Time-resolved PIV measurements inside and downstream of a 180° bend

The flow in a 180° bend with a radius of curvature 1.5 time the pipe diameter (Rc /D of 1.5) was studied for a Reynolds number of 34,800 to characterize the development of the mean velocity and Reynolds stresses and the unsteady motions that may affect the heat or mass transfer in the latter part of the bend. 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, transients of the coefficients and low-order reconstructions were used to characterize the time scales and dynamics of the motions. The results showed that the initial development of the flow was similar to that in a 90° bend but the development of the mean secondary flow, and the low-speed region associated with this feature, appeared delayed relative to the 90° bend in the middle section of the 180° bend. The results in the second half of the 180° bend showed that the mean longitudinal velocity near the inner wall was larger than that near the outer wall in the latter part of the bend. Changes in the unsteady interaction of the flow with the outer wall in the middle of the 180° bend also appeared to cause significant unsteadiness in the flow near the inner wall in the latter part of the bend. The mean secondary flow towards the end of the 180° bend was significantly diminished relative to the flow in the middle of the bend and largely suppressed by 1D downstream of the bend. Reconstructions of the instantaneous secondary flow showed occurrences of large secondary velocities along the sidewalls up to 170°, but these were substantively reduced even at 0.25D into the downstream bend.