Uncertainty Evaluation of Friction Velocity Measurements by Oil-Film Interferometry

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.