Collaborative Research: Temporal and spatial scaling of dissipation under non-breaking surface waves

对表面波消散能量的速率进行量化对于正确模拟上层海洋混合、环流和海浪本身是必要的。到目前为止，大部分工作的一个基本假设是，只要波浪不破裂，就很少或根本没有能量从波浪进入湍流。这项研究将使用一种新的仪器--光学湍流传感器(OTS)来探索这一假设的有效性限度。该仪器可以在很短的时间内测量湍流，以便在实验室中解析波不同部分的湍流强度。该项目将导致对波浪作用下上层海洋湍流的更丰富的理解，并导致用于模式的改进的参数化。该项目还将支持两名研究生，包括通过两所大学正在进行的社区项目面向普通公众的外联活动，并通过学生运营的视频项目Waterlust分发波浪和湍流的可视化图像。在大多数情况下，波浪破碎已被证明是波能消散的主要机制，但也已清楚地表明，为了与非破碎条件下的观测结果相匹配，还需要额外的消散。为了解决这一问题，推导了剪切波诱导的Stokes漂移与预先存在的湍流相互作用引起的涌浪能量耗散方程。另一种方法认识到水具有有限的粘性，因此必然存在由波的轨道运动引起的湍流和耗散。不幸的是，在平均流、波浪破碎、切变风流和浮力效应共存的领域中，基本上不可能分离出这些所提出的耗散源。一些实验室研究已经证实了不破碎波诱导湍流的存在。然而，流动可视化研究没有看到湍流的证据，直到波幅达到微破裂可能是重要的。要解决关于不破裂表面波的湍流消散的这一重大知识鸿沟，将需要一项全面的实验室研究。已建立的测量技术将在第一年用于校准和验证OTS。OTS可以测量短至1ms的时间窗口的温度耗散谱。这使得能够测量非平稳条件下的湍流，并可以解决波的相位相关性。接下来的实验将量化当不破裂的波在最初静止的流体上传播时发生湍流的时间和空间尺度。将使用额外的仪器来观测平均流量、波高和波斜率。最后的实验将量化在更真实的条件下，当存在明显的预先存在的湍流时的湍流的产生和放大。首先，单色机械波将在含有网格产生的湍流的水中传播。然后将包括能量分布在~0.2到2赫兹之间的波。最后，波浪场将在变风的情况下被强迫，并将被采样更长的持续时间，以研究当存在切变平均流时的湍流产生。这项研究将解决先前存在的湍流的作用，解决是否存在与波浪有关的湍流阈值，并确定湍流发展的时间、深度和空间尺度。

Quantifying the rate at which surface waves dissipate energy is necessary to properly model upper ocean mixing, circulation and the waves themselves. A fundamental assumption for much of the work so far is that little to no energy goes from waves into turbulence as long as the waves are not breaking. This study will explore the limits of validity of this assumption using a novel instrument, the Optical Turbulence Sensor (OTS) that can measure turbulence over very short time periods, so that turbulence intensity over different parts of a wave can be resolved in the laboratory. This project will result in a richer understanding of upper ocean turbulence under waves and lead to improved parameterizations to be used in models. The project will also support two graduate students, include outreach activities for the general public through ongoing community programs at both institutions, and distribute wave and turbulence visualizations through the student-run video project Waterlust. Breaking of waves has been shown to be the dominant mechanism for dissipation of wave energy in most circumstances, however it has also become clear that additional dissipation is required to match observations in non-breaking conditions. To address this, equations were derived for the dissipation of swell wave energy due to interaction of the sheared wave-induced Stokes drift with pre-existing turbulence. An alternative approach recognizes that water has finite viscosity and as such there must be turbulence and dissipation induced by the wave orbital motions. Unfortunately, it is essentially impossible to isolate these proposed sources of dissipation in the field where mean flows, wave breaking, sheared wind-drift currents and buoyancy effects co-exist. Some laboratory studies have confirmed the existence of non-breaking wave-induced turbulence. However, flow visualization studies did not see evidence of turbulence until the wave amplitudes were such that microbreaking could be important. Addressing this significant knowledge gap regarding the turbulent dissipation of non-breaking surface waves will take a comprehensive laboratory study. Established measurement technologies will be used to calibrate and validate the OTS in the first year. The OTS can measure the temperature dissipation spectrum over temporal windows as short as 1 ms. This enables the measurement of turbulence in non-stationary conditions and can resolve wave phase dependence. The next experiments will quantify the temporal and spatial scales at which turbulence occurs when non-breaking waves propagate over a fluid that is initially at rest. Additional instrumentation will be used to observe mean flows, wave heights and wave slopes. The final experiments will quantify turbulence generation and amplification in the more realistic condition when there is significant pre-existing turbulence. First, monochromatic mechanical waves will be propagated over water containing grid generated turbulence. Then waves with energy distributed from ~0.2 to 2 Hz will be included. Finally, the wave fields will be forced with variable winds and will be sampled for longer duration to investigate the turbulence generation when sheared mean flows are present. This study will resolve the role of pre-existing turbulence, address whether there is a wave related threshold for turbulence and determine the time, depth and space scales over which turbulence develops.