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An application-agnostic procedure is outlined for checking the validity of momentum-deficit-based drag measurements performed under different turbulent conditions in a wind tunnel. The approach defines a two-step methodology: the first stage characterizes the turbulent flowfield generated downstream a passive grid through a set of statistical parameters. Acceptable values for such parameters are determined by means of two criteria: compliance with the threshold value set by an analysis of the experimental uncertainties, and fulfilment of the isotropic condition for ensuring a well-established turbulent flowfield. Those two prerequisites define a set of turbulent configurations for which the momentum-deficit-based technique applies feasibly. The second stage of the procedure is configuration-specific, and undertakes drag measurements upon a NACA0021 airfoil subjected to a set of different turbulent configurations. It is shown that performing measurements under invalid turbulent conditions leads to inconsistent drag curves, which serves for defining a validity map based on the testable cases. [ABSTRACT FROM AUTHOR]

• A breakdown of the uncertainty analysis of measurands providing individual contributions. • Operative condition variations take aerodynamic coefficients outside uncertainty intervals. • Temperature variations due to wind tunnel operation constitute the critical factor. • The effect of operative condition variations on aerodynamic coefficients is non-linear. • An analysis of the operative conditions is to be performed regardless of the experimental set-up. A twofold analysis is performed on an experimental set-up comprising a low-Reynolds wind tunnel, with the aim of quantifying the effect of measurand uncertainties and operative condition variations in the lift and drag curves of a NACA0021 airfoil. The first part of the study focuses on an uncertainty analysis of basic measurands used for calculating standard aerodynamic magnitudes, showing the individual contributions of each of the factors affecting the overall uncertainty ranges. The second part intends to show how those uncertainties are propagated to lift and drag coefficients, and to calculate the effect of varying operative conditions on those curves. It is found that, when changes in operative conditions are left unattended, their influence on derived magnitudes may grow as large as to cause the resultant data to lie outside the estimated uncertainty intervals. Additionally, those variations are shown to depend non-linearly on the operative condition changes that produce them. [ABSTRACT FROM AUTHOR]

Wind tunnel tests are carried out upon a NACA0021 airfoil subjected to transitional Reynolds numbers. Transitionally-operating airfoils show a high sensitivity to external conditions and pose relevant measurement issues for capturing the physical processes adequately. On the global side, the employed set of techniques measures lift forces directly and uses the momentum-deficit method for drag coefficients. Locally, the development of transitional structures is acknowledged via surface pressure measurements carried out by pressure taps together with oil-flow visualizations. The coupling of such techniques with a well-founded uncertainty analysis shows two relevant aspects of the measurement protocolization: on the one hand, the limitations of either the global or local methods for completely accounting for all transitional phenomena. On the other hand, the fact that combining the proposed set of different measurement techniques with a systematic protocol is a mandatory requirement for achieving a holistic characterization of transitionally-operating airfoils. • Load and pressure measurements, momentum-deficit method and oil-flow visualizations. • Uncertainty analysis addresses scale-effect on transitionally-operating airfoils. • Limitations of the piezoelectric balance for carrying aerodynamic drag measurements. • Surface pressure measurements and oil-flow visualizations limited to local analysis. • Different measurement techniques for performing a thorough aerodynamic characterization. [ABSTRACT FROM AUTHOR]