使用隱式曲面造型與體積霧呈現異重流體三維網格數據

Abstract

電腦圖學對計算流體力學最大的助益，莫過於科學計算視覺化的應用。但現有科學視覺化圖像的擬真性通常不如三維多媒體繪圖的真實感圖形，且部分視覺化技術會受限於缺乏網格組態或節點關係而難以成像。本研究的目的是結合多媒體電腦繪圖常使用的隱式曲面造型法與三維噪訊紋理呈現的體積霧技巧，根據所需加以調整，以利於應用在計算流體力學異重流體三維網格數據的視覺化成像上。重點在於提出一種偏向擬真、視覺化的真實感圖形成像，有別於現有科學視覺化商業軟體講求的易讀性與精確性，以便運用於不具備異重流知識背景的非相關專業人士宣傳與介紹之用途。研究進行二維與三維固定網格間距數據的隱式曲面生成參數測試，歸納出元球法有效半徑與理想門檻值的關係式。並選擇四筆具有指標性特徵的數值資料進行實例應用，包括潰壩、二相流、水槽異重流、水庫異重流等不同數值方法生成的模擬結果。由測試的結果可知隱式曲面造型法生成的表面具有良好的幾何型體連通性，易於呈現計算流體力學模擬結果包含封閉水體的自由液面資料，或是異重流臨界輪廓的描述。且可藉由調整參數修改曲面的平滑與精緻度，藉此控制顯示效能與成像結果。同時可知體積霧技巧可以用於描述雲霧或異重流邊界等模糊物，增加細節層次以及景物的真實感。藉由噪訊紋理與景物的融合，改善傳統視覺化三維多邊形模型難以呈現模糊效果。

The most important benefit of computer graphics in the field of computational fluid dynamics is to visualize the data results from numerical computations. However, an image created by computer graphic techniques is not so realistic and some techniques can not be easily applied due to lack of mesh profile or node relationship. The purpose of this study is to apply the implicit surfaces modeling method and the volumetric-foggy rendering technique for visualizing computed data, and describing the movement of three-dimensional density current. The obtained images and animations are helpful for those who have no hydraulic background for understanding the phenomena of fluid flow.n this study, parameters of implicit surfaces modeling method are estimated, and a regressed relation between influence radius and ideal threshold for Metaballs scalar function is also established. Four cases of applications are performed including dam-break flow, two-phase flow, density current in the flume, and the density current in the Shihmen Reservoir. The results suggest that the implicit surfaces modeling method can generate continuous surfaces describing free surface of fluid and density current. Furthermore, the smoothness of implicit surfaces and computational efficiency can be adjusted through parameters of scalar function. Volumetric fog can exhibit the object with fuzzy boundary such as cloud, fog or muddy water. Blending noise texture with three-dimensional scene can improve object more realistic, especially interface between density current and clear water. The fuzzy effect of volumetric fog display is different from the polygon model in current visualization technique.