改善空氣動力特性是飛具設計的一大重點。氣流經過凹槽如彈藥艙時會產生複雜的流體動態，高長深比之凹槽會導致阻力的增加並且阻礙內部儲存物的投放。氣流流經低長深比之凹槽時，凹槽前端分離產生的剪力層會直接衝擊凹槽後面，受剪力層於凹槽末端產生的擾動傳遞至上游並形成回饋機制，導致強烈的自我維持震盪，影響飛行器結構體的安全。雖然已有大量關於凹槽流的研究，為了對流場動態有更深入的洞見則須更進一步的探討，其中包含凹槽的幾何效應及流場條件的影響。

本研究在穿音速風洞中進行可壓縮流於圓柱形凹槽及具轉角之矩形凹槽的實驗研究。實驗條件之自由流馬赫數為0.64至0.83，其紊流邊界層厚度約為7毫米。圓柱形凹槽之直徑深度比為4.43至43.00，而矩形凹槽的長深比為4.43至21.50、轉角為5度至45度。實驗結果亦與無轉角之矩形凹槽的研究進行比較。動態壓力傳感器沿著凹槽的中心線及跨距貼平表面分布進行壓力量測，實驗結果除了進行平均值及擾動量的統計分析外，也利用了快速Fourier轉換得到功率譜密度的分布來探討凹槽的流場動態，並將功率譜密度結果與文獻中典型的Rossiter's formula比較。

圓柱形凹槽流的類型分界與矩形凹槽流大致相同，轉角亦不會影響矩形凹槽流之類型。比起過渡型的矩形凹槽流，過渡型的圓柱形凹槽流會有較高的表面壓力係數，但是兩者的壓力梯度差異並不大。圓柱形凹槽產生的最大壓力擾動比矩形凹槽低，當增加矩形凹槽的轉角，亦減緩凹槽後端的擴張及表面壓力的最大擾動。邊界層厚度與凹槽深度比只對接近後凹槽面之表面壓力係數有明顯影響，包含流場在凹槽後端的擴張及表面壓力的最大擾動。在自我維持震盪特性方面，經過最佳化分析，不管是圓柱形凹槽或有轉角之矩形凹槽，其延遲時間以及對流速度與自由流速度比皆低於Rossiter's formula的參數值。此外對一大轉角之矩形凹槽流，流場受較低的Rossiter模態主導，而且可能完全破壞自我維持的震盪。本研究對凹槽流的流場動態有更廣的瞭解，並且支持更進一步的探討，像是時頻分析、計算流體力學、震盪抑制的設計等。

Improvement of the aerodynamic performance is one of the major goals for aircraft design. A flow over cavity such as the weapon bay of an aircraft suffers from its complex fluid dynamics. A cavity with a large ratio of length to depth induces drag and inhibits the deployment of storage. For a cavity with a small ratio of length to depth, the shear layer separates from the leading edge and impinges on its rear face. The feedback mechanism with the upstream propagation of the disturbance leads to strong self-sustained oscillation, which results in unsteady structural loading. There have been enormous studies on the rectangular cavities. However, further investigation is inevitable for more insight, including the geometric effect and the flow condition.

The experimental study determines the characteristics for a compressible turbulent flow over cylindrical cavities and rectangular cavities at yaw. The freestream Mach number ranges from 0.64 to 0.83 and the thickness of incoming turbulent boundary layer is about 7 mm. The ratio of diameter to depth for the cylindrical cavities are 4.43--43.00. The ratio of length to depth and the yaw angles for the rectangular cavities are 4.43--21.50 and 5 deg.--45 deg., respectively. The results for unswept rectangular cavities are used for comparison. Flush-mounted dynamic pressure transducers for mean and fluctuating pressure measurements are installed along the streamwise and spanwise directions. The self-sustained oscillation phenomenon is examined by spectral density analysis with fast Fourier transform. The empirical constants are also evaluated and compared with those in the classical Rossiter's formula.

The present study shows that the boundaries of flow types for cylindrical cavities are similar to those for rectangular cavities. Yaw angle for a rectangular cavity also has a minor effect on the flow types. A transitional cylindrical cavity has a higher surface pressure coefficient near the rear face than a transitional rectangular cavity, but there is a minor difference on the amplitude of pressure gradient. The maximum pressure fluctuations that generated by the cylindrical cavities are less than those generated by the rectangular cavities. Yaw angle of a rectangular cavity alleviates the trailing-edge expansion and surface pressure fluctuation near the rear corner. The ratio of the incoming boundary layer thickness to the depth of the cavity has an evident influence only near the rear corner of cavity, including the trailing-edge expansion and the maximum pressure fluctuation. For the empirical constants, there are smaller time lags and lower ratios of convection velocity to freestream velocity for either cylindrical cavities or rectangular cavities at yaw than those used in the Rossiter's formula. The power spectral density shows that low mode dominates for a rectangular cavity with large yaw angle.

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