Self sustained oscillations and collective behaviours in a lattice of jets

Strongly interacting aligned multiple jets are produced behind a perforated plate placed in a uniform flow. The performation patterns investigated experimentally are a square and a triangular lattice of holes with diametersd ranging from 1 mm to 10 mm and of mesh sizeM ranging from 2.54 mm to 25.4 mm. At moderate Reynolds numbers (Re=ud/<3000), each laminar jet develops instabilities causing its effective diameter to increase, thus leading the parallel jets to merge at a distanceL from the plate. The merging distanceL is shown to exhibit a low frequency self sustained oscillation around its mean value with a lateral correlation length much larger than the mesh size. Both the merging distanceL and the oscillation frequency are shown to be functions ofM and of the jet velocity. At larger values ofRe, the merging distance approaches a constant mean value and the amplitude of the oscillations becomes vanishingly small.At the scale of the mesh of the lattice, the oscillating phenomena is shown to result from the local confinement of the jet by its nearby neighbours. This observation is consistent with the fact that when the effect of the nearby jets is simulated by rigid walls, the frequency of the jet's oscillations is found to be of the same order. The influence of the hydrodynamical régime of the individual 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 non linear saturation of the instabilities of the jet.