Experiments and numerical simulations of mass transfer and flow evolution in transient rectangular free jet of air

The paper investigates the transient flow evolution, up to the steady state, of a rectangular free jet of air with aspect ratio AR = 6.3. The Reynolds number, Re, defined according to the hydraulic diameter, D, of the rectangular slot of height H, equal to about D = 2H, spans in the range from Re = 48,000 to Re = 3400. A centrifugal fan generates the jet flow with a velocity increasing, as a ramp, from zero up to a constant steady state value. In the transient state of the flow the jet is composed of a primary vortex (PV) and a quasi-stable stem. In the transient 2D experimental instant images the stem of the jet has a height equal to that of the slot and a length increasing with the time up to a maximum value, which is greater than the lengths observed in the steady state, i.e. Negligible Disturbance Flow (NDF) and Small Disturbance Flow (SDF). The experimental jet evolution, observed with measurements and image processing, show that the primary vortex pinch-off does not occur for all the Reynolds numbers investigated, while the leading vortex structure becomes progressively incoherent and breaks down into turbulence. Two-dimensional (2D) Large Eddy Simulations (LES) of mass transfer and fluid flow evolution confirm the experimental images. The transient LES simulations are in good agreement with the experimental results.