| Summary: | 博士 === 國立成功大學 === 航空太空工程學系 === 105 === The purpose of this research was to explore the two-dimensional backward-facing step flow perturbed by a synthetic jet actuator. The step was placed in a low-speed wind tunnel, and it exhibited a flow passage expansion ratio of 1.07. The Reynolds number was determined to be Reh = 1.18 × 104 on the basis of the step height and the upstream velocity before the step edge. An electromagnetic loudspeaker was used to generate synthetic jet flows in the direction of the main flow through a slot 1 mm in width.
To investigate how the operating properties of a synthetic jet device affect the control of 2D flow over a backward-facing step, an electromagnetic speaker, which served as the actuator, and the internal space of a backward-facing step were employed to create a jet flow through a spanwise slit. The jet flow travelled in the same direction as the backward-facing step flow. The experimental results showed that the frequency and amplitude of the actuating voltage notably affected the nozzle flow rate. When the actuating frequency was 100 Hz, the flow rate demonstrated an even spanwise distribution. When a low frequency was applied, employing a high actuating voltage substantially attenuated the outlet flow rates of the two slit ends, and a 3D phenomenon was observed. When the actuating voltage was 10 V, the instantaneous nozzle flow rate demonstrated a vibration phenomenon at the peak flow rates. This phenomenon might be attributed to the excessively high actuating voltage, which caused the thin film of the speaker to quickly reach its maximum displacement and prevented it from moving regularly. Therefore, the maximum driving peak voltage is 8V of this experiment.
The drive frequency of the experiment is 80 Hz, 100 Hz and 120 Hz. A hot-wire anemometer was adopted to measure the cross-sectional average flow rate and turbulent properties of the backward-facing flow from 0 h to 12 h . The results revealed that under the effect of the synthetic jet actuator, the flow field characteristics of the backward-facing step flow occurred 2–3 h ahead of their normal occurring distances. Regarding the development of the downstream flow field, the turbulent intensity and momentum transfer subsided and dispersed as the flow moved further downstream.
A phase averaging method was employed to further examine the effect of the synthetic jet actuator on the development of the flow over a backward-facing step. The changes in the main flow structure were analyzed using the phase averaging flow rate. Moreover, a flow rate distribution graph with respect to time was plotted using the phase averaging measurement data. Accordingly, a model describing the effect of a synthetic jet actuator on the flow over a backward-facing step was established. Examining the flow rate distribution graph revealed that at the slit nozzle, a periodic change in the flow rate and a mixing phenomenon between the synthetic jet flow and external main flow field were observed. Using the flow rate gradient to determine the location of the shear layer showed that the synthetic jet actuator generated a strong vibration in the shear layer and effectively restricted the size of the recirculation zone.
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