Influence of elbow curvature on flow and turbulence structure through a 90° elbow

The influence of elbow curvature on the flow field of a 90° elbow was studied experimentally to understand the flow and turbulence structure in the elbow using planar and stereo particle image velocimetry (PIV) measurements. Three elbows with different radius to diameter ratios (1.0, 1.2, and 1.5) were investigated in the Reynolds number range of Re = (3-10) × 104. The experimental results showed that the mean velocity decreased and turbulence intensities increased along the inner wall in the second half of the elbow, and similar observations were made in all of the elbows. However, variations in these quantities increased with decreasing radius ratio. This was caused mainly by the occurrence of flow separation on the inner wall, near the elbow outlet, for smaller radius ratios. Similarly, flow separation on the inner wall was promoted at lower Reynolds numbers. Cross-sectional velocity field measurements at the elbow outlet using stereo PIV showed the presence of a pair of counter-rotating vortices, which increased in mean velocity and turbulence energy on the inner wall with decreasing radius ratios of the elbow. This result was well reproduced in the contour of the first proper orthogonal decomposition (POD) mode. In the POD analysis, opposite signs of the high POD mode were formed on both sides of the inner wall and the peak of the POD mode increased with decreasing radius ratio.