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A numerical investigation is presented to study the flow
characteristics behind an axially-mounted center-body of an
confined annular swirling jet flow. The numerical scheme solves
the nonlinear, axisymmetric Navier-Stokes equations. The
governed equations are transferred to the difference equations
by using the finite difference method. The projection method is
used to solve the pressure Poisson equation to retain the
constraint of the equation of continuity. The velocity is then
solved by using the explicit Adams-Bashforth scheme. Bythis
numerical algorithm, second order accuracy in both time and
space discretizations can be obtained. The advantage of using
the linearized convective outflow boundary condition is that the
length of the computational domain in theaxial direction can be
reduced significantly. Observing from the velocityprofile,
it is found that the length of recirculation bubble is strongly
associated with the strength of vortex shedding. There are seven
different flow patterns which vary with the shape of
recirculation bubble, and the structures within the
recirculation bubble are effected by the Reynolds number and
swirl number. Besides, different flowfield geometric parameters
（blockage ratio & expansion ratio）also have large-scale
effects on the recirculation bubble.
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