Characterization of the secondary flow in hexagonal ducts

In this work we report the results of DNSs and LESs of the turbulentflow through hexagonal ducts atfrictionReynolds numbers based on centerplane wall shear and duct half-heightReτ,c≃ 180, 360, and 550. The evolution of the Fanningfriction factorf withRe is in very good agreement with experimental measurements. A significant disagreement between the DNS and previousRANS simulations was found in the prediction of the in-plane velocity, and is explained through the inability of theRANSmodel to properly reproduce the secondaryflow present in the hexagon. The kinetic energy of the secondaryflow integrated over the cross-sectional area 〈K〉yz decreases withRe in the hexagon, whereas it remains constant withRe in square ducts at comparableReynolds numbers. Close connection between the values ofReynolds stressuw¯ on the horizontal wall close to the corner and the interaction of bursting events between the horizontal and inclined walls is found. This interaction leads to the formation of the secondaryflow, and is less frequent in the hexagon asRe increases due to the 120∘ aperture of its vertex, whereas in the square duct the 90∘ corner leads to the same level of interaction with increasingRe. Analysis of turbulence statistics at the centerplane and the azimuthal variance of the meanflow and the fluctuations shows a close connection between hexagonal ducts and pipeflows, since the hexagon exhibits near-axisymmetric conditions up to a distance of around 0.15DH measured from ...

Source: Physics of Fluids - December 6, 2016 Category: Physics Authors: O. Marin, R. Vinuesa, A. V. Obabko and P. Schlatter Source Type: research