Explain boundary layer development over a flat plate.

When fluid flows over a flat plate, viscous effects create a boundary layer where velocity varies from zero at the surface (no-slip condition) to freestream velocity at boundary layer edge (δ). Initially, flow is laminar with parabolic velocity profile; boundary layer thickness grows as δ ∝ √(νx/U). At critical Reynolds number (Re_x ≈ 5×10⁵), transition to turbulent boundary layer occurs with fuller velocity profile and faster growth. Turbulent boundary layer has higher skin friction but better momentum transfer. Understanding boundary layers is crucial for drag prediction, heat transfer, and flow separation analysis.