Direct numerical simulation of coflowing rough and smooth turbulent channel flows

Fully developed turbulent flows through ribbed channels have been simulated using direct numerical simulation (DNS). Square ribs were arranged transversely to the flow on one of the channel walls, and based on their spanwise extent, resulted in two configurations - two-dimensional (2D) configuration resulting from full-span-width ribs and a three-dimensional (3D) configuration, where the ribs extend only up to half the span-width of the channel, leaving the other half smooth. The 3D configuration thus produced a unique problem of coflowing rough and smooth turbulent channel flows. A striking phenomenon has been observed of the secondary roll cells, exhibiting a strong updraft in the smooth half of the channel. The mean velocity profile from the smooth half surprisingly possesses a linear region of constant slope at the channel core. Comparisons were also drawn with DNS of a smooth channel at the same friction Reynolds number 400 and it was found that the roll cells on the smooth half not only affect the bulk flow negatively but also attenuate the turbulence significantly. In spite of having a higher bulk velocity, the rough half of the 3D configuration is more turbulent than the 2D configuration; this has been attributed to the momentum transfer from the smooth half to the rough half. Statistical turbulence quantities, and the production rates of turbulent kinetic energy and enstrophy have been used to arrive at the inferences. In essence, the roll cells buffer the variations between the rough and the smooth halves in the 3D configuration. The instantaneous streamwise velocity fluctuations in the 3D configuration displayed a wavelike form.