Near-inertial parametric subharmonic instability of internal wave beams
Near-inertial parametric subharmonic instability of internal wave beams
复制标题
内波束的近惯性参数次谐波不稳定性
DOI：
10.1103/physrevfluids.2.074801
复制
发表时间：
2017
期刊：
影响因子：
--
通讯作者：
T. Akylas
中科院分区：
文献类型：
--
作者：
H. Karimi;T. Akylas
讨论了均匀分层流体中内波光束的参数亚谐不稳定性（PSI），其中光束频率几乎是由于背景旋转引起的惯性频率的两倍。与一般的PSI相比，有限宽度的光束在物理上更容易受到近惯性频率的次谐波扰动的影响，因为这些扰动具有较小的群速度，并且与底层光束保持接触的时间更长，从而提取更多的能量。这种近惯性PSI的弱非线性理论是在“杰出的极限”，其中三重非线性相互作用，色散和粘性耗散的影响是同样重要的。这个模型是用来检查一个均匀的光束的线性稳定性的无穷小扰动下的“泵浦波”近似，以及PSI的非线性发展，考虑到不断增长的扰动对光束的演化的影响。近惯性PSI是可能的一般局部限制的轮廓的光束，形成鲜明对比的通用PSI，可以只出现准单色光束，其轮廓包括一个正弦载波调制的局部限制的包络。理论预测是一致的，与早期的数值模拟的半日内潮束产生的大陆架断裂在纬度以上和以下的临界值，在该次谐波半日频率匹配当地的惯性频率。
Parametric subharmonic instability (PSI) of internal wave beams in a uniformly stratified fluid is discussed, for the case where the beam frequency is nearly twice the inertial frequency due to background rotation. Compared with generic PSI, beams of finite width are expected on physical grounds to be more vulnerable to subharmonic perturbations of near-inertial frequency, as these disturbances have small group velocity and stay in contact with the underlying beam longer, thus extracting more energy. A weakly nonlinear theory for such near-inertial PSI is developed in the “distinguished limit” where the effects of triad nonlinear interactions, dispersion, and viscous dissipation are equally important. This model is used to examine the linear stability of a uniform beam to infinitesimal perturbations under a “pump-wave” approximation, as well as the nonlinear development of PSI that takes into account the effect of the growing perturbations on the beam evolution. Near-inertial PSI is possible for beams of general locally confined profile, in sharp contrast to generic PSI which can arise only for quasimonochromatic beams whose profile comprises a sinusoidal carrier modulated by a locally confined envelope. The theoretical predictions are consistent with earlier numerical simulations of semidiurnal internal tide beams generated over the continental shelf break at latitudes above and below the critical value, at which the subharmonic semidiurnal frequency matches the local inertial frequency.