| Summary: | 博士 === 國立臺灣科技大學 === 機械工程系 === 93 === The vee-shaped fluidic oscillator is developed by inserting a target blockage with a specially designed crescent surface into the downstream cavity of a slit vee-gutter. Stable, self-sustained, periodic fluidic oscillations can be induced by the dynamic Coanda effect when the geometric parameters of the target blockage and the Reynolds number are properly tuned. The fluidic oscillations are directed through two slit passageways and injected into the near wake of the vee-shaped fluidic oscillator like the pulsing jets. The oscillation behaviors, frequency selection, streamline patterns, and turbulence properties of the unsteady flows in the near wakes of the vee-shaped fluidic oscillator are studied experimentally in a wind-tunnel by using the smoke-wire flow visualization technique, hot-wire anemometer, and laser Doppler velocimeter. Flow fields of the slit vee-gutter and the closed-tip vee-gutter, which are the counter parts of the vee-shaped fluidic oscillator, are measured as well for comparisons. The Strouhal number of the fluidic oscillation based on the slit width of the presently developed oscillator can attain about 0.56 at large Reynolds numbers, which is about 80 times larger than the previous results for the enhancement of heat transfer and about 60 times larger than that for the fluidic flowmeter. The fluidic oscillations act as an excitation source to the wake. The kinetic energy of the fluidic oscillations is transferred to turbulent fluctuations and therefore causes increases in the size of the recirculation bubble and the turbulence intensity in the wake of the vee-shaped fluidic oscillator. The turbulence intensity in the near wake can be increased by about 7% when compared with that of the closed-tip vee-gutter. In order to further enhance the understanding and to increase the functionality of the developed fluidic oscillator, the particle tracking flow visualization method (PTFV) and the particle image velocimetry (PIV) are employed to diagnose the flow field in the forced fluidic oscillator. The experiments are conducted in a water tunnel. The Reynolds number ranges from 1332 to 2333. The forced fluidic oscillator is made of transparent acrylic so that the flow visualization and the PIV measurements are possible. The time-evolving instantaneous and time-averaged flow and streamline patterns are obtained. The statistical properties of the turbulence, e.g., the probability density function, autocorrelation coefficient, power spectrum density function, time and length scales of the turbulence, turbulence intensity, shear stress, vorticity, are extracted from the measured raw data of the PIV experiments. A technique of jet-deflection-plate is developed to control the injection path and excitation situation of the oscillation jets on the wake flow. The results reveal that the vee-shaped fluidic oscillator with the installation of the jet-deflection-plate can effectively modulate both the small-scale, large-scale flow structure and the turbulence.
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