When a fluid encounters a solid surface, a boundary layer forms due to the interaction between the fluid's motion and the stationary surface. This phenomenon is characterized by a thin region adjacent to the surface where viscous forces dominate, influencing the fluid's velocity profile. The development of the boundary layer begins at the leading edge of the surface and evolves as the fluid moves downstream.

As the fluid flows over the surface, friction between the fluid and the wall slows down the particles nearest the surface. This effect creates a velocity gradient, where the velocity of the fluid increases from zero at the wall to the free-stream velocity farther away. The region where this velocity gradient exists defines the boundary layer. Over distance, the thickness of the boundary layer increases as more fluid is affected by the surface's resistance.

In the region close to the leading edge, the boundary layer is typically laminar. Here, fluid particles move in smooth, parallel layers with minimal mixing. The flow remains stable under certain conditions, but as it progresses downstream, factors such as surface roughness, velocity increases, or pressure gradients can destabilize the flow. Once the flow reaches a critical Reynolds number, the laminar boundary layer transitions to a turbulent state. The turbulent boundary layer is characterized by chaotic mixing of fluid particles, increased energy dissipation, and a thicker boundary layer compared to its laminar counterpart.

The laminar boundary layer's velocity profile can be described mathematically using the Blasius equation, a solution to the Navier-Stokes equations under specific assumptions. For practical applications, the Momentum Integral Boundary Layer equation provides a way to analyze momentum changes in the boundary layer without requiring a detailed velocity profile. These models are critical for understanding fluid dynamics near surfaces, particularly in engineering and aerodynamic applications.