In this study, to investigate and observe the heat transfer phenomena in a circular pipe transition region to turbulence region caused by single or multiple small obstacles mounted on the wall, DNS is carried out, in which the presence or absence of transition influenced by the obstacle height is mainly investigated. DNS results show that an increase in the number of obstacles leads to a faster transition to a turbulent state from a laminar. Also, the Reynolds number, Rek, based on the obstacle height and the bulk svelocity Ub is arranged as Rek=539 with k/R=0.2, or Rek=293 with k/R=0.1, where it is found that the flow transition is clearly observed in the former case, but not observed in the latter case.
Futamura et al. (2025) studied this question.