LES study of global instability in annular jets

The paper presents results of numerical investigations of annular non–swirling jets. The research is performed with the use of large eddy simulations (LES) and a high–order code based on the compact differences and the Fourier approximation combined with a projection method for pressure–velocity coupling. We consider various Reynolds numbers and inner shear layer thicknesses and observe that both parameters affect the dynamics of the flow in such a way that the formation of the turbulent structures follows two different scenarios. For the particular group of the inlet conditions the flow pattern is found to be similar to low–swirling jets. The instantaneous flow–fields reveal the formation of the spiral structures located in the inner and outer mixing layers. The analysis shows that the structures are the result of the instability that leads to the precession of the recirculation region formed in the near–field. The winding sense of the spirals are clockwise or counterclockwise depending on the case and azimuthal distribution of perturbations.