本文主要研究為將二維背向階梯流流場(Backward-Facing Step Flow) 加入合成噴流制動器用於背向階梯流場之控制，藉由風洞實驗觀察背向階梯流流場的剪流層趨勢。實驗環境設定為階梯模型高度20mm，流道突張比為1.07，安置以電磁喇叭為驅動元件的合成噴流制動器，結合於背向階梯內部空間，經由寬度1 mm之展向狹縫朝主要流動方向產生噴流。在風洞風速為9m/s時，以階梯高度為特徵長度及自由流流速為特徵速度所得之雷諾數Reh=1.18×104。
為探討合成噴流裝置之操作特性於二維背向階梯流場之控制，以電磁喇叭為驅動元件，結合背向階梯內部空間，經由展向狹縫朝主要流動方向產生噴流。由實驗結果可知，驅動電壓之頻率與振幅對噴口速度有明顯影響。就展向均勻度而言，在驅動頻率100 Hz時，展向均勻度較佳。頻率較低時，狹縫兩端噴出速度在高電壓驅動下會有明顯的衰減，三維現象較為明顯。當驅動電壓為10V時，噴口的瞬時速度變化，在較高速度峰值處會產生震盪現象，原因可能是給予的電壓過大，合成噴流裝置的擴音器薄膜很快接近位移極限，而無法正常移動。因此本實驗驅動峰值電壓最大為8V。
實驗之驅動頻率為80 Hz、100 Hz及120 Hz，使用熱線測速儀測量0h~12h等截面於背向階梯流場中的平均速度與紊流統計資訊。由實驗結果可知，受合成噴流制動器所激擾後背向階梯流流場流場特性都會提前約2~3h的距離發生。在下游流場的發展中，速度擾動值的紊流強度或動量傳遞，均會隨著流場越往下游越呈現消退且擴散的趨勢。
為更進一步了解合成噴流制動器作用於背向階梯流場之演變而利用相位平均法加以分析與討論，流場主要結構演變主要由相位平均速度判定。本研究亦由相位平均量測中所得隨時間演變之速度分佈圖提出合成噴流制動器作用於背向階梯流場系統之模型。在隨時間演變之速度分佈圖中可得知在背向階梯狹縫噴口處可以看到週期性變化及合成噴流與外在主流場的混合情形。如果以速度梯度來界定剪流層位置，則合成噴流制動器明顯可以產生很強的剪流層震盪，並有效的限制背向階梯流場迴流區的大小。

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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