前導下沉N型海嘯波溯升之OpenFOAM數值模擬

Abstract

海嘯是一種需要高度防範的自然災害。傳統文獻中常以孤立波作為海嘯替代模型，然而，孤立波僅有單一波峰的特性，許多學者對此提出質疑並提出了新興的N型波海嘯替代模型。N型波的特點在於具有波峰和低於水面的波谷，其中前導下沉N型波 (Leading Depression N-waves, LDNs) 能夠模擬海嘯來臨前的退潮現象。 本研究使用基於計算流體力學軟體OpenFOAM開發的流體力學模擬工具 olaFlow，建立了二維數值水槽，對LDNs與相對應波峰之孤立波的溯升現象進行比較與分析。數值模擬結果量化並證實了Lo et al. (2024)實驗中的假說：破碎劇烈的type A LDNs由於能量損失大，其溯升值小於相應的孤立波；然而，在實驗中type A LDNs非破碎型態則在模擬結果中發現其有輕微破碎導致能量損失。模擬結果中type B LDNs之溯升值則大於相對應孤立波。此外，模擬結果顯示，LDNs沿著斜板的沖刷速度也大於相應的孤立波。因此，若使用孤立波作為海嘯替代模型，可能會低估海嘯對沿岸的影響範圍和沖刷程度。

Tsunamis are natural disasters that require significant preventive measures. Traditionally, solitary waves have been used as substitute models for tsunamis in the literature. However, due to the solitary wave's characteristic of having a single crest, many scholars have raised concerns and proposed the emerging N-wave tsunami substitute model. The distinguishing feature of N-waves is their crest and trough below the water surface, with Leading Depression N-waves (LDNs) capable of simulating the receding tide phenomenon that precedes a tsunami. This study utilized olaFlow, a computational fluid dynamics simulation tool developed based on OpenFOAM, to establish a two-dimensional numerical wave tank for comparing and analyzing the runup phenomena of LDNs and corresponding solitary waves at wave crests. The numerical simulation results quantified and confirmed the hypothesis from the experiment by Lo et al. (2024): type A LDNs with violent breaking have smaller runup values than the corresponding solitary waves due to significant energy loss. However, the simulation results indicated that non-breaking type A LDNs in the experiment exhibited slight breaking, leading to energy loss. The runup values of type B LDNs in the simulation were found to be greater than those of the corresponding solitary waves. Additionally, the simulation results showed that the swash velocity of LDNs along the slope was also greater than that of the corresponding solitary waves. Therefore, using solitary waves as a substitute model for tsunamis may underestimate the impact range and scouring extent on the coast.