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Research Papers: Techniques and Procedures

Uncertainty Evaluation of Friction Velocity Measurements by Oil-Film Interferometry

[+] Author and Article Information
René Thibault

Civil Engineering Department,
Université de Moncton,
Moncton, New Brunswick E1A 3E9, Canada
e-mail: rene.thibault@umoncton.ca

Gérard J. Poitras

Civil Engineering Department,
Université de Moncton,
Moncton, New Brunswick E1A 3E9, Canada
e-mail: gerard.poitras@umoncton.ca

Contributed by the Fluids Engineering Division of ASME for publication in the JOURNAL OF FLUIDS ENGINEERING. Manuscript received July 26, 2016; final manuscript received November 14, 2016; published online March 14, 2017. Assoc. Editor: Mark R. Duignan.

J. Fluids Eng 139(5), 051401 (Mar 14, 2017) (10 pages) Paper No: FE-16-1480; doi: 10.1115/1.4035461 History: Received July 26, 2016; Revised November 14, 2016

Wind loads on structures and the wind environment around buildings are based on tests in boundary layer wind tunnels with corresponding scale parameters. The lower part of the troposphere boundary layer was simulated inside a small wind tunnel located at the Wind Engineering Centre of the Université de Moncton. The correct scale ratios of the boundary layer thickness combined with the roughness height are two of the most important scales to match. For small wind tunnels, roughness parameters related to the model boundary layer can be difficult to measure since scale ratios for wind load studies are expected to be in the range of 400–1000. Oil-film interferometry was used to determine the roughness parameters (shear stress, friction velocity, and roughness height) of the forced turbulent boundary layer inside the wind tunnel. In this work, the International Organization for Standardization (ISO) Guide to Expression of Uncertainty in Measurements was used to evaluate the standard uncertainty of the roughness parameters on the bottom wall of the wind tunnel. The standard uncertainty of the roughness parameters depends strongly on the oil viscosity and on the accurate measurement of the fringe spacing. Results show that the standard uncertainty of the shear stress and friction velocity determined by the interferometry technique can be less than 5% when the oil viscosity and the fringe spacing can be accurately measured with a standard uncertainty lower than 4% and 1%, respectively.

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Figures

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Fig. 1

Wind tunnel configuration

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Fig. 3

Measured velocity profile and turbulence intensity compared with the requirements of a type A wind velocity profile from NBC-2010

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Fig. 4

Normalized velocity von Kármán power spectrum at Ys = 200 mm

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Fig. 5

Velocity profile linearization (R = 0.996)

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Fig. 6

Oil control volume [28] (Copyright 2002. Reproduced with Permission from Elsevier, New York)

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Fig. 7

Fringe interference principles: (a) constructive interference producing white stripes and (b) destructive interference producing black stripes [28] (Copyright 2002. Reproduced with Permission from Elsevier, New York)

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Fig. 9

Interferometry technique configuration aligned with a monochromatic LED

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Fig. 10

Diagram depicting the analysis line for the interferometry technique for t = 570 s and a viscosity ν = 100 cS

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Fig. 11

Analysis phase of an interferogram: (a) intensity along the analysis line; (b) FFT modulus on the signal of figure (a); (c) signal used for cross-correlation; (d) cross-correlation results between figures (a) and (c); and (e) height of oil at the surface

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