Effects of Shear-Layer Roll-Up on Axisymmetric DNS Velocity Signals in a Coaxial Jet Configuration

Abstract

For the characterization of developing turbulent flows, which are dominated by the dynamics of vorticity structures of different scale, it would be very useful to connect the features of the velocity fluctuations detected at a certain point with the evolution of the structures that are present in the nearby regions. To this end, outputs of DNS calculations, which can provide simultaneously both the evolution of the vorticity and the “velocity signals” corresponding to the different regions of the flow field, may be used. Obviously, only low Reynolds number conditions can be analysed, but the influence of basic mechanisms, as roll-up, passage and pairing of vortical structures, can be singled out, both as regards their “signatures” on the velocity time histories, and their effect on different statistical quantities. In particular, indications may be obtained on the processes that contribute to the production of Reynolds stresses, and thus to the average mass transport between two streams at different velocities. This type of analysis might then be considered as a very first step towards the identification of possible procedures for the recognition of the presence and evolution of the dominating vortical structures from velocity signals obtained experimentally at higher Reynolds number.