In this paper, turbulent flow through a longitudinally rib-roughened square duct is studied using direct numerical simulation at a nominal bulk Reynolds number of ${\text{Re}}_b^N=4410$. To understand the rib effects on the velocity field, DNS of a smooth-wall duct flow is also performed, which serves as a baseline case of comparison. The influences of longitudinal ribs on the flow structures, statistical moments of the velocity field, and turbulence kinetic energy budget balance are investigated. Through analyses of the streamwise two-point autocorrelation coefficients of the velocity fluctuations and swirling strength, it is found that ribs can effectively disrupt the growth of turbulence structures. In each corner of the smooth and ribbed ducts, secondary flow appears as a pair of counterrotating vortices. However, in the longitudinally ribbed duct flow case, it is interesting to observe tertiary vortex structures which significantly alter the distributions of the viscous and turbulent stresses. Owing to the shearing effects from the ribs, turbulence transport of momentum is substantially enhanced near the rib edges. By examining the third- and fourth-order statistical moments of the velocity field, it is also interesting to observe that sweep and ejection events are intensified and the turbulence is more intermittent near the rib regions.

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• Shear flows
• Turbulence
• Turbulence simulations
• Turbulent boundary layers