| Budget Amount *help |
¥2,100,000 (Direct Cost: ¥2,100,000)
Fiscal Year 1992: ¥600,000 (Direct Cost: ¥600,000)
Fiscal Year 1991: ¥1,500,000 (Direct Cost: ¥1,500,000)
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| Research Abstract |
The present experimental study is concerned with active control of circular free jets, whose initial conditions are in the fully-developed turbulent state. Two different jet-flow configurations were considered; (1)"pipe jets" which discharge from a long straight pipe, and (2)"pseudo-nozzle jets" which discharge from a contraction nozzle but have thick initial boundary layer thickness.
Main results of the research are summarized bellow. The following notations will be used. D: pipe diameter, Ue: jet exit speed, f: frequency of velocity fluctuations, fe: excitation frequency, x: axial distance from jet exit plane.
a) The Strouhal number (based on the pipe diameter) of dominant frequency of velocity fluctuations at x/D=0.5 - 2.5 increases with decreasing the initial momentum thickness, and decreases with increasing x/D.
b) The jet preferred mode fnD/Ue, defined at x/D=3 - 4, falls in the range of 0.3 - 0.5 for all the initial conditions tested, but tends to decrease when the initial boundary layer thickness exceeds about 0.15D.
c) The development of turbulent pipe jets depends on the frequency and level of excitation. For feD/Ue=0.1 - 1.0, the jet spreading rate increases; the maximum effect of excitation occurs at feD/Ue=0.45 - 0.5. For feD/Ue=1.2 - 2.0, the so-called "turbulence suppression phenomenon" occurs, and the jet spread slightly decreases.
d) The response of the psuedo-nozzle jet to periodic forcing is more sensitive than that of the turbulent pipe jet.
e) The maximum spreading of the pseudo-nozzle jet is achieved by excitation at feD/Ue=0.55 - 0.65, which is higher than the corresponding value of the pipe jet. Under this excitation condition, spectral contents around the frequency of 0.5fe in velocity fluctuations are significantly amplified, and the predominant mode observed in the unexcited case disappears.
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