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| Boundary Layer Theory-1 |
- D’Alembert’s paradox: Even for low viscosity fluids drag is experienced
- Prandtl introduced the concept of a boundary layer: there is a region very close to the wall where viscous effects are important
- No slip condition is valid at the wall.
- Even though viscosity of the fluid may be small, steep velocity gradients exist close to the wall since velocity goes from zero at the wall to its free stream value far away from the wall.
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| Boundary Layer Theory-2 |
- The region close to the wall where the velocity gradients are important is the boundary layer
- Velocity goes from zero to free stream and the thickness of this boundary layer can be defined as that, at the outer edge of which the velocity reaches 99%, 99.5%, 99.9% …… of the free stream velocity. The thickness is often denoted as δ99, δ99.5, δ99.9 ......
- The physical thickness of the boundary layer can vary significantly therefore.
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