N型海嘯波溯升與沖流之CFD模擬與影像辨識驗證

Abstract

本研究結合實驗影像辨識與 OpenFOAM 數值模擬，探討前導下沉 N 型波與其相對應孤立波於 1:10 斜坡上之溯升與沖流特性。為補足傳統僅以最大溯升高度進行比較之不足，本文將最大溯升位置視為波浪可能影響範圍之指標，並進一步利用實驗影片逐幀擷取溯升前線位置，計算最大溯升前之沖流速度，以分析水體上爬過程中的動態推進特徵與潛在衝擊程度。影像處理部分，透過魚眼校正、HSV 顏色分割、反光抑制與邊界擷取等程序，自動判定溯升前線位置。後續驗證影像辨識所得之最大與最小溯升值多數落於 ±5% 容許誤差範圍內，顯示此方法具有良好可靠性。 數值模擬部分，本文以 OpenFOAM v8 搭配 olaFlow 建立二維數值水槽，並透過局部網格加密、靜水預跑與場量映射等方式改善初始條件造成之異常流速問題。修正後的數值模型可合理重現斜坡底部水位歷時與溯升前線變化，並作為後續代表案例比較之依據。最後以案例 N25 前導下沉 N 型波及其相對應孤立波作為代表案例進行比較。結果顯示，不論實驗影像辨識或數值模擬結果，相對應孤立波之最大溯升位置皆略高於案例 N25；然而案例 N25 在最大溯升前之主要上爬階段仍呈現較高之前線推進速度。顯示前導下沉 N 型波即使未必具有較高之最大溯升位置，仍可能具有較快之沖流特性。

This study combines experimental image recognition and OpenFOAM numerical simulation to investigate the runup and swash characteristics of leading-depression N-waves and their corresponding solitary waves on a 1:10 slope. To supplement the limitation of conventional analyses that mainly compare the maximum runup height, this study extracts the runup front position from experimental videos frame by frame and further calculates the swash velocity to describe the dynamic characteristics of the uprush process. For the image processing, the runup front position is automatically identified through fisheye correction, HSV color segmentation, reflection suppression, and boundary extraction. The validation results show that most of the maximum and minimum runup values obtained from image recognition fall within the allowable error range of ±5%, indicating that the proposed method has good reliability. For the numerical simulation, a two-dimensional numerical wave flume is established using OpenFOAM v8 with olaFlow. Local mesh refinement, still-water pre-running, and field mapping are applied to reduce the abnormal velocity caused by the initial condition. After the initial-condition treatment, the numerical model can reasonably reproduce the water-level history near the slope toe and the runup-front evolution, providing the basis for the subsequent representative-case comparison. Finally, the leading-depression N-wave N25 and its corresponding solitary wave are selected as representative cases for comparison. The results show that, in both the experimental image-recognition results and the numerical simulation results, the corresponding solitary wave reaches a slightly higher maximum runup position than N25. However, both results indicate that N25 generally has a higher runup-front velocity during the main uprush stage before maximum runup. This suggests that a leading-depression N-wave may still exhibit faster swash motion even when it does not necessarily produce a higher maximum runup position.