Quote by brambram
Hi
I cannot find an equation for a boundary layer in a pipe flow (laminar). I am looking for an equivalent of the equation δ(x)=4.91x/(√Re) that works for a flow between plates (x is the distance downstream). The thing is I am looking for BL thickness for still undeveloped flow. I would be very grateful for any help
Best regards

The BL thickness for a pipe should be the same as for parallel plates (for the same free stream velocity) because the BL thickness is small compared to the pipe radius, so, to the BL, the pipe wall looks like a flat plane.