Passive control of annular jet instabilities studied by Proper Orthogonal Decomposition
Résumé
Shear flows are complex turbulent flows which are widely used in the industrial domain. An annular jet is an example of these particular flows (used in burners, cooling processes, inlet valve in a combustion chamber, processing glass fibers...): an obstacle for the flow, placed in the center of a round nozzle creates two axisymmetric shear layers at the jet exit. These shear layers are significant for the organization and the evolution of the flow. This study talks about coherent structures developed in shear layers and their effects on three-dimensional instabilities of the annular jet flow. A method to detect and identify coherent structures, called Proper Orthogonal Decomposition (P.O.D.) is applied on velocity fields recorded by Particle Image Velocimetry (P.I.V.) measurement to understand how a modification of the nozzle geometry could change the flow morphology. The P.O.D. post-processing is used to compare different central obstacle geometries (a conical, cylindrical or ellipsoidal shape). The Reynolds number based on the outer diameter Do is equal to 107 800 (for a velocity U0 at the exit of the nozzle equal to 30m.s-1). Instabilities of annular jets are a real handicap for industrial processes. Studying the influence of a passive control could improve understanding of shear flows mechanisms and then determining a new way to reduce or suppress annular jets instabilities in order to meet manufacturer's needs.
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