Computational study of jetlet structures in perforate silencers

Reynolds Averaged Navier Stokes allow for the steady state solutions while Large eddy simulation is a useful tool for predicting the spatial and temporal behavior of flow structures generated by complex geometries. However, many predictions suffer from poor grid resolution and initial conditions resulting in poor development of the initial jet shear layer and consequent incorrect prediction of critical flow behaviour. In this work, two 1mm thick perforate plate geometries of 23% and 40% porosity with 2mm diameter holes at an overall pressure ratio of 1.45 have been investigated. Results presented in this paper show the initial jetlet and fully merged jet flow-field to be sensitive to the porosity and the presence of partial holes around the circumference of the plate. The increase in porosity reduces the available entrainment flow, and increases the local jetlet interaction and resultant turbulence levels. This interaction fundamentally changes the flow structure from coherent vortex rings (found at low porosity) to a helical structure. The 2nd and 4th order spatiotemporal correlation Rij and Rij,kl are presented as evidence of the associated impact on acoustic source modeling.