Understanding Boundary Layer Theory When a real fluid flows over a solid body, the particles at the surface cling to it, resulting in zero velocity relative to the body's surface—a phenomenon known as no-slip condition. Moving away from this boundary, velocity gradually increases until reaching free stream velocity (U), creating a region with varying velocities called the boundary layer. This gradient within the boundary layer generates shear stress proportional to viscosity and rate of change of velocity (du/dy). Beyond this thin layer, there is no variation in flow speed or production of shear stress.
Characteristics Within and Beyond Boundary Layers The height up to which variations occur defines the extent of the boundary layer where du/dy exists along with corresponding shear stresses. Above this zone lies uniform flow moving entirely at free stream velocity without any gradients or associated stresses. The transition marks distinct behavior between constrained motion near surfaces versus unrestricted movement further away.