Hi, can you suggest me your opinion on this question: when an external body with roughness surfaces has a lower drag than a smooth body (skin of dolphin or shark), can I use the same idea for an internal flows in pipes? Can I decrease a pressure drop of pipe with roughness walls? If yes, can I solve it by CFD codes and what turbulence model for core and near wall I must use?

Many thanks for your commentary

I suppose it depends on the mechanism of drag reduction and if the same mechanism is still effective in pipe flow. As far as I remember, in your example of shark skin, the spatial scale of significance is so small that most people who did CFD on this kind of problem applied at least LES or even DNS. The purpose of those studies was to explain the mechanism of drag reduction. Maybe you should survey the literature. A turbulence model may not be good enough to capture such a small-scale effect.