在過去的數十年裡，飛機的幾何外型一直是研究的主要焦點。近年來，為了兼顧氣動性能和隱身能力，創新的設計理念不斷地出現。其中便包含了後平板矩型噴嘴，其特徵是進氣口為圓形，出口形狀為矩形。然而，這種設計可能會導致非預期的尾流偏轉，辨別和理解該偏轉原因便是本研究的主要目標。過往的研究顯示，不同的噴嘴進氣壓力和後甲板長度會導致尾流不穩定。然而，關於噴嘴出口的寬高比參數的研究仍很有限。本文將對進氣壓力、甲板長度和寬高比三者之間的關係進行量化，以探討尾流偏移的機制。本研究設計了一個研究矩陣，噴嘴進氣壓力比範圍為2至4倍大氣壓，甲板長度為30毫米、55毫米（定義為基準模型）和70毫米，以及三種不同的寬高比（6.766、7.54和8.354）。研究運用實驗和數值模擬方法進行基準模型的壓力測量和尾流的震波成像。其餘案例，採用商業軟體STAR-CCM+來輔助分析。
研究顯示，55毫米和70毫米長度的後甲板噴嘴模型較能使尾流保持水平穩定的射出。這歸因於兩個主要原因：在高強度震波情形，流體在較長的甲板上經歷完整的分離和重合，或者在低強度震波中，流體始終附著在甲板上。兩種強度的震波配合較長的甲板有助於穩定的水平尾流射出。除了足夠長的甲板確保尾流穩定性之外，噴嘴的寬高比也具有類似的效果。當噴嘴進氣壓力比等於3時、基準模型的寬高比為6.766時尾流會向下偏轉。隨著寬高比的增加，偏轉角度從8度減少到4度，隨後恢復到水平射出。對於70毫米的甲板長度，在噴嘴進氣壓力等於4時，增加長寬比也可將向下偏轉的尾流（5度）恢復到水平方向。在甲板長度至少為55毫米，透過增加寬高比可有效地抑制尾流偏轉。對於30毫米的甲板長度，噴嘴壓力為2和4時，增加寬高比並不能抑制尾流偏轉。
另外，本研究利用無因次化的均方根速度、無因次化雷諾剪應力、紊流動能、渦流強度和速度剖面來闡述噴流偏移的成因，發現增加寬高比可以抑制尾流偏移，主因是紊流強度會減低而減少偏移，但只在甲板長55 mm和70 mm時增加寬高比才有抑制偏移的效果。矩型噴嘴的流場特徵如典型的馬鞍形速度分布和噴嘴兩側角落的渦流，尾流下游的軸對稱、分岐和融合現象，也一併透過數值模擬方法的剖面圖做觀察。

In the past few decades, the geometric shape of aircraft has been a major focus of research. In recent years, to balance aerodynamic performance and stealth capabilities, innovative design concepts have gradually been introduced to the public. A notable design is the aft deck nozzle with transitions from a circular inlet to a rectangular exit shape. However, this practice may result in undesirable plume deflection. Identifying and understanding the causes of this deflection is the primary objective of this study.
Previous investigations have demonstrated that varying nozzle inlet pressures and aft deck lengths can lead to plume instability. However, there are limited studies that comprehensively incorporate the aspect ratio factor at the nozzle exit into the discussion. In this dissertation, the relationships among these three parameters will be quantified, various permutations and combinations will be systematically analyzed.
This study designs a research matrix with nozzle pressure ratios ranging from 2 to 4, deck lengths of 30 mm, 55 mm (defined as baseline model), and 70 mm, and three different aspect ratios (6.766, 7.54 and 8.354). The pressure measurements and shock wave imaging of the baseline model were conducted using both experimental and numerical simulation methods. For the remaining cases, commercial software STAR-CCM+ was employed to assist in the analysis.
The investigation revealed that the plume maintains stable emanation in most cases with 55 mm and 70 mm length aft deck nozzle models. This is attributed to two main reasons: in a high-intensity shock flow field, the fluid undergoes complete separation and reattachment over the longer deck, or in a low-intensity shock flow field, the fluid remains attached to the deck throughout. Both fluid regimes contribute to stable plume emanation.
In addition to a sufficiently long deck ensuring plume stability, the nozzle's aspect ratio also has a similar effect. The study shows that for the baseline model with a nozzle pressure ratio of 3, an aspect ratio of 6.766 results in downward plume deflection. As the aspect ratio increases, the deflection angle decreases from 8 degrees to 4 degrees, subsequently restoring to a horizontal emanation. With a 70 mm deck length, an increase in aspect ratio at a nozzle pressure ratio of 4 can restore the downward-deflected plume (by 5 degrees) to a horizontal direction. Suppressing plume deflection by increasing the aspect ratio is feasible, provided that the deck length is at least that of the baseline model (55 mm). For a deck length of 30 mm at nozzle pressure ratio of 2 and 4, increasing the aspect ratio does not mitigate plume deflection.
Moreover, this study employs non-dimensional root-mean-square velocity, non-dimensional Reynolds shear stress, and turbulent kinetic energy, along with vortex intensity and velocity profiles, to elucidate the mechanisms behind jet deflection. It was observed that increasing the aspect ratio can mitigate plume deflection, primarily because the turbulence intensity decreases, leading to a reduction in deflection. However, this suppression of deviate is effective only when the deck length is 55 mm or 70 mm.
Finally, the flow field characteristics of the rectangular exit nozzle are presented using numerical simulation methods, specifically through velocity and vorticity contour plots. These diagrams illustrate the axis-switching and bifurcation phenomena downstream of the plume, as well as the vortex characteristics at the corner.

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