On the use of Irwin probes to measure wall shear street in rectangular ducts of variable section using Openfoam

Abstract eng:
The present work is performed with the aim of evaluating the applicability of using Irwin probes and Preston tubes to study incompressible turbulent flow through rectangular ducts and in the presence of pressure gradient conditions. A rectangular plywood duct with 0.12 meters width and 5 meters length was used to perform the experimental tests. A hinge and movable top wall allowed the study of several cases: constant crosssection with an aspect ratio of AR=1:2 (horizontal (H) top wall), convergent longitudinal section with 1º slope (C1), and divergent longitudinal section with 1º slope (D1). The numerical simulations performed using mainly the open source OpenFOAM CFD toolbox. The mesh with y+ ≈ 1 was the one selected in the model verification study. To validate the numerical model, predictions for both pressure and friction coefficients were compared against the experimental values and good agreement was observed with deviations below 8%. The calibration of the in-house built Irwin probes is discussed in some detail. For the tested cases, the calibration curve is formulated in terms of a power function with differences from each other lower than 5% in the range of Irwin probe pressure difference (∆𝑝𝐼 ). Thus, one single calibration curve can be employed regardless the case, being the curve obtained for the horizontal case the one with the best overall results. For larger ranges of ∆𝑝𝐼 , some caution is advised in the use of a single calibration curve, especially in the presence of adverse pressure gradients.

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