Originally Posted by LuckyTran (Post 856697)

Let me rephrase the question a bit.

Given that I have velocity data only in the inner region, how can I determine where the inner region stops?

You can't. The inner region has no info about what is happening in the outer region except for the wall shear stress. You need more info, a priori knowledge.

But why this obsession with Spalding's profile? Spalding himself found multiple fits. Whatever you are trying to accomplish in your line of work, I need you to understand (just like Spalding did) that this one curve-fit is not "42". It is simply one possible curve fit of one guy's data. I recommend you to look at more generalized/unified wall functions that do go into the outer region. See Lumley's work for example.

Originally Posted by avaharper (Post 856778)

It states that the dimensionless velocity close to a pipe wall is proportional to the logarithm of the distance from the wall, and it is valid until a certain distance from the wall known as the "y-plus" value.