[Mason liu]

Hi, all,

I'm starting to estimate the drag force of cylinder-like body which axis is perpendicular to flow.

And recently I have a basic question about the Reynolds Number and Y-plus in external flow. Could you guys please kindly help me? Thank you all really. Details as below.

1) For example, flow through a finite length cylinder-like rod(the rod perpendicular to flow), how should we calculate the Reynolds number, ie use which size as length scale: diameter(D), length of rod(L) or flow-field size(W)? I think should use the diameter(D).
Length Scale.jpg

2) Use above estimated Reynolds number to calculate the Y-plus and calculation result gives bigger Y-plus on some wall-area where has high velocity region close to wall. And according to Y-plus's definition:
Y-plus=ro*frictional velocity*first cell height/dynamic viscosity
This definition seems tell us that for a uniform first cell height, Y-plus should be bigger on those wall-area which has high velocity region nearby(because this region gives bigger frictional velocity). Is this guess right?
External flow.jpg

3) If this is right. Then those Y-plus values on wall will vary with velocity nearby, so if we use a low-Reynolds number model(Like SST) and we need a Y-plus=~1, do we just need the maximum Y-plus on the wall around 1 or what? If this is true, then the method we used to calculate Y-plus based on Reynolds number seems not that accurate, the maximum Y-plus will be obviously bigger than what we expected. Because we use a normal velocity to calculate the Reynolds number, then Y-plus, but the actual flow will give local high velocity region, on those wall close to this region Y-plus will be bigger.

This really confused me long time.

4) And in flow simulation(like drag force estimate) do we need to guarantee all Y-plus value the same as what we need on wall? This seems extremely hard to realize. Usually we can only keep the first cell height same, does this acceptable?

My English is not good and expression may be not that clear, beg your pardon and really want to receive your guide. Thanks a lot all.