Shock-cell structures and corresponding sound pressure levels emitted from closely spaced supersonic jet arrays

Shockwave structures and overall sound pressure levels from 8 × 8 arrays of axisymmetric supersonic jets were experimentally characterized for 14 different arrays. The results were also compared to behavior for single jets of similar dimensions. The jet diameters ranged from 3.2 to 6.4 mm and the jet spacing to diameter ratio ranged from 1.44 to 3. The arrays were tested at nominal net pressure ratios ranging from 2 to 24. It was found that above a certain net pressure ratio level, the array of jets coalesced into an interacting lattice of oblique shock interactions. These shock structures consisted of oscillatory shocks at lower pressure ratios and then developed into stable shock structures as the pressure ratio increased. The formation of the interacting shock lattices is strongly correlated with the measured sound pressure levels. Initially the arrays emitted low sound pressure levels, similar to that of a single jet within the array. As the shock structure transitioned from weak to strong the sound pressure levels transitioned to a level similar to that of a single nozzle with an exit area equivalent of the entire array. Shadow graph images of the flow in the arrays are used to correlate to the sound pressure levels measured.