In the present study, the turbulent flow from an annular jet is investigated by using large eddy simulation. Particularly, the effect of the ratio of the inner and outer diameters is one of the main interests of this study. The instantaneous fields presented in this paper showed that behind the jet exit the backflow region, as well known in literatures, exists, and its detailed behavior depends on the ratio of the inner and outer diameters (D 1 /D 2 ). The dependence on D 1 /D 2 is attributed

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... the different shear layer development according to D 1 /D 2 . At small D 1 /D 2 , the development of the outer shear layer is similar to that from the circular jet. However, with increasing D 1 /D 2 , the interaction between the outer and inner shear layers becomes strong, resulting in fast transition to turbulence.