Low Reynolds Numbers LDA-Experimental Analysis of the Near-Field of an Isothermal Laminar Round Free Jet

Jet transition towards a turbulent state is an interesting topic requiring a detailed analysis of the process leading to the onset and amplification of small flow disturbances. Here we examine experimentally the transition process for an isothermal laminar round free jet at low values of the Reynolds number. Close to the inlet nozzle, the turbulence intensity is assumed to be small enough so that the initial shear layer can be considered laminar and the velocity profile uniform. Experimental data are obtained using a Laser Doppler Anemometry (LDA) technique at various longitudinal and transversal coordinates, (x,y). Spectral analysis of the instantaneous streamwise velocity component u (y,t), at fixed stations x measured from the nozzle exit, reveals that the entrainment physical mechanism, which occurs by engulfment, is caused by the presence of coherent structures. However, in proximity to the jet center, the energy spectrum of the u (x,y = 0,t) velocity component proves the existence of a preferred mode (most unstable mode) of instability that has a convective nature. Our results compare well with those obtained using another experimental method based on laser tomography.