LAMINAR-TURBULENT BOUNDARY-LAYER-TRANSITION

The boundary layer growing on a wing of an airfoil is generally laminar near the leading edge. Then, along the chord wing, unstable waves appear, grow and finally break down into turbulent spots. As the drag is higher in turbulent motion than with a laminar fluid, this is a reason which explains why the researchs on the laminar-turbulent transition are very important from a practical point of view. The linear stability theory is a very classical tool but it remains efficient. A fluctuation is superimposed to a given mean flow; this fluctuation takes a wave form which is convected with the main flow. The theory shows that the boundary layer is dispersive: the phase velocity depends on the wavelength, moreover, the theory is able to predict the most dangerous waves, their frequencies, wave-lengths, amplification rates, but it is unable to predict something about the transition onset location. Therefore an empirical criterion, the so-called e'' method, is very useful for practical applications. Some examples for two- and three-dimensional flow at low and high speed are given below. The suction is a well-known example of Laminar Flow Control technic and some results are given in the present paper in order to show the high efficiency of the suction when it is applied at ''good'' locations. Two aspects are not taken into account with the e'' method: firstly the receptivity problem and secondly the non-linearities which are present just before the transition onset. These two points are currently developed and the first results, even if they are not straightforward, seem to be very promising.