表面重力波破碎起始的能量学特征
An energetic signature for breaking inception in surface gravity waves
AI总结：
针对表面重力波破碎动力学机制不明的问题，通过数值波浪水槽模拟研究动能平衡方程演化，发现破碎前四分之一周期的动能收敛速率突变特征，可同时预测破碎发生及强度。
摘要：
对表面重力波破碎物理过程的动力学理解仍是流体动力学领域一个未解决的问题。从概念上讲，破碎可由起始（inception）和发生（onset）描述，其中破碎起始指波峰内未知不可逆过程的启动，它早于可见的破碎发生现象。为寻找破碎起始的能量学指标，我们利用在数值波浪水槽中模拟的、非定常波包内演化的一组非破碎与破碎波峰，研究动能平衡方程中各项的演化过程。我们观察到，在破碎发生前约四分之一波周期时，动能收敛速率会快速上升，触发动能增长率的不可逆加速。这一仅出现在后续会发生破碎的波峰中的能量学特征，在动能增长率超过临界阈值时产生。此时，额外的动能收敛无法通过将多余动能转化为势能或摩擦耗散来抵消。我们的结果表明，该能量学特征出现时动能平衡方程主导项的比值与波峰破碎强度成正比。因此，这一能量起始点既能预测破碎发生的可能性，也能指示破碎事件的强度。
英文摘要：
A dynamical understanding of the physical process of surface gravity wave breaking remains an unresolved problem in fluid dynamics. Conceptually, breaking can be described by inception and onset, where breaking inception is the initiation of unknown irreversible processes within a wave crest that precede the visible manifestation of breaking onset. In the search for an energetic indicator of breaking inception, we use an ensemble of non-breaking and breaking crests evolving within unsteady wave packets simulated in a numerical wave tank to investigate the evolution of each term in the kinetic energy balance equation. We observe that breaking onset is preceded by around one quarter of a wave period by a rapid increase in the rate of convergence of kinetic energy that triggers an irreversible acceleration of the kinetic energy growth rate. This energetic signature, which is present only for crests that subsequently break, arises when the kinetic energy growth rate exceeds a critical threshold. At this point the additional kinetic energy convergence cannot be offset by converting excess kinetic energy to potential energy or by dissipation through friction. Our results suggest that the ratio of the leading terms of the kinetic energy balance equation at the time of this energetic signature is proportional to the strength of the breaking crest. Hence this energetic inception point both predicts the occurrence of breaking onset and indicates the strength of the breaking event.
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