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Research Papers

The Effect of Surface Roughness on Laminar Separated Boundary Layers

[+] Author and Article Information
Mark P. Simens

School of Aeronautics,
Universidad Politécnica de Madrid,
Madrid 28040,Spain
e-mail: mark@torroja.dmt.upm.es

Ayse G. Gungor

Faculty of Aeronautics and Astronautics,
Istanbul Technical University,
Istanbul 34469, Turkey
e-mail: ayse.gungor@itu.edu.tr

1Corresponding author.

Contributed by the International Gas Turbine Institute (IGTI) of ASME for publication in the JOURNAL OF TURBOMACHINERY. Manuscript received June 30, 2013; final manuscript received July 8, 2013; published online October 22, 2013. Editor: Ronald Bunker.

J. Turbomach 136(3), 031014 (Oct 22, 2013) (8 pages) Paper No: TURBO-13-1117; doi: 10.1115/1.4025200 History: Received June 30, 2013; Revised July 08, 2013

Roughness effects on a laminar separation bubble, formed on a flat plate boundary layer due to a strong adverse pressure gradient similar to those encountered on the suction side of typical low-pressure turbine blades, are studied by direct numerical simulation. The discrete roughness elements that have a uniform height in the spanwise direction and ones that have a height that is a function of the spanwise coordinate are modeled using the immersed boundary method. The location and the size of the roughness element are varied in order to study the effects on boundary development and turbulent transition; it was found that the size of the separation bubble can be controlled by positioning the roughness element away from the separation bubble. Roughnesses that have a height that varies in a periodic manner in the spanwise direction have a great influence on the separation bubble. The separation point is moved downstream due to the accelerated flow in the openings in the roughness element, which also prevents the formation of the recirculation region after the roughness element. The reattachment point is moved upstream, while the height of the separation bubble is reduced. These numerical experiments indicate that laminar separation and turbulent transition are mainly affected by the type, height, and location of the roughness element. Finally, a comparison between the individual influence of wakes and roughness on the separation is made. It is found that the transition of the separated boundary layer with wakes occurs at almost the same streamwise location as that induced by the three-dimensional roughness element.

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Figures

Grahic Jump Location
Fig. 1

From top to bottom, these figures show Δx/η, Δy/η, and Δz/η for R3df. The solid line indicates the zero contour of the streamwise velocity.

Grahic Jump Location
Fig. 2

Difference between the shape factor obtained using 1537 × 301 × 768 points Hfg (R3df) and Hig (R3d), discretized with 1537 × 301 × 384 points and between Hfg and Hcg (R3d6c) discretized with 513 × 301 × 192 points. The solid line indicates ΔH = |Hfg − Hig|/Hfg and the dashed line indicates ΔH = |Hfg − Hcg|/Hfg.

Grahic Jump Location
Fig. 3

Effect of roughness on the instantaneous streamwise velocity. (a),(c), and (e) A longitudinal plane z/θ0 = 200. The solid white line indicates the separated region. (b),(d), and (f) A wall-parallel plane y/θ0 = 0.93. The solid magenta line indicates the two-dimensional roughness element. (a),(b) R0, (c),(d) R2d1, and (e),(f) R2d1h.

Grahic Jump Location
Fig. 4

Effect of roughness on the streamwise velocity fluctuations. The solid white line indicates the mean separated region. (a) R0, (b) R2d1, and (c) R2d1h.

Grahic Jump Location
Fig. 5

Effect of the roughness height on the mean flow properties. (a) Skin friction coefficient, and (b) wall-pressure coefficient. Black: R0; red: R2d1h, hr/θ0=0.7; and blue: R2d1, hr/θ0 = 1.8. The solid lines indicate the separated flow Cf < 0 and the dashed lines indicate the attached flow Cf > 0 (see online version for color).

Grahic Jump Location
Fig. 6

Effect of the roughness location on the zero contour of the streamwise velocity. Black: R0; blue: R2d1, xr/θ0 = 36; green: R2d2, xr/θ0 = 74; magenta: R2d3, xr/θ0 = 110; and red: R2d4, xr/θ0 = 210.

Grahic Jump Location
Fig. 7

Effect of the roughness location on the mean flow properties. (a) Shape factor, (b) Reynolds number based on momentum thickness, (c) maximum turbulent intensity, and (d) skin friction coefficient. Black: R0; blue: R2d1, xr/θ0 = 36; green: R2d2, xr/θ0 = 74; magenta: R2d3, xr/θ0 = 110; and red: R2d4, xr/θ0 = 210. The solid lines indicate the separated flow Cf < 0 and the dashed lines indicate the attached flow Cf > 0.

Grahic Jump Location
Fig. 8

The zero contour of the streamwise velocity. Black: R0; green: R3df; and red: R3d6c.

Grahic Jump Location
Fig. 9

Maximum turbulent intensity. Black: R0; green: R3df; and red: R3d6c. The solid lines indicate the separated flow Cf < 0 and the dashed lines indicate the attached flow Cf > 0.

Grahic Jump Location
Fig. 10

Instantaneous snapshots of the U-velocity in the yz-plane for R3df. In the clockwise direction the x-portion varies as x/θ0 = 47 (slightly upstream of the roughness element), x/θ0 = 58 (within the roughness element), x/θ0 = 106 (slightly downstream of the roughness element), and x/θ0 = 271 (slightly upstream of the separation point). Note the different scale of the y axis in comparison with the z axis.

Grahic Jump Location
Fig. 11

An instantaneous snapshot of the U-velocity at y/θ0 = 0.9. Left: R3df; and right: R3d6c. The solid line marks the beginning of the roughness element and the end, respectively.

Grahic Jump Location
Fig. 12

Individual effects of roughness and wakes on the mean flow properties. (a) Shape factor, and (b) maximum turbulent intensity. Black: smooth, unforced case R0; blue: two-dimensional roughness element R2d1; red: three-dimensional roughness element R3dp; green: wake forcing St = 0.78; and magenta: wake forcing St = 2.90. The solid lines indicate the separated flow Cf < 0 and the dashed lines indicate the attached flow Cf > 0.

Grahic Jump Location
Fig. 13

The effect of the roughness element on the size of the separation bubble as a function of the streamwise location (bottom x-axis). The filled circles indicate the two-dimensional elements h = 0.7θ0, the open square indicates the two-dimensional element h = 1.8θ0, the red triangle indicates the three-dimensional element h = 0.7θ0, the green triangle indicates the three-dimensional element without inflow disturbances h = 0.7θ0, and the dashed line indicates the separation location of the bubble due to the pressure gradient. The effect of the wake passing period (TUref/θ0) on the size of the separation bubble (top x-axis). Blue diamonds indicate the cases presented in Ref. [4] and the solid line indicates the time required for the separation bubble to regenerate itself 5000θ0/Uref.

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