On the evolution of instabilities in the near field of a plane jet

Abstract

The dynamic behaviors of coherent structures in the near field of a plane jet are extensively studied by hot-wire measurements. The instability evolution for both streamwise and transverse velocity fluctuations are investigated simultaneously. The shear stresses contributed by the individual instabilities are detected by using the cross-spectrum technique. Experimental results show that the jet spreading is governed by the vortex interaction mechanism, and the evolution of the instabilities for the two velocity components is found to perform distinct patterns. The characteristics for the instability evolution cannot be adequately identified from only streamwise velocity fluctuations. The vortex formation and merging locations should be determined from the saturation condition of the transverse velocity fluctuations v′( f ). Results also indicate that the instabilities cannot only absorb energy from the mean flow, but also transfer their energy to the subharmonics, or even back to the mean flow. The variations of these instability shear stresses are associated with the resulting formation and merging processes of the coherent structures. The subharmonic evolution model proposed by Ho [in Proceedings of the Symposium on Numerical and Physical Aspects of Aerodynamic Flow, edited by T. Cebeci (Springer, New York, 1982), p. 521] is then further demonstrated and extended from the energy transfer characteristics between the instability waves. Finally, the near-field acoustic properties are investigated to verify that the interaction processes of the coherent structures are the dominant mechanism for the production of sound.