TURBULATORS

The effect of the sharp leading edge is very similar to that of a turbulator wire in the main stream ahead of the leading edge. A similar effect is obtained by mounting, on or just behind the leading edge, a raised ‘trip strip’ or leading edge turbulator, which may be of various forms and sizes. In each case, what is required is a brief separation bubble followed by turbulent re-attachment downstream. A ‘turbulator’ which is too small will not achieve the early transition, but one which is too large may itself cause flow separation.

Once the boundary layer has been forced into turbulence, it remains important that it should not separate from the upper surface. A profile with a turbulator or sharp leading edge still requires the air to flow against an adverse pressure gradient once it has passed the minimum pressure point A thin profile presents a less formidable task to the boundary layer, so separation may be avoided, on die upper surface. On the underside, at high angles of attack flow separation is unlikely since once the point of maximum pressure is passed, the flow speeds up and tends to follow the surface of a thin profile closely. At low angles of attack under-side separation is very likely behind the leading edge, but re­attachment is still probable before the trailing edge (compare Fig. 2.3).