A new type of self-excited flapping jets due to a flexible film at the nozzle exit

This paper reports a new type of flapping turbulent jets that are self-excited by flutter of a flexible film with a leading edge fixed axially and centrally at a round nozzle exit. Both flutter conditions of a film and flapping jet characteristics are investigated by varying the film material and dimensions of length (L), span (S) and thickness (delta). Visual observation, flow visualization and hot-wire anemometry are used to examine the flapping jet versus the free counterpart without involving any film. Experiments are made at L/D = 0.5 similar to 2.0 and the jet Reynolds number of Re = 10,000 to 50,000; where Re = UoD/v with D, U-o and v being the jet-exit diameter, exit-averaging velocity and fluid viscosity, respectively. Results show that the film's flutter domain reduces with increasing its overall stiffness and varies with the film's shape, size and thickness. The jet-flapping frequency f (F) rises as either L decreases or U-o increases. For the rectangular FEP film of L/D = 0.5 similar to 2.0, the jet-flapping Strouhal number St(F) = f(F)D/U-o varies over the range of 0.05 <= St(F) <= 0.23. This St(F) is noticeably lower than that (approximate to 0.5 similar to 0.7) of the primary vortex passage in the non-flapping free jet, but extraordinarily one to two orders of magnitude higher than those for the self-excited oscillation of a jet from conventional fluidic nozzles.