Coannular jets—Are they really quiet and why?

Abstract

The noise reductions from both normal- and inverted-velocity-profile (IVP) shock-free coannular jets are critically assessed on the basis of existing data. The fully mixed equivalent (FMEQ) single jet, defined as having a uniform exit profile and the same thrust, mass flow rate and exit area as the coannular jet, is used throughout as the reference for noise comparisons. It is shown that uninverted-velocity-profile coannular jets are consistently noisier than FMEQ jets, whereas IVP jets are slightly quieter. The practical implications are discussed in detail. The “inverted velocity” effects are isolated experimentally from the “inverted temperature” effects and the resulting noise reductions or increases are explained by using the measurements of mean flow, turbulence structure, and source location in coannular jets.