Stereo PIV study of laminar-turbulent transition at 180o sharp bend inside a rectangular duct

Evolution of fully developed laminar flow (𝑅𝑒 = 800) around 180° sharp bend of a rectangular duct, and its subsequent transition into turbulent flow has been characterized by using the Stereo Particle Image Velocimetry (stereo-PIV, 2D3C) system. Results have been captured for numerous imaging planes to develop a fundamental understanding of flow progression inside the duct. A shear layer that separates the region of mainstream turning flow from the recirculation region (created near the divider wall) is observed at the recirculation boundary. Centrifugal instabilities in the mainstream turning flow arise by the sharp bend, and thus the secondary flows in the form of a counter-rotating pair of Dean vortices get induced. Two pairs of counter-rotating Dean vortices are being formed after 90° turning (at plane P7y−z), which merge into single pair after 180° turning (at plane P8x−y). It is observed that these set of Dean vortices plays a very significant role in the laminar-turbulent transition at the bend. The complete 180° turn results in a sharp 1825% increment in spatially averaged turbulent kinetic energy (k), and a 287% rise in spatially averaged wall normal root mean square (RMS) velocity fluctuations (vrms), at vertical planes in the direction of flow progression.