SELF-SUSTAINED OSCILLATIONS AND COLLECTIVE BEHAVIORS IN A LATTICE OF JETS

Strongly interacting aligned multiple jets are produced behind a perfo rated plate placed in a uniform flow. The perforation patterns investi gated experimentally are a square and a triangular lattice of holes wi th diameters d ranging from 1 mm to 10 mm and of mesh size M ranging f rom 2.54 mm to 25.4 mm. At moderate Reynolds numbers (Re = ud/nu < 300 0), each laminar jet develops instabilities causing its effective diam eter to increase, thus leading the parallel jets to merge at a distanc e L from the plate. The merging distance L is shown to exhibit a low f requency self sustained oscillation around its mean value with a later al correlation length much larger than the mesh size. Both the merging distance L and the oscillation frequency are shown to be functions of M and of the jet velocity. At larger values of Re, the merging distan ce approaches a constant mean value and the amplitude of the oscillati ons becomes vanishingly small. At the scale of the mesh of the lattice , the oscillating phenomena is shown to result from the local confinem ent of the jet by its nearby neighbours. This observation is consisten t with the fact that when the effect of the nearby jets is simulated b y rigid walls, the frequency of the jet's oscillations is found to be of the same order. The influence of the hydrodynamical regime of the i ndividual jets on the oscillations and the role of the lattice pattern on the collective behaviour is discussed on hand of an original model which focuses on the role of the recirculation zone on the delayed no n linear saturation of the instabilities of the jet.