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Abbassi A.,Unit of Metrology and Energetic systems | Aissia H.B.,Unit of Metrology and Energetic systems
Fluid Dynamics and Materials Processing | Year: 2014

This paper describes the transition from forced to mixed convection in a jet flow with variable properties. The classical laminar layer analysis is extended by taking into account the dependence of physical properties on temperature. The related model relies on the assumption that the variations of Prandtl number and specific heat at constant pressure are sufficiently small to be neglected. A second-order finite-difference numerical method based on a staggered grid is used to analyze transition, hydrodynamic and heat transfer phenomena in the jet. The dimensionless control parameter, L = T0/T¥, is limited to values less than 1. It is found that the variation of physical properties with temperature has a significant effect on jet flow velocity, whereas this dependence has an almost negligible influence on the expansion characteristics of the flow. © 2014 Tech Science Press.

Abbassi A.,Unit of Metrology and Energetic systems | Kechiche N.,Unit of Metrology and Energetic systems | Ben Aissia H.,Unit of Metrology and Energetic systems
Fluid Dynamics and Materials Processing | Year: 2014

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. © 2014 Tech Science Press.

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