使用有限尺寸之受局限噴嘴形成連續噴流衝擊平板，在不同的出口至平板距離及噴流雷諾數條件下，使用實驗方法研究衝擊噴流的流場特徵。使用雷射輔助煙霧流場可視化技術觀察衝擊噴流中心平面上的特徵流場照片;利用熱線風速儀量測剪流層之不穩定性的瞬時速度;藉由質點影像速度儀獲得的流場的平均速度場，分析衝擊噴流的速度向量、流線、渦度輪廓及速度分佈;使用壓力轉換器量測平板上方分佈的壓力孔，記錄受噴流衝擊之壁面上的衝擊壓力。根據觀察流場圖片，在不同的噴流雷諾數範圍內，可以辨識出三種特徵流場模態。在低噴流雷諾數時，流場特徵為層流渦漩流模態，衝擊噴流的行為是兩個軸對稱的層流環狀渦漩。在中噴流雷諾數時，流場特徵為過渡流動模態，噴流衝擊平面後由軸向往徑向偏折，然後沿著徑向往外流動，在衝擊噴流上沒有渦漩結構形成。在高雷諾數時，流場特徵為紊流渦漩流模態，平板上之噴流之剪流層上形成渦漩，衝擊噴流的特徵行為轉變為兩個軸對稱的紊流環狀渦漩。在紊流渦漩流模態之渦漩的尺寸較層流渦漩流模態小。渦度輪廓圖沿著噴流中心軸呈現軸對稱的輪廓分佈，在中心軸上的渦度值趨於零。當噴流衝擊平板時，噴流的動量在平板上轉換為停滯壓力，所以形成較大的衝擊壓力。

The flow characteristics of the continuous jet impinging on a flat plate using a finite confined nozzle at various exit-to-plate distances and jet Reynolds number were experimentally investigated. The characteristic flow patterns in the median plane of the impinging jet were examined using the laser-assisted smoke flow visualization technique. The instantaneous velocities of instability in shear-layer were detected by the hot wire anemometer. The time-averaged velocity fields were carried out by particle image velocimetry (PIV) were applied to analyze the velocity vectors, streamline patterns, vorticity contours, and velocity distributions of the impinging jet. The impingement pressure of the impinging jet was determined using the pressure transducer through the pressure tap arrangement at the flat plate. According to the flow patterns observed on the flat plate, three characteristic flow modes are identified within the different ranges of the jet Reynolds number. At low jet Reynolds number, the “Laminar vortical flow” appeared. The impinging jet behaves two axisymmetric laminar vortex rings. At moderate jet Reynolds number, the “transitional flow” appeared. The jet impinges on the flat plate, deflects from the axial direction to radial direction, and then flows outward along the radial direction. No vortical flow structures are formed in the impinging jet. At high jet Reynolds number, the “turbulent vortical flow” appeared. The vortices evolve in the shear layer of the impinging jet on the flat plate. The impinging jet behaves two axisymmetric turbulent vortex rings. The vortex rings in turbulent vortical flow mode are smaller than those in laminar vortical flow mode. The vorticity contour of the impinging jet is distributed symmetrically with respect to the jet axis and the vorticity along the jet axis is zero. As the jet flow impinges the flat plate, the jet momentum is transformed to stagnation pressure on the flat plate so that the impingement pressure is large.
Keywords: Continuous impinging jet, wall jet, PIV, Laser-assisted Flow visualization

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