Large eddy simulations (LES) of two hot three-stream jets are used to assess key assumptions made in recent volumetric and surface-based models of jet noise sources based on Reynolds-Averaged Navier Stokes (RANS) flow fields, and to explore the proper definition of a linear surface-based model. The jets studied are at the conditions of a supersonic turbofan engine at takeoff.;A new method for the detection of the RANS-based surface of peak stress (OSPS) for an eccentric jet is employed. Direct measurements of the Reynolds stress and convective velocity show that the RANS-based OSPS and convective velocity are in good agreement with the LES results. In addition, evaluation of the instantaneous pressure fields suggests that the most energetic events happen at the OSPS. These results lend credence to the use of the RANS-based OSPS to simulate the convective Mach number distribution in a multi-stream jet.;Efforts are made on the study of the proper location of a "radiator surface" for a linear surface-based model. This surface follows the "edge" of the jet, defined as the boundary between the rotational and irrotational fields. Previously employed definitions for the location of this surface based on thresholds of the gradient of mean axial velocity place it too far on the acoustic field, and therefore fail to produce a surface with the desired property. A surface on which the convective velocity matches that on the OSPS falls exactly on this boundary. This surface also appears to follow the outer band of a layer of negative pressure skewness on both of the nozzles studied.
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