For the study of round jet flow at low Reynolds number, a fine analysis of instabilitiesis necessary. In this paper, we present a frequency analysis of temporal and spatialvelocity variations in a round jet with a Reynolds number equal to 830. The data wereobtained from Laser Doppler Anemometry (LDA) measurements for various downstreampositions. Attention is focused on the characteristics of the instantaneous signal of thelongitudinal velocity component u(t) in order tocharacterize the transition from laminar state to turbulent state. The spectral analysisof the LDA signals of the axial component of the exit velocities highlights the presenceof a single peak of energy around a characteristic frequencyf0. This frequency corresponds to the amplified mode of theinstability. The jet develops a sinuous mode which is the most unstable anti-symmetricmode. The results obtained in this study concerning the sinuous mode frequency, agree wellwith those obtained by laser tomography technique and images analysis.
