile at Rer = 4200

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Fig. 3

Examples of meshes around roughness elements (left-Circ, right-Rectkmax)

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Fig. 4

Mesh dependency study for C1 and C3. Reference (used) mesh sizes are 66,000 (2D) and 13 × 106 (3D), respectively.

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Fig. 5

Mesh dependency study for case 4. Reference (used) mesh size 2 × 106.

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Fig. 6

C1: Cylinder validation test cases (ReD = 40 from Ref. [23], rest from Ref. [22])

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Fig. 7

Results for validation cases C2 and C3: (a) C2: results for three elements, (b) C2: results for five elements, and (c) C3: results

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Fig. 8

Drag contribution of each roughness element. Each marker corresponds to an element: (a) Rer = 4200 (k–ω SST) and (b) Rer = 210 (laminar).

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Fig. 9

Drag increase with Reynolds number (k−ω SST)

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Fig. 10

Drag increase with Reynolds number (k−ω SST)

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Fig. 11

Flow patterns at midspan of Circ-last two elements downstream-(k−ω SST): (a) Rer = 1000 and (b) Rer = 2700

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Fig. 12

Drag increase with Reynolds number (k−ω SST)

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Fig. 13

Spanwise spacing effect at Rer = 4200 (k−ω SST): (a) drag and (b) heat transfer

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Fig. 14

Streamwise spacing effect at Rer = 4200 (k−ω SST): (a) drag and (b) heat transfer

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Fig. 15

Iso-surfaces of Λ2 criterion for Circ (left) and Rectkmax (right) when λst=6 (top) and 3 (bottom) at Rer = 4200: (a) Circ λst = 6r, (b) Rect λst = 6r, (c) Circ λst = 3r, and (d) Rect λst = 3r

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Fig. 16

Flow pattern at midspan for Circ (left) and Rectkmax (right) when λst=6 (top) and 3 (bottom) at Rer = 4200. Element numbering reflects element id as presented in Fig. 8(a) (k−ω SST).

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Fig. 17

Metrics increase with Reynolds number. •—onset of unsteady flow (implicit LES): (a) drag and (b) heat transfer.

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Fig. 18

Streamwise spacing effect at Rer = 200 (laminar): (a) drag and (b) heat transfer

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Fig. 19

Flow patterns at midspan for Circ (left) and Rectkmax (right) when λst=3 (bottom) and 6 (top) at Rer = 200. Element numbering reflects element id as shown in Fig. 8(b) (laminar).

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Fig. 20

Iso-surfaces of vorticity of Circ (left) and RectRa/λ (right) at Rer = 200 (laminar)

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Fig. 21

Iso-surfaces of Λ2 criterion (instantaneous) of Circ (left) and RectRa/λ (right) at Rer = 2700 (implicit LES)

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Fig. 22

Power spectra density of axial velocity downstream of the last element of Circ at Rer = 2700 (implicit LES)

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Fig. 23

Mesh dependency study for Circ and RectRa/λ at Rer = 2700 for implicit LES. Reference (used) mesh size 7 × 106.

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Fig. 24

Flow patterns (time averaged) at midspan for Circ-last two elements downstream (implicit LES)

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Fig. 25

Flow patterns (time averaged) at midspan for RectRa/λ-last two elements downstream (implicit LES)

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