Abstract

Rotating-drum tests are widely employed for the characterization of powder flowability through image-derived free-surface observables. However, the statistical stability of the selected observable is rarely examined, although it directly determines the number of images required for reliable evaluation. In the present study, two observables obtained from rotating-drum tests of a cohesive metal powder are compared: the dynamic angle of repose (AoR) and a contour-length-based index, R, derived from the extracted free-surface contour. Using identical experimental conditions and image sequences, the observables are evaluated in terms of their mean values, coefficients of variation (CVs), and the sample sizes required to achieve a prescribed relative precision. The CV of AoR ranged from 0.095 to 0.186, whereas that of R remained below 0.03. For a target relative error of 3%, the required numbers of cases were estimated to be 21, 84, and 189 for AoR at the 1σ, 2σ, and 3σ confidence levels, respectively, whereas those for R were 1, 1, and 2. These results demonstrate that, even under identical apparatus and powder conditions, the statistical burden of characterization depends strongly on observable choice. In the present study, the contour-length-based index R is interpreted as a geometry-based descriptor of free-surface irregularity rather than as a direct measure of cohesion or adhesion. Because the analysis is limited to a single cohesive metal powder, the present findings should be interpreted as a case-specific demonstration within the current experimental framework rather than as a general law for all cohesive powders.

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