Boundary Layer and Its Separation

Boundary Layer Separation

• A boundary layer forms around a solid object in a flowing fluid.
• Velocity within the boundary layer increases from zero at the surface to the free-stream velocity.
• Boundary layer thickness increases along the object's length.
• Friction at the surface slows down fluid, reducing kinetic energy.
• Momentum exchange recovers this energy.
• Separation occurs when the boundary layer detaches from the surface, potentially leading to backflow and energy loss.
• The point where this happens is called the separation point.

Preventing Boundary Layer Separation

• Separation is undesirable, causing energy loss and undesirable flow patterns.
• Methods to prevent separation include:
  • Suction of slower-moving fluid.
  • Energy input via a blower.
  • Modifications to wing designs (bypass).
  • Rotating the boundary layer in the flow direction.
  • Creating a slight divergence in diffusers.
  • Adding guide vanes in bends.
  • Introducing a trip wire in laminar flow (e.g., over a sphere).

Boundary Layer Types

• Laminar Boundary Layer:

  • Smooth, parallel flow.
  • Occurs at lower Reynolds numbers.
  • Lower skin friction.
  • Less stable; prone to transitioning to turbulent flow.

• Turbulent Boundary Layer:

  • Chaotic, irregular flow with eddies.
  • Occurs at higher Reynolds numbers.
  • Higher skin friction.
  • More stable; less prone to separation.

Transition from Laminar to Turbulent

• Factors affecting transition include:
  • Reynolds number (the ratio of inertial to viscous forces).
  • Surface roughness (rough surfaces promote turbulence).
  • Pressure gradient (adverse pressure gradients can lead to transition).