Controlling the type of boundary layer

Since the type of boundary layer influences both surface friction drag and flow separation, it is important to know what factors control the transition from laminar to turbulent boundary layer flow.

We have already mentioned that if the pressure is decreasing in the direction of flow (a favourable pressure gradient) transition is delayed. Transition is also delayed if the surface is smooth and without undulations.

The position of transition to turbulent flow on an aerofoil moves forwards with increasing speed V and also if the air density p is increased. It moves rear­wards if the coefficient of viscosity R (a measure of the stickiness) increases. The distance of the transition from the leading edge also depends on the aero­foil chord length c for a given section shape, since increasing the chord, and hence the overall size, will increase the length of the region of favourable pres­sure gradient.

The dependence of the transition position on the speed, density viscosity and chord, as described above, can be expressed in terms of a single quantity known as the wing Reynolds number, where

density x speed x wing chord

Wing Reynolds number is

viscosity coefficient

or in mathematical symbols

Re = (PVC)

R

The transition position moves forward as the Reynolds number increases.

Reynolds number is just a number with no dimensions, like a ratio. It is a term that frequently crops up in aerodynamic literature, and always has the form (pVl)/R where l is a length.