Strongly interacting aligned multiple jets are produced behind a perforated plate placed in a uniform flow. The perforation patterns investigated experimentally are a square and a triangular lattice of holes with diameters d ranging from 1 mm to 10 mm and of mesh size M ranging from 2.54 mm to 25.4 mm. At moderate Reynolds numbers (Re = ud/nu < 3000), each laminar jet develops instabilities causing its effective diameter to increase, thus leading the parallel jets to merge at a distance L from the plate. The merging distance L 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 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 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 regime 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.
