為控制與影響等勢核末端之後流場之結構，本研究以熱線測速器量測及流場觀測之實驗方法探討低速平面噴流在反對稱低頻激擾下之渦流結構特性。擾動設備為裝置於噴嘴出口上下的兩片金屬平板，以出口風速及出口寬度為特徵長度之雷諾數為8.2103。因在下游區所誘發的渦流波長尺度較初始動量厚度或不穩定波之尺度大至少兩個尺度以上，所以此類激擾又可稱為長波激擾。雖然長波激擾對下游區之相干性結構演變有顯著的影響，但在近場區卻有著與自然噴流相似的流場結構。因激擾使得勢流區末端之後的噴流擺盪特性被增強，所以激擾噴流的擴展特性被顯著的提升。能量頻譜中顯示遠場區只有激擾頻率及其倍頻具有較高的能量，搭配流場觀測結果顯示遠場區的渦流結構為大尺度且為非對稱模態。為探討相干性在空間上的演變特性，所以本研究以實驗髮量測渦流傳遞速度，其結果顯示在近場之值與理論值相同，因為渦流模態在遠場區會轉變成接近反對稱之激擾模態大尺度渦流，所以其傳遞速度會降低至一穩定值，且其值會相當接近該位置之局部平均速度(0.35 )。根據頻譜及渦流傳遞速度，經換算後可得渦流之空間尺度且其尺度與流場觀測之結果相當類似，且亦證實了長波激擾確實在下游區會誘發出大尺度的渦流結構，這樣的渦流結構發生的位置並會隨著激擾頻率降低而往下游移動。
為探討渦流模態的演變，本研究以雙頻激擾的方式同時強化上游具主頻特徵及下游具激擾頻率特徵的渦流模態。一系列廣泛的平均流場量測用於分析同時受到聲波及長波激擾影響之平面噴流流場，整個相干性結構的發展則由頻譜分析及不穩定模態演變進行確認。結果將由不同激擾條件進行比較與分析，其顯示自然噴流中之顯著頻率模態被低頻激擾模態抑制，而且整個噴流由上游基頻模態發展到下游激擾模態的相關性因非線性交互關係所以並不大。
最後，下游區具備激擾頻率特徵之大尺度相干性結構的發展與交互作用則是利用相位平均法加以分析與討論，大尺度渦流結構位置主要由相位平均之紊流強度及剪應力判定，由渦流中心隨時間的空間位置所計算得到的渦流傳遞速度之值與平均量測所得之值相符。結果指出下游區大尺度渦流的存在確為造成擴展增加的主要原因。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噴流-渦流系統之模型，藉此便可以完整解釋中心線擺動主要原因。儘管在所量測與觀測區域中在不同激擾頻率有不同的渦流結構，較低的激擾頻率所誘發的大尺度渦流會在更下游的位置形成，所以量測區域內噴流擴展特性在較低頻率的增加主要由上下劇烈振動所造成。

In order to control and influence the far field flow structures, the vortical characteristics of a low-speed plane jet under anti-symmetric low-frequency excitation are well investigated experimentally by means of hot-wire anemometric measurements and smoke flow visualization. The jet flow velocity in the exit can be operated in the range of =0~15m/sec with the turbulence intensity below 0.9% of the center velocity. However, the jet is normally operated at 10m/sec for most of experiments, which has the corresponding Reynolds number of 8.2103 based on the nozzle exit width of 12mm. The perturbations for flow excitation are introduced with two oscillating mental strips which are flush mounted right at the nozzle exit, and five kinds of excitation frequencies are conducted, which are 5, 7.5, 10, 12.5 and 15Hz. The excitation frequency can be controlled by changing the rotating speed of the servo motor and is monitored by an optical tachometer. The excitation mode is anti-symmetric with the jet center and the excitation amplitude is fixed at 1mm. The mean velocity and the fluctuation intensity are obtained by long-time averaging calculations. In addition, because the wavelength of the oscillating perturbations is larger than the initial momentum thickness and initial instability wavelength at least two orders of magnitude, this kind of excitation method can be claimed as long-wave excitation.
The basic flow properties of the natural plane jet are firstly examined. The results of the initial Strouhal number, based on the initial momentum thickness, initial instability frequency and jet exit velocity are well fitted with the theoretical value of a laminar jet flow. In addition, the values of shape factor in the initial shear layers of the jet exit are all below 2.5, indicating that the jet flow properties are initially laminar in the jext exit. In addition, the velocity profile near the jet exit is a top-hat shape and will become bell-shape after end of the potential core where the flow has gradually transformed into turbulent in the downstream. The mixing and spreading characteristics have slight influence under different exit velocities.
The spatial developments of the jet flow under long-wave excitation will be spread out from the experimental data of mean velocity distributions and spectral evolution. The experimental results further indicate that the evolution of coherent structures is significantly influenced by long-wave excitation in the downstream, but it is similar to the natural jet in the near field. The spreading properties are influenced obviously due to the enhancement of the jet flapping motion after the end of potential core. Only the excitation frequency and its harmonics are observed in the far field region and the vortical structures at this region are of large-scale and in asymmetric mode. Results of convective speed show that the value in the near field fitted with the theoretical value of . The initial vortical mode will change to the near anti-symmetric, low-frequency excitation mode in the far field (x/H>9) and its convective speed will decrease to a fixed value of about 35% of and similar to the value of local velocity. Results of spectrum and convective speed indicate that the spatial scales of vortices can be easily obtained and agreeable with the data from the flow visualization. It also reveals that there are induced large-scale vortices in the downstream under long-wave excitation. The occurrence locations of these kinds of vortical structures will move to downstream when the excitation frequency decreases.
According to the evolution study of vortical structures, a serious of extensive mean velocity results is proposed to analyze the influence of acoustic excitation in the near field. The developments of coherent structures are identified by the spectral and instability evolutions. Finally, the general comparison for all excitation cases is addressed and the results indicate that the preferred mode frequency is suppressed by the low-frequency excitation mode and the developments of fundamental and low-frequency excitation modes are less dependent due to their nonlinear interactions.
The phase-averaged technique is employed to analyze the developments and interactions of large-scale coherent structures in the far field which possess the features of excitation frequency mode. The developments of large-scale vortical structures are calculated by the locations of vortex cores obtained from the phase-averaged fluctuation intensity and shear stress. When the spatial locations of large-scale vortices vary with time, the averaged convective speed obtained is very much similar with the data from the previous measuring results. It also shows that the occurrence of lager-scale vortices in the far field is a primary cause of huge increment of spreading. This study also proposes a vortex-jet system model to well explain the centerline vibration. In spite of having two different kinds of vortical structures after the end of potential core when and , the results also indicate the induced large-scale vortex will take place over the measuring domain. Therefore, the increment of spreading when is mainly caused by the strong oscillation up and down.

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