Читать книгу Wind Energy Handbook - Michael Barton Graham - Страница 128

The velocity of the flow adjacent to the surface of any solid body, and in particular wind turbine blades and aerofoils, reduces to zero relative to the body at its surface (the no‐slip condition) due to viscous stresses in the fluid. At usual flow Reynolds numbers [O(10⁵) to O(10⁸)] occurring in practice, diffusion is much slower than streamwise convection. As a result nearly all of the change in velocity takes place in very thin regions next the body surface called boundary layers, which therefore exhibit a strongly sheared velocity profile; see Figure A3.2. These boundary layers grow in thickness from the attachment point and are shed eventually into the wake of the body. They convect downstream as free shear layers, forming a wake where viscous stresses are similarly significant. Outside the boundary layers and wake the flow behaves almost as if inviscid. The integrated streamwise component of the skin friction on the body surface due to the viscous stresses gives rise to an important component of the drag on the body, the skin friction drag. The other component is the pressure drag (the integrated streamwise component of the normal forces on the body surface). This component is small because the front half streamwise component of the pressures on the body nearly balances the downstream half; the thinner the boundary layer, the nearer they are in balance. The pressure drag is usually similar in size to the skin friction drag for streamlined bodies, such as aerofoils, but becomes much larger if boundary layer separation occurs. The combined skin friction and pressure drag for an aerofoil section in 2‐D flow is known as the profile drag. The profile drag coefficient of an aerofoil is quite small for these Reynolds numbers while the flow remains attached, depending weakly on the Reynolds number and the angle of attack.

Figure A3.2 Boundary layer showing the velocity profile.