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Research Papers: Fundamental Issues and Canonical Flows

PIV Study of Separated and Reattached Open Channel Flow Over Surface Mounted Blocks

[+] Author and Article Information
Martin Agelinchaab

Department of Mechanical and Manufacturing, University of Manitoba, Winnipeg, MB R3T 5V6, Canada

Mark F. Tachie

Department of Mechanical and Manufacturing, University of Manitoba, Winnipeg, MB R3T 5V6, Canadatachiemf@cc.umanitoba.ca

J. Fluids Eng 130(6), 061206 (Jun 12, 2008) (9 pages) doi:10.1115/1.2911677 History: Received August 24, 2007; Revised January 15, 2008; Published June 12, 2008

A particle image velocimetry is used to study the mean and turbulent fields of separated and redeveloping flow over square, rectangular, and semicircular blocks fixed to the bottom wall of an open channel. The open channel flow is characterized by high background turbulence level, and the ratio of the upstream boundary layer thickness to block height is considerably higher than in prior experiments. The variation of the Reynolds stresses along the dividing streamlines is discussed within the context of vortex stretching, longitudinal strain rate, and wall damping. It appears that wall damping is a more dominant mechanism in the vicinity of reattachment. In the recirculation and reattachment regions, profiles of the mean velocity, turbulent quantities, and transport terms are used to document the salient features of block geometry on the flow. The flow characteristics in these regions strongly depend on block geometry. Downstream of reattachment, a new shear layer is formed, and the redevelopment of the shear layer toward the upstream open channel boundary layer is studied using the boundary layer parameters and Reynolds stresses. The results show that the mean flow rapidly redeveloped so that the Clauser parameter recovered to its upstream value at 90 step heights downstream of reattachment. However, the rate of development close to reattachment strongly depends on block geometry.

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Copyright © 2008 by American Society of Mechanical Engineers
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Figures

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Figure 1

Schematic diagram of experimental setup and coordinate system: (a) side view of test section, (b) and (c) are sketches showing dividing streamlines and the various flow regions for square and semicircular blocks, respectively. All dimensions are in mm. Xr is the reattachment length, and Lr is length of separation bubble.

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Figure 2

Characteristics of upstream boundary layer: mean velocity profiles in outer (a) and inner (b) coordinates; (c) variation of the freestream velocity, Ue with x; (d) profiles of u+, v+, and −⟨u+v+⟩. Dotted line in (b) is U+=y+, and solid line is U+=2.44lny++5.

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Figure 3

Streamlines in recirculation zone: (a) square, (b) rectangular, and (c) semicircular blocks

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Figure 4

(a) Loci of the dividing streamlines, (b) mean streamwise velocity (U) along the dividing streamlines, (c) mean transverse velocity (V) along the dividing streamlines, and (d) flow angle V∕U along the dividing streamlines. Note that x′=0 at separation point. Symbols: square block, ◼; semicircular block, ○; rectangular block, △.

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Figure 5

The variation of (a) velocity difference ΔU=Umax−Umin, maximum slope ∂U∕∂ymax, and (b) vorticity thickness δω in recirculating and reattachment regions. In (b), the solid line is dδω∕dx=0.33 and the dash line is dδω∕dx=0.15. Symbols are as in Fig. 4.

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Figure 6

Distributions of turbulence intensities, Reynolds shear stress, and correlation coefficient along the dividing streamlines for the square (◼) and semicircular (○) blocks. (a) u∕Ur, (b) v∕Ur, (c) −⟨uv⟩∕Ur2, and (d) ρuv.

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Figure 7

Mean velocity profiles: streamwise components in (a) recirculation and (b) reattachment regions; wall-normal components in (c) recirculation and (d) reattachment regions. Symbols as in Fig. 4. Definition of the stations is provided in text.

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Figure 8

Profiles of turbulence intensities: streamwise component in (a) recirculation and (b) recovery regions; wall-normal component in (c) recirculation and (d) recovery regions. Symbols are as in Fig. 4.

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Figure 9

Profiles of −⟨uv⟩ in (a) recirculation and (b) recovery regions. Symbols are as in Fig. 4.

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Figure 10

Distributions of triple products (normalized by Ur3) at selected x-locations: (a) u3, (b) u2v, (c) v3, and (d) uv2. Symbols are as in Fig. 4.

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Figure 11

Production and mixing length at selected x-locations: (a) total production, PT=[u2∂U∕∂x+v2∂V∕∂y]+[⟨uv⟩(∂U∕∂y+∂V∕∂x)]; (b) mixing length, Lm=(−⟨uv⟩)1∕2∕∂U∕∂y. Symbols are as in Fig. 4.

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Figure 12

Variation of the boundary layer parameters with X∕h: (a) shape factor H, (b) momentum thickness θ, (c) skin friction coefficient Cf, and (d) Clauser parameter G. Symbols are as in Fig. 4.

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Figure 13

Typical mean profiles in inner coordinates at selected locations for ((a) and (b)) semicircular block and ((c) and (d)) square block. The dashed lines are U+=y+, the solid lines are U+=2.44×lny++5.

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Figure 14

Profiles of (a) ⟨u+u+⟩, (b) ⟨v+v+⟩, and (c) −⟨u+v+⟩ for upstream boundary layer and at selected X-locations for square and semicircular blocks

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