The Effect of Oblique Waves on Jet Turbulence

Coaxial jets are used in many technological applications such as burners, turbofans, etc. They are important also for fundamental research, since they represent a basic flow configuration. However, its full characterisation is governed by numerous parameters [1]. The prevailing model for more than a decade was that coaxial jets could be considered as a simple combination of single jets [2], where the two shear layers develop independently from each other. This simple view was however modified when Dahm et al. [3] in their flow visualisation study evinced the existence of different topological flow regimes for different velocity ratios, ru = Uo/Ui (hereby Ui and Uo denote the maximum absolute velocity of the inner and outer streams at the nozzle exits, respectively), and absolute velocities. One of the most important results of Dahm et al. [3] was the finding of a mutual interaction between the two shear layers. The authors found that the development of the inner shear layer was ‘locked’ into the development of the outer shear layer and is henceforth known as the ‘locking phenomenon’. Their flow visualisation study revealed that the vortices from the inner shear layer were trapped into the free spaces left in between the consecutive outer layer vortices. Buresti et al. [1] showed that the each other, has a crucial role in the evolution of transitional coaxial jets. They found that for a certain range of ru, two trains of alternating vortices are shed from both sides of the inner wall with a frequency, which is related to the vortex shedding frequency. In a recent study Talamelli and Gavarini [4] formulated a theoretical background for this experimental finding. They showed, by means of linear stability analysis, that the alternate vortex shedding behind the inner wall can be related to the presence of an absolute instability, which exists for a specific range of velocity ratios and for a finite thickness of the wall separating the two streams. The authors proposed that this absolute instability may provide a continuous passive forcing mechanism for the destabilisation of the whole flow field even if the instability is of limited spatial extend. In the present abstract we show results from an experimental investigation, which aims to prove the possibility to use this mechanism to control the development of coaxial jets not only in the inner mixing region, but also in the outer one, where the annular jet interacts with the ambient fluid. The absence and presence of the vortex shedding phenomenon behind the inner separating wall is hereby introduced by a sharp and thick wall geometry, respectively. It is evident from figures 1–2 that the ‘locking phenomenon’ is reversed, i.e. the outer shear layers vortices are trapped into the spaces left free by the inner ones and their passage frequency collapses with that of the vortex shedding frequency (cf. left plot in fig. 3). To our knowledge this is the first time that the reverse mechanism of the ‘locking phenomenon’ was shown experimentally, confirming the results of the linear stability analysis of Talamelli and Gavarini [4] and underlining the importance of the geometry of the inner separating wall. The experiments also showed that the vortex shedding mechanism enhances the turbulence intensity both within the inner and outer shear layer (cf. right plot in fig. 3). Further insight into this passive control mechanism and its effect on the turbulence statistics will be presented in the paper.