Boundary Layer

Boundary Layer

1. Shear Stress

   • It has a meaningful effect on the flow where the velocity gradients are substantial.
     • Division of flow into two regions, one where friction is much more important than the other
       • Recognized by Ludwig Prandtl in 1904
       • VIscous
         • Thin region of the flow adjacent to the body
           • Large velocity gradients
           • Friction plays a defining role
             • Due to friction, molecules stick to the surface, thus it has zero velocity relative to the surface
               • No-slip condition
                 • Cause of large velocity gradients within the boundary layer
       • Inviscid
         • Vast region of the flow away from the body
           • Friction plays virtually no role
           • Relatively small velocity gradients

2. Thin viscous region adjacent to the body

   • Source of friction drag

3. Flow separation

   • Changes dramatically the pressure distribution over the surface
     • Pressure drag