Improving the Accuracy of Multihole Probe Measurements in Velocity Gradients

This study describes an implementation and verification of an effective and reliable correction for the finite-size effects of pressure probes. A modified version of correction by Ligrani et al. (Exp. Fluids, vol. 7, 1989, p. 424) was used. It is shown that the correction procedure can be implemented in two steps as in Ligrani et al. or in a single step, either for probe pressures, or for velocity components. The latter correction method is found to have the best performance and studied in very detail. The effect of the correction in application to the highly three-dimensional flow downstream of the outlet guide vanes is scrutinized through detailed side-by-side comparison with corresponding cross hot-wire data. The influence of the correction on all three velocity components, flow streamlines and streamwise vorticity fields is thoroughly examined. Two flow cases with different incoming turbulence intensities are considered. The study demonstrates a very good efficiency and reliability of the correction, which lead to a significant improvement of the corrected velocity data. The improvement in crossflow velocity components has allowed correct description of the flow streamlines, and as a result, the secondary flow field structures were resolved more accurately. The considered correction does not affect the streamwise vorticity component, which is clarified as well. A very important fact is that the correction is not found to over-correct and distort the data, thus can be used safely. A very good performance of the correction for the finite-size effects of pressure probes presented in this study allows us to recommend it as a mandatory step in postprocessing procedures for multihole pressure probes.