Research Papers: Multiphase Flows

PIV Study of Adverse and Favorable Pressure Gradient Turbulent Flows Over Transverse Ribs

[+] Author and Article Information
M. Agelinchaab, M. F. Tachie

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

J. Fluids Eng 130(11), 111305 (Sep 23, 2008) (12 pages) doi:10.1115/1.2969448 History: Received September 28, 2007; Revised June 12, 2008; Published September 23, 2008

This paper reports an experimental study of the combined effects of rib roughness and pressure gradient on turbulent flows produced in asymmetric converging and diverging channels. Transverse square ribs with pitch-to-height ratio of 4 were attached to the bottom wall of the channel to produce the rib roughness. A particle image velocimetry technique was used to conduct measurements at several streamwise-transverse planes located upstream, within, and downstream of the converging and diverging sections of the channel. From these measurements, the mean velocities and turbulent statistics at the top plane of the ribs and across the channel were obtained. The data revealed non-negligible wall-normal motion and interaction between the cavities and overlying boundary layers. The different drag characteristics of the rough bottom wall and the smooth top wall produced asymmetric distributions of mean velocity and turbulent statistics across the channel. The asymmetry of these profiles is most extreme in the presence of adverse pressure gradient. Because of the manner in which pressure gradient modifies the mean flow and turbulence production, it was found that the streamwise turbulence intensity and Reynolds shear stress in the vicinity of the ribs are lower in the adverse pressure gradient than in the favorable pressure gradient channel. The results show also that the combined effects of rib roughness and adverse pressure gradient on the turbulent intensity statistics are significantly higher than when roughness and adverse pressure gradient are applied in isolation.

Copyright © 2008 by American Society of Mechanical Engineers
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Figure 1

Experimental setup: schematic side views of the adverse pressure gradient (a) and favorable pressure gradient (b) test sections; measurement planes (c). P1–P5 denote x-y planes in which PIV measurements were made, L1–L5 correspond to locations where detailed data analysis was performed; and (d) pitch and top plane of adjacent ribs. All units are in millimeters.

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Figure 2

Profiles of the mean velocity and turbulent quantities at the cavity center for two interrogation window sizes and three sample sizes. (a) Mean velocity, U, (b) streamwise, u, and (c) transverse, v, turbulent intensities, and (d) Reynolds shear stress, −uv. The symbols in (b), (c), and (d) as in (a).

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Figure 3

Variation of boundary layer (BL) parameters with x-locations. x*=x∕1000. (a) Local maximum velocity, Umax. Umax* is Umax normalized by the maximum velocity at location L1, (b) y-location of the maximum velocity, ymax, (c) BL thickness, δ, (d) BL displacement thickness, δ* (e) BL momentum thickness, θ, and (f) the shape factor, H. The vertical dash lines indicate the start and end of the converging/diverging sections. Note. ymax, δ, δ*, and θ are normalized by the local channel height, h. Symbols in (a): inviscid flow in APG (- - -), inviscid flow in FPG (—), APG (●), FPG (◻), SM-FPG (☆), and SM-APG (★). Symbols in the rest of the figure: (--◻--) FPG upper, (--◼--) FPG lower, (--○--) APG upper, and (--●--) APG lower.

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Figure 4

Streamlines and their corresponding isocontours of the mean vorticities for selected locations. The vorticities are normalized by local maximum velocity, Umax, and rib height, k. ((a) and (b)) Location L1 without a pressure gradient, ((c) and (d)) location L3 in a favorable pressure gradient, and ((e) and (f)) location L3 in an adverse pressure gradient.

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Figure 5

Mean velocities, turbulent intensities, and momentum fluxes at the interface, y=0, for selected test conditions. (a) Streamwise velocity, (b) mean momentum flux, (c) streamwise turbulent intensity, and (d) turbulent momentum flux.

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Figure 6

Development of spatially averaged mean and turbulent quantities. (a) Streamwise mean velocity, (b) streamwise turbulent intensity, and (c) Reynolds shear stress. The symbols are as follows: smooth wall (⊕), upstream of the converging section (☆), within the converging section (◻), upstream of the diverging section (★), and within the diverging section (●).

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Figure 7

Profiles of spatially averaged turbulence production terms and stress ratios at selected locations. (a) Pk=−uv∂U∕∂y, (b) P−uv=v2∂U∕∂y, (c) stress ratio. −uv∕u2, and (d) stress ratio. v2∕u2. Pk and P−uv are normalized by local maximum velocity, Umzx and height, h. Symbols in (b) as in (a) and in (c) and (d) as in Fig. 6.

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Figure 8

Spatially averaged mean velocity and mean velocity defect profiles in the lower boundary layer. Outer coordinates. (a) favorable pressure gradient (FPG) and (b) adverse pressure gradient (APG). Inner coordinates. (c) FPG and (d) APG. Defect profiles for the FPG normalized by (e) Umaxδ*∕δ and (f) Uτ, and for both FPG and APG normalized by (g) Umaxδ*∕δ and (h) Uτ. The solid lines in (c) and (d) are as follows. U+=2.44lny++5 and U+=2.44lny++5−ΔB+. The Dash lines are U+=Ci[(y++a+)]γ. Symbols in (f) as in (e) and in (h) as in (g). Note. U*=(Umax−U)∕Umaxδ*∕δ; U**=(Umax∕U)∕Uτ.

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Figure 9

Spatially averaged turbulent intensities and Reynolds shear stresses in the lower boundary layer in the inner coordinates. (a) Streamwise turbulent intensity, (b) transverse turbulent intensity, and (c) Reynolds shear stress. All symbols are as in (a).




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