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Research Papers: Fundamental Issues and Canonical Flows

An Experimental Investigation on the Characteristics of Turbulent Boundary Layer Flows Over a Dimpled Surface

[+] Author and Article Information
Wenwu Zhou

Department of Aerospace Engineering,
Iowa State University,
Ames, IA 50011

Yu Rao

Gas Turbine Research Institute,
School of Mechanical Engineering,
Shanghai Jiao Tong University,
Shanghai 200240, China
e-mail: yurao@sjtu.edu.cn

Hui Hu

Department of Aerospace Engineering,
Iowa State University,
Ames, IA 50011
e-mail: huhui@iastate.edu

1Corresponding author.

Contributed by the Fluids Engineering Division of ASME for publication in the JOURNAL OF FLUIDS ENGINEERING. Manuscript received March 15, 2015; final manuscript received July 29, 2015; published online September 10, 2015. Assoc. Editor: Mark F. Tachie.

J. Fluids Eng 138(2), 021204 (Sep 10, 2015) (13 pages) Paper No: FE-15-1174; doi: 10.1115/1.4031260 History: Received March 15, 2015; Revised July 29, 2015

An experimental investigation was conducted to quantify the characteristics of the turbulent boundary layer flows over a dimpled surface in comparison to those over a conventional flat plate. In addition to measuring surface pressure distributions to determine the friction factors of the test plates and to map the surface pressure inside the dimple cavity, a high-resolution digital particle image velocimetry (PIV) system was used to achieve detailed flow field measurements to quantify the characteristics of the turbulent boundary layer flows over the test plates and the evolution of the unsteady vortex structures inside the dimple cavity at the middle of the dimpled test plate. It was found that the friction factor of the dimpled plate would be about 30–80% higher than that of the flat plate, depending on the Reynolds number of the test cases. In comparison with those over a conventional flat surface, the flow characteristics of the turbulent boundary layer flows over the dimpled surface were found to be much more complicated with much stronger near-wall Reynolds stress and higher turbulence kinetic energy (TKE) levels, especially in the region near the back rims of the dimples. Many interesting flow features over the dimple surface, such as the separation of oncoming boundary layer flow from the dimpled surface when passing over the dimple front rim, the formation and periodic shedding of unsteady Kelvin–Helmholtz vortices in the shear layer over the dimple, the impingement of the high-speed incoming flow onto the back rim of the dimple, and the subsequent generation of strong upwash flow in the boundary flow to promote the turbulent mixing over the dimpled surface, were revealed clearly and quantitatively from the PIV measurement results. The quantitative measurement results are believed to be the first of its nature, which depict a vivid picture about the unique flow features over dimpled surfaces and their correlations with the enhanced heat transfer performance reported in previous studies.

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Figures

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Fig. 1

Schematic of the two test plates used in the present study (unit in mm): (a) a dimpled test plate and (b) a conventional flat test plate

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Fig. 2

Experimental setup for PIV measurements

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Fig. 3

Measured velocity profiles of the oncoming boundary layer flow at the entrance of test plate for the test cases of Re = 8.2 K, Re = 36.7 K, and Re = 50.5 K, respectively (where U0 is the maximum flow velocity inside the channel)

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Fig. 4

Measured friction factors of the test plates a function of the Reynolds number

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Fig. 5

Measured surface pressure coefficient distributions inside the dimple cavity: (a) Re = 8.2 K, (b) Re = 36.7 K, and (c) Re = 50.5 K

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Fig. 6

The profiles of the surface pressure coefficient along the centerline of the dimple

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Fig. 7

PIV measurements of the flow field over the dimpled and flat surfaces at Re = 50.5 K: (a) Instantaneous velocity field over the dimpled (left) and flat (right) surfaces, (b) instantaneous vorticity distributions over the dimpled (left) and flat (right) surfaces, (c) mean velocity field over the dimpled (left) and flat (right) surfaces, and (d) in-plane TKE distribution over the dimpled (left) and flat (right) surfaces

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Fig. 8

PIV measurement results of the flow field inside the dimple at Re = 8.2 K: (a) instantaneous velocity field, (b) instantaneous vorticity distribution, (c) ensembles-averaged velocity field, (d) streamlines of the mean flow field, (e) normalized in-plane TKE distribution, and (f) normalized Reynolds stress distribution

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Fig. 9

PIV measurement results of the flow field inside the dimple at Re = 36.7 K: (a) instantaneous velocity field, (b) instantaneous vorticity distribution, (c) ensembles-averaged velocity field, (d) streamlines of the mean flow field, (e) normalized in-plane TKE distribution, and (f) normalized Reynolds stress distribution

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Fig. 10

PIV measurement results of the flow field inside the dimple at Re = 50.5 K: (a) instantaneous velocity field, (b) instantaneous vorticity distribution, (c) ensembles-averaged velocity field, (d) streamlines of the mean flow field, (e) normalized in-plane TKE distribution, and (f) normalized Reynolds stress distribution

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Fig. 11

Schematic of the flow structures inside a dimple cavity

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Fig. 12

Distributions of the normalized in-plane TKE (i.e., 0.5(u′2¯+w′2¯)/U¯2) in the horizontal plane near the upper surface of the dimpled test plate: (a) Re = 36.7 K and (b) Re = 50.5 K

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Fig. 13

(a) Measured TKE profiles of the present study and (b) Measured local Nusselt number data reported in Mahmood and Ligrani [15]

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