Describe the thermal and velocity boundary layers in convection and their relationship.

The velocity boundary layer is the region where fluid velocity increases from zero at the wall to 99% of free-stream velocity. The thermal boundary layer is where temperature changes from wall temperature to 99% of free-stream temperature. Their relative thickness depends on Prandtl number: for Pr > 1 (oils), thermal BL is thinner; for Pr < 1 (liquid metals), thermal BL is thicker; for Pr ≈ 1 (gases), they are similar. This relationship affects heat transfer: thinner thermal BL means steeper temperature gradient and higher heat transfer. For turbulent flow, both boundary layers have similar structures with a viscous sublayer, buffer zone, and turbulent core.