LAMINAR-TURBULENT BOUNDARY-LAYER-TRANSITI ON

The boundary layer growing on a wing of an airfoil is generally lamina r near the leading edge. Then, along the chord wing, unstable waves ap pear, grow and finally break down into turbulent spots. As the drag is higher in turbulent motion than with a laminar fluid, this is a reaso n which explains why the researchs on the laminar-turbulent transition are very important from a practical point of view. The linear stabili ty theory is a very classical tool but it remains efficient. A fluctua tion is superimposed to a given mean flow; this fluctuation takes a wa ve form which is convected with the main flow. The theory shows that t he boundary layer is dispersive: the phase velocity depends on the wav elength, moreover, the theory is able to predict the most dangerous wa ves, their frequencies, wave-lengths, amplification rates, but it is u nable to predict something about the transition onset location. Theref ore an empirical criterion, the so-called e'' method, is very useful f or practical applications. Some examples for two- and three-dimensiona l flow at low and high speed are given below. The suction is a well-kn own example of Laminar Flow Control technic and some results are given in the present paper in order to show the high efficiency of the suct ion when it is applied at ''good'' locations. Two aspects are not take n into account with the e'' method: firstly the receptivity problem an d secondly the non-linearities which are present just before the trans ition onset. These two points are currently developed and the first re sults, even if they are not straightforward, seem to be very promising .