Boundary Layer BL Thickness Why the advantages did Prof Pra

Boundary Layer (BL) Thickness - Why (the advantages) did Prof. Prandtl introduce the BL concept? Define (with a plot) and explain the physical meaning of: (a) Thermal BL Thickness, delta T, and (b) Displacement Thickness, delta_1(or delta\'). Can BL thickness delta be measured?

the prandtl introduce bl concept