Wall roughness
 

We are trying to streamline the prescription of wall roughness and I realise that you can't just prescribe Ra value as the wall roughness because it may not always be equivalent to sand-grain roughness term that exists in the boundary layer formulae. If you go and ask for sand-grain roughness on the shopfloor or to quality manager, they look at you with blank face. I have seen some papers that explore this concept:

http://ijmem.avestia.com/2012/008.html
http://rsta.royalsocietypublishing.o.../1852/699.full
http://arrow.dit.ie/cgi/viewcontent....t=engschmecart

But none gives a straight-forward guide for determining the equivalent sand-grain roughness value from Ra or Rz value obtained from optical profilometer. Has anyone done some study in this area?

I'm also confused about this question···