Assessing the statistical validity of momentum-deficit-based CD measurements in turbulent configurations

Abstract

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.