Visualization of secondary flow in a corner of a channel

• Main Authors: Bém, Jindřich, Duda, Daniel, Kovařík, Jiří, Yanovych, Vitalii, Uruba, Václav
• Format: Conference Proceeding
• Language: English
• Subjects: Aerodynamics; Air flow; Boundary layer transition; Broken symmetry; Fluid dynamics; Fluid flow; Particle image velocimetry; Reynolds number; Secondary flow; Turbulence; Turbulent boundary layer; Velocity; Velocity distribution; Vortices
• ISSN: 0094-243X; 1551-7616
• DOI: 10.1063/1.5138615

We report observation of secondary flow in one corner of developing channel air flow. Length of the channel, i.e. length of boundary layer, is 400 mm, which is 3.2 times the channel cross-sectional size. Three components of velocity are measured by using a Stereo Particle Image Velocimetry (PIV) technique in the measurement area of size 24×22 mm, which is perpendicular to the direction of main flow in the channel. The Reynolds number based on the length of the channel ranged from 4·104 to 8·105 and has been controlled via imposed velocity. At low Reynolds number we observe a laminar corner vortex having at all velocities the same orientation. This symmetry breaking is probably caused by an imperfectness of the experimental device. At Reynolds number around 8·104 this vortex starts to slightly variate its strength and position causing transition of boundary layers into turbulence at Re = 1.1·105. At higher Re this laminar vortex disappears from the instantaneous velocity fields, but it is still apparent in the averaged ones. It gets smaller and another oppositely oriented vortex forms; note that the second vortex is not observed in the instantaneous velocity fields, only in the ensemble average. At even higher Re, this secondary flow structure is smaller than the turbulent boundary layers, but its shape of a pair of counter-rotating vortices is conserved probably being a seed for secondary flow between fully developed boundary layers reported in the literature for longer channels with fully developed flow.