Experimental and numerical investigation of a variable density swirling-jet stability

International audienceLinear stability of a variable density, and incompressible, swirling jet is studied at moderate Reynolds numbers, both experimentally and numerically. Mean flow characteristics are obtained experimentally. They show a correlation between axial velocity and density profiles. For low density ratios, s, the presence of self-sustained oscillations of the flow (global modes) is highlighted. Using direct numerical simulations (at low azimuthal wavenumbers m = 0, −1, −2) to study the local convective/ absolute transition at the jet exit-nozzle, it is shown that frequencies of the local absolute-instability are in good agreement with those of the experimentally obtained global modes. Furthermore, it is found that increasing the swirl number q enhances the absolute instability for helical modes m=−1 and−2. Finally, it is also shown that the unstable selected mode, experimentally obtained, has the spatio-temporal properties of the absolutely unstable one obtained numerically