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

Abstract

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 (δ). 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 ∼ 2.0 and the jet Reynolds number of Re = 10,000 to 50,000; where Re ≡ UoD/ν with D, Uo and ν 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 fF rises as either L decreases or Uo increases. For the rectangular FEP film of L/D = 0.5 ∼ 2.0, the jet-flapping Strouhal number StF ≡ fFD/Uo varies over the range of 0.05 ≤ StF ≤ 0.23. This StF is noticeably lower than that (≈ 0.5 ∼ 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.