Abstract: In this paper, the wavelet analysis method is used to perform the time frequency double local decompositions for the signal of streamwise fluctuating velocity both in time space and in frequency space. The signal is obtained from a hot film anemometer output in an experimental study of turbulent boundary layer over a smooth flat surface. The energy evolution of fluctuating velocitywith scales is studied using the wavelet coefficients. The maximum energy criterion is proposed to determine the time scale th...