Research Papers: Fundamental Issues and Canonical Flows

Entropy Generation for Bypass Transitional Boundary Layers

[+] Author and Article Information
Richard S. Skifton

Department of Mechanical Engineering,
University of Idaho,
Idaho Falls, ID 83401
e-mail: skif8744@vandals.uidaho.edu

Ralph S. Budwig

Center for Ecohydraulics Research,
University of Idaho,
Boise, ID 83702
e-mail: rbudwig@uidaho.edu

John C. Crepeau

Department of Mechanical Engineering,
University of Idaho,
Moscow, ID 83844
e-mail: crepeau@uidaho.edu

Tao Xing

Department of Mechanical Engineering,
University of Idaho,
Moscow, ID 83844
e-mail: xing@uidaho.edu

Contributed by the Fluids Engineering Division of ASME for publication in the JOURNAL OF FLUIDS ENGINEERING. Manuscript received January 28, 2016; final manuscript received November 4, 2016; published online February 14, 2017. Assoc. Editor: Oleg Schilling.

J. Fluids Eng 139(4), 041203 (Feb 14, 2017) (13 pages) Paper No: FE-16-1061; doi: 10.1115/1.4035223 History: Received January 28, 2016; Revised November 04, 2016

The principal purpose of this study is to understand the entropy generation rate in bypass, transitional, boundary-layer flow better. The experimental work utilized particle image velocimetry (PIV) and particle tracking velocimetry (PTV) to measure flow along a flat plate. The flow past the flat plate was under the influence of a negligible “zero” pressure gradient, followed by the installation of an adverse pressure gradient. Further, the boundary layer flow was artificially tripped to turbulence (called “bypass” transition) by means of elevated freestream turbulence. The entropy generation rate was seen to behave similar to that of published computational fluid dynamics (CFD) and direct numerical simulation (DNS) results. The observations from this work show the relative decrease of viscous contributions to entropy generation rate through the transition process, while the turbulent contributions of entropy generation rate greatly increase through the same transitional flow. A basic understanding of entropy generation rate over a flat plate is that a large majority of the contributions come within a wall coordinate less than 30. However, within the transitional region of the boundary layer, a tradeoff between viscous and turbulent dissipation begins to take place where a significant amount of the entropy generation rate is seen out toward the boundary layer edge.

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Fig. 1

Schematic of flat plate configurations for the three experiments performed. Not to scale. ZPG: flat plate only (solid black), ZPG with TG: turbulence generator added (solid gray), and APG with TG: diverging plate added (dashed gray).

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Fig. 2

PIV field of view schematic diagram for both the large (a) and mezzo (b) fields of view

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Fig. 3

Data at the extreme downstream location of the APG with TG flow configuration. It shows a considerably large viscous sublayer within the MFOV that finally breaks away from the y+ = U+ curve at around y+ = 5 – 6, and stitches together with the LFOV data setup to a large wake region.

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Fig. 4

Convergence study for PIV and PTV sample size

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Fig. 5

Streamwise variations of integral quantities. (a), (b), and (c) boundary layer and integral parameters of flat plate flow for three flow conditions. (d), (e), and (f) freestream velocity, acceleration parameter, K, Reynolds number based on distance from leading edge and based on momentum thickness, and FSTI for all three cases. ((a) and (d)) ZPG without TG. ((b) and (e)) ZPG with TG. ((c) and (f)) APG with TG. Note: Vertical supports positioned in between each of the test section windows (exterior to the tunnel) blocked the camera optics, and the effect can be seen as small gaps in between datasets.

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Fig. 6

Wall-normal mean velocity profile and Reynolds shear stress × 1000 ((a) through (c)), and normal to wall turbulence intensity profiles of x- and y-components of the flow ((d) through (f)). Colors show progression down the length of the plate, x. Solid curves correspond, generally, to the left label of the abscissa, while the dotted curves correspond to the right label. Results in figure are from the LFOV. ((a) and (d)) ZPG without TG. ((b) and (e)) ZPG with TG. ((c) and (f)) APG with TG.

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Fig. 7

Lin-log curve fit using Eq. (19) to measure streamwise-normal velocity gradient at the wall. Showing U+ versus y+ as the viscous sublayer. Data from APG with TG, MFOV, and at x = 855 mm.

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Fig. 8

Streamwise variation of skin friction coefficient and shape factor (H = δ*/θ) for the three cases shown. (a) ZPG without TG, (b) ZPG with TG, and (c) APG with TG.

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Fig. 9

Dissipation coefficient, Cd, for each flow condition. Also, individual contributions of each term of Eq. (5). A slight smoothing was applied: (a) ZPG w/out TG, (b) ZPG w/TG, and (c) APG w/TG.

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Fig. 10

Conceptual drawing showing the trends of the Reynolds shear stress and viscous terms in the dissipation coefficient, Cd, for a “bypass” transitional boundary layer for ZPG (–) and APG (- -) flow conditions

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Fig. 11

Typical uncertainty profile of streamwise velocity for the APG flow configuration at x = 0.15 m




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