Abstract

This study investigates a new type of the flapping jets `self- excited' by a thin FEP film fixed at the leading edge axially and centrally to the jet nozzle exit. Hot-wire anemometer and flow visualization are used to measure the flapping-jet flows induced by different-length films and also the non-flapping counterpart, i.e., the free jet without any film involving. The rectangular film length (L) is varied for L/D = 0.5 ~ 2.0 at the constant width W = D while the jet Reynolds number Re ≡ UoD/v is taken between 10,000 and 50,000; here, Uo and D are the jet-exit velocity and diameter while v is the fluid viscosity. It is found that the effective length of a rectangular film for the flapping to occur depends upon the exit velocity. The flapping frequency fF reduces as L is increased and rises with increasing Uo. For the present nozzle integrating with a rectangular FEP film of L/D = 0.5 ~ 2.0 and , the flapping Strouhal number StF ranges in 0.05 <= StF <=0.23. These Strouhal numbers are substantially lower than that (≈0.65) for the primary vortex shedding in the free jet, but one to two orders of magnitude higher than those from the self-exciting fluidic devices. Results also show that the flapping jet decays and spreads far more rapidly than the free jet. The decaying and spreading rates vary with L and Re.