What is the reason for loss in total temperature in Boundary layer?

The loss of total temperature in the boundary layer is primarily due to viscous effects and energy dissipation caused by friction between the fluid and the surface. As the fluid moves over a solid boundary, momentum transfer results in turbulence and a reduction in the kinetic energy of the flow, which manifests as a decrease in total temperature. Additionally, heat exchange with the surface can further contribute to this temperature loss, particularly in cases where the surface is cooler than the fluid.