Direct numerical simulation of turbulent convection in a channel roughened with circular-arc ribs

Turbulent flows through a channel roughened with circular-arc ribs of different pitch-to-height ratios (P/H=3.0, 5.0 and 7.5) at a fixed Reynolds number are studied using direct numerical simulations (DNS). It is interesting to observe that in contrast to the classical channel flow over transverse square bars, the flow separation point in a channel roughened with circular-arc ribs is sensitive to the testing conditions and varies along the rib arc. The influences of the pitch-to-height ratio on the flow separation, statistical moments of the velocity field, and the anisotropy of turbulence stresses are investigated. The dynamics of coherent structures are studied by examining the skewness of the flow field, characteristics of the joint probability density functions (JPDF) of velocity fluctuations, quadrant analysis of Reynolds stresses, instantaneous velocity field, swirling strength, and temporal–spatial two-point auto-correlations. It is found that the flow anisotropy becomes suppressed as the value of P/H increases due to significant enhancement of the spanwise Reynolds normal stress 〈w′w′〉. It is also interesting to observe that the lifespan of turbulence structures becomes increasingly shortened at the midspan between two adjacent ribs as the pitch-to-height ratio increases.