- 學位論文
粒子影像測速技術開發—窗口偏移法之應用
Development of Particle Image Velocimetry — Phase II: Applications of Window Offset Methods
摘要
粒子影像測速技術（PIV），為流場可視化技術，具非侵入式測量瞬時區域流場之優勢。綜觀我國PIV研究領域中，大多使用開源或商業軟體，鮮少探討PIV理論。因此本研究將開發採用先進詢問技術（窗口偏移法與網格細化法）之PIV系統，探討該技術對PIV精確度與空間解析度之提升。進行風洞實驗，拍攝三個實驗案例，以本系統進行分析，同時研究二維矩柱搭配圓柱之周圍流場特性。 本研究開發之PIV系統（PIVLIU），理論包含影像預處理之減去背景、高通濾波;影像評估之標準詢問及本文重點，窗口偏移法及其延伸，網格細化法;影像後處理之歸一化中位數檢驗、3×3平均修正。將分析結果和已知位移之合成影像對比，還與開源軟體PIVlab（Thielicke,2014）比較，透過位移向量場、流線、位移分布等分析確定本系統正確性。 最後設計三個實驗案例以探討圓柱尾流對矩柱周邊流場之影響，分別為：無上游圓柱、矩柱前緣中心上游2單位寬度處設置圓柱、以及矩柱前緣邊角上游2單位寬度處設置圓柱。此處圓柱直徑皆為0.1單位矩柱寬度。主要發現如下：(1)由相對風速分佈可知：模型前端與中心圓柱尾流會形成一低風速區，(2)由分析時間平均流線可知：在圓柱於矩柱中心前的請況下，分離泡長度最長且厚度最薄;在圓柱於矩柱邊緣前的請況下，分離泡長度最短；在無圓柱的情況下，分離泡厚度最大，而長度居中。(3)由分析渦度可知：剪切層邊界具正渦度，且在上游無圓柱情況下強度最強；在矩柱中心上游有圓柱下，渦度範圍最廣；在矩柱邊角前有圓柱的情況下，渦度範圍最小。
並列摘要
Particle Image Velocimetry (PIV) is a flow visualization technique used for non-intrusive measurement of instantaneous regional flow fields. In the field of PIV research in our country, it is observed that open-source or commercial software is mostly used, and there is little exploration of PIV theory. Therefore, this study aims to develop a PIV system using advanced interrogation techniques such as window offset and grid refinement to investigate the improvement in PIV accuracy and spatial resolution. Wind tunnel experiments will be conducted for three cases, and analyzing them using the developed system. The characteristics of the flow field around a 2D rectangular prism with a cylinder are studied. The PIV system developed in this study (PIVLIU) includes the following key components: Image preprocessing with background-subtraction and high-pass-filtering; Image evaluation using standard interrogation and method of window offset, which is the main focus of this thesis. By extending this technique with grid refinement, the resolution of the derived velocity field is increased by a factor of 2. The post-processing involves a normalized median check to identify erroneous vectors, which are then corrected using the averaged value obtained from the 3x3 neighbors. The developed PIV evaluation system (PIVLIU) is validated by successfully comparing the particle displacement fields of synthetic images. It is further used to derive the displacement fields in experimental images and compared them to those obtained from open-source software (PIVlab, Thielike, 2014). Finally, three experimental cases are designed to explore the influence of the cylinder wakes on the flow field around the rectangular model: (i) No upstream cylinder; (ii) Circular cylinder placed at two prism widths upstream to the center of the model windward face; (iii) Circular cylinder placed at two prism width upstream to the prism windward edge. In all cases, the cylinder diameter is 10% of the prism width. The main findings are: (1) From the distribution of relative wind speed, there is a low wind speed area observed in the wake circular cylinder and front of the rectangular prism. (2) From the analysis of the time average of streamlines, the length of the separation bubble is the longest and the thickness is the thinnest when the cylinder is in front of the center of the prism windward face; The length of the separation bubble is the shortest when the circular cylinder is placed upstream to the edge. The thickness of the separation bubble is the largest when no upstream circular cylinder is presented. (3) From the analysis of vorticity, it can be seen that the shear layer is defined by a band of strong vorticity. The vorticity is the strongest when there is no perturbation of the cylinder wake; Widest distribution of vorticity is observed when the cylinder is placed upstream to the center of the prism windward face; In the case of the windward edge cylinder, the distribution of shear layer vorticity is narrowed.