Turbulence Enhancement in Coaxial Jet Flows by Means of Vortex Shedding

For many years investigations have been conducted to understand the flow instabilities that lead to transition in jets. The inviscid linear stability analysis of Batchelor & Gill [1] has shown that immediately at the jet exit, where the velocity profile is close to a ’top-hat’ one, all the instability modes are able to be exponentially amplified while in the far field region only the helical mode seems to be unstable. The transition between these two different instability regions is still unclear and the analysis is complicated by the presence of several unstable modes embedded in the turbulence background. Therefore, several experiments have been done with active or passive excitation methods in order to highlight the role of a single or few modes in the evolution of the flow. Previous investigations in naturally and artificially excited jets have determined the importance of two instability lengthscales: one associated with the initial shear-layer thickness at the exit of the nozzle, and the other associated with the jet diameter which governs the shape of the mean velocity profile at the end of the potential core. The instability modes in the first region develop through continuous and gradual frequency and phase adjustments to produce a smooth merging with the second region. This process makes this problem from a fundamental point of view interesting, and for that reason it has received a great deal of attention. Axisymmetric excitation by means of acoustic forcing has been able to highlight several important aspects of the complex dynamics involved, like the role played by the so called shear layer and jet column mode acting in the near field of the jet at the nozzle exit and at the end of the potential core, respectively. However, fewer works have been devoted to the investigation of higher azimuthal modes principally due to the higher complexity of the excitation facility (see [2]). This work is aimed to show the effect of two oblique modes (m = ±1) at the nozzle lip generated by means of acoustic forcing. Different excitation amplitudes and frequencies have been tested regarding their effects on the flow dynamics. The relative decrease, with respect to the unexcited case, in the rms value of the streamwise velocity fluctuations in the shear layer centerline at x/D =3 has been reported in figure 1. It is evident that the amplitude of the exciting wave does not play an important role (except at very high levels) and that the rms shows a maximum reduction (around 15%) close to 1500 Hz that is equal to a Strouhal number St = f/U0 = 0.029, i.e. a superharmonic of the shear layer mode. Velocity measurements have been performed in the natural (A) and excited cases with f =1500 Hz (B) and with f =180 Hz (C). (A) corresponds to the unexcited case, (B) to the one with maximum turbulence suppression and (C) to the one associated to the jet column mode. It is evident from figure 2(a) that only in case (B) several nonlinear interactions take place in the near field region leaving some imprint further downstream (see figure 2(b)). The measurements in the mixing layer region confirm this reduction showing that in case (B) (reported in figure 3(b)) the shear layer attains earlier a self similar state in the velocity fluctuation after a faster evolution inside the first diameter and a peak value of the fluctuation intensity 20% less than in case (A) (reported in figure 3(a)) and (C), underlining the important effect of this kind of excitation. This reduction of turbulence could be connected to the same phenomenon described by Elofsson & Alfredsson [3] about the effect of oblique waves in laminar boundary layers, where the authors showed that the interaction of two waves is able to generate streamwise streaks by means of nonlinear interaction. Whether or not oblique waves can be observed in jets will be further investigated by means of several measurement techniques and will be reported in th...