Three-dimensional receptivity of boundary layers

The paper presents a review of results of some recent (mainly experimental) studies devoted to a quantitative investigation of the problem of receptiv ity of the 2D and 3D boundary layers with respect to various 3D (in general ) external perturbations. The paper concentrates on the mechanisms of excit ation and development of stationary and travelling instability modes in a 3 D boundary layer on a swept wing, as well as in 2D boundary layers includin g the Blasius Row and a self-similar boundary layer with an adverse pressur e gradient. In particular, the following problems of the boundary-layer rec eptivity are discussed: (i) receptivity to localized 3D surface roughness, (ii) receptivity to localized 3D surface vibrations, (iii) acoustic recepti vity in presence of 3D surface roughness, and (iv) acoustic receptivity in the presence of 3D surface vibrations. All experiments described in the pap er were conducted using controlled disturbance conditions with the help of simulation of the stationary and non-stationary perturbations by means of s everal disturbance generators. This approach gives us the possibility to ob tain quantitative results which are independent of any uncontrolled backgro und perturbations of the how and the experimental model. In contrast to the data obtained at "natural" environmental conditions these results can be d irectly compared with calculations without any significant assumptions abou t the physical nature of the disturbances under investigation. The complex (amplitude and phase) coefficients of the boundary-layer receptivity to ext ernal perturbations, obtained as functions of the disturbance frequency and the spanwise wavenumber (or the wave propagation angle), represent the mai n results of the experiments described. These results can be used for the e valuation of the initial amplitudes and phases of the instability modes gen erated by various external perturbations, as well as for quantitative verif ication of linear receptivity theories. Several examples of the comparison of experimental results with calculations are also presented in this paper. A brief analysis of the state-of-art in the held is performed and some gen eral properties of different receptivity mechanisms are discussed. (C) 2000 Editions scientitiques et medicales Elsevier SAS.