Time-resolved refractive index matched PIV measurements inside and downstream of a 90° bend

The flow in a 90° bend with Rc /D = 1.5 is studied at a Reynolds number of 34,800 to characterize the unsteady motions that may affect mass transfer in the latter part of the bend and downstream pipe where differences have been reported. Two-component, time-resolved measurements were performed on the axial symmetry plane and on cross sectional planes in the bend and up to 1D downstream of the bend using refractive index matched Particle Image Velocimetry. The unsteady flow structures were examined using the Snapshot Proper Orthogonal Decomposition (POD) technique. Frequency spectra of the POD coefficients and low-order reconstructions were used to characterize the time scales and dynamics of the motion. Unsteady variations in the secondary flow, consisting of two vortices, were found throughout much of the bend and the downstream pipe. The largest effect of the secondary motions within the bend occurred near the outer wall and side walls of the bend. Low to moderate frequency motions ( St ≤ 0.17 ) were prevalent at the 10° and 20° cross sectional planes. Moderate frequency motions (St of 0.17–0.35) played an increasing role in the latter part of the bend. The instantaneous longitudinal and swirling velocity near the inner centerline were small at 70° into the bend, but increased near the end of the bend as the unsteady motions on the side walls extended to the inner centerline. The unsteady secondary motions near the outer wall decreased at the beginning of the downstream pipe, while the role of the moderate and high frequency secondary motions near the inner wall (St of 0.35 and 0.4–0.6) increased. The secondary flow near the inner wall in the downstream pipe appears responsible for the unsteady recovery of the streamwise velocity observed near the inner wall. The unsteady interaction of the high-speed flow with the outer wall appears responsible for the streamwise velocity fluctuations near the outer wall in the downstream pipe.