Spectral analysis of round jet instabilities at low Reynolds number

Abstract

For the study of round jet flow at low Reynolds number, a fine analysis of instabilities is necessary. In this paper, we present a frequency analysis of temporal and spatial velocity variations in a round jet with a Reynolds number equal to 830. The data were obtained from Laser Doppler Anemometry (LDA) measurements for various downstream positions. Attention is focused on the characteristics of the instantaneous signal of the longitudinal velocity component u (t ) in order to characterize the transition from laminar state to turbulent state. The spectral analysis of the LDA signals of the axial component of the exit velocities highlights the presence of a single peak of energy around a characteristic frequency f 0 . This frequency corresponds to the amplified mode of the instability. The jet develops a sinuous mode which is the most unstable anti-symmetric mode. The results obtained in this study concerning the sinuous mode frequency, agree well with those obtained by laser tomography technique and images analysis.