Direct Numerical Simulation of Thermal Turbulent Boundary Layer Flow over Multiple V-Shaped Ribs at Different Angles

Direct numerical simulations (DNSs) of spatially developing thermal turbulent boundary layers over angle-ribbed walls were performed. Four rib angles (? = 90 degrees, 60 degrees, 45 degrees and 30 degrees) were examined. It was found that the 45 degrees ribs produced the highest drag coefficient, whereas the 30 degrees ribs most improved the Stanton number. In comparison to the transverse rib case, streamwise velocity and dimensionless temperature in the V-shaped cases significantly increased in the near wall region and were attenuated by secondary flows further away from the ribs, which suggested a break of the outer-layer similarity in the scenario presented. The surprising improvement of heat transfer performance in the 30 degrees rib case was mainly due to its large dispersive heat flux, while dispersive stress reached its peak value in the 45 degrees case, emphasizing the dissimilarity in transporting momentum and heat by turbulence over a ribbed surface. Additionally, by calculating the global and local Reynolds analogy factors, we concluded that the enhancement in heat transfer efficiency was attributed to an increasing Reynolds analogy factor in the intermediate region as the rib angle decreased.