An analysis is made of the turbulent boundary layer in the sense of the well known phénoménologie semi-empiric plane boundary-layer theory, based on an analogy with the power rheologic laws widely used in the study of the non linear viscous flows. So a simple ’’one layer method” to calculate the plane turbulent boundary layer using Prandtl’s model of turbulence is first prepared, and then tested in the case of a plane diffusor with the satisfactory results.