VISUALIZED FLOW PATTERNS OF DOUBLE CONCENTRIC JETS AT LOW ANNULUS VELOCITIES

The flow structures in the near wake region of the unducted double con centric jets at low Reynolds numbers are studied by smoke-wire flow vi sualization technique. Four typical characteristic flow regimes-weak f low, pre-penetration, transition, and penetration-are identified for d ifferent jet velocities in the near disk region. Flow patterns and beh avior in each characteristic flow regime are investigated. The contour s of the separation surfaces and the lengths of the recirculation zone in various flow regimes are correlated with nondimensional parameters . The recirculation length reaches a maximum in the transition region. The toroidal recirculation region exhibits both expelling and shear-l ayer vortex shedding. The shedding processes are presented, and the fr equencies are correlated with Strouhal number and central-to-annular j et velocity ratio.