Unsteady wall pressure characteristics of a 90 degree elbow in high Reynolds numbers

摘要

Wall pressure measurements were conducted for a 90 degree elbow of which the axis curvature coincided with its inner diameter (125 mm). Reynolds numbers examined were 1.0 × 10~(5) (available only for steady components), 3.2 × 10~(5) and 5.0 × 10~(5). Results showed that distributions of fluctuating normalized pressures obtained here and those made by Shiraishi et al . (2006) for the Reynolds number of 3.25 × 10 ~(6) coincided within 0.04 of the dynamic pressure. These distributions had the same tendency: The strong fluctuating region existed in the curvature inside and had concave/convex shapes at the upstream/downstream ends, respectively. Power spectral density functions of fluctuating pressures mostly exhibited the slope of the minus seven-third law, which is seen in the inertial range of turbulence, at large frequencies. The peak spectrum with the Strouhal number of 0.5 could be found in the curvature inside downstream of the elbow. They corresponded to the vortex shedding from the boundary layer developed in the inner and aft part of the elbow. The slope at large Strouhal numbers became negatively steep near the region where the peak spectrum was seen. The peak intensity having the Strouhal number of 0.5 was quantitatively in accordance with that of the data obtained in the experimental setup that Shiraishi et al . used (Yamano et al ., 2011), suggesting that the law of dynamical similarity could be applied with regard to this oscillation. Cross correlations of pressure fluctuations showed that the pressure fluctuation having the Strouhal number of 0.5 propagated as a planar wave with the bulk velocity.