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

量化表面波耗散能量的速率对于正确模拟上层海洋的混合、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，只要波浪没有破裂，很少甚至没有能量从波浪进入湍流。本研究将使用一种新型仪器，光学湍流传感器（OTS）来探索这种假设的有效性的局限性，该仪器可以测量很短时间内的湍流，因此可以在实验室中解决波的不同部分的湍流强度。该项目将使我们对波浪下的上层海洋湍流有更丰富的了解，并改进模式中使用的参数化。该项目还将支持两名研究生，包括通过两所机构正在进行的社区项目为公众开展外展活动，通过学生运营的视频项目“水癖”分发波浪和湍流可视化。在大多数情况下，波浪的破碎已被证明是耗散的主要机制，但也很清楚，在非破碎条件下，需要额外的耗散来匹配观测值。为了解决这个问题，导出了由于剪切波诱导的斯托克斯漂移与预先存在的湍流相互作用而导致的膨胀波能量耗散的方程。另一种方法认识到水具有有限的粘度，因此必然存在由波轨道运动引起的湍流和耗散。不幸的是，在平均流、波浪破碎、剪切风漂移流和浮力效应共存的地区，基本上不可能分离出这些提出的耗散源。一些实验室研究已经证实了非破碎波引起的湍流的存在。然而，流动可视化研究并没有看到湍流的证据，直到波的振幅如此之大，以至于微破碎可能是重要的。解决这一重要的知识差距，关于湍流耗散的非破碎表面波，将采取全面的实验室研究。既定的测量技术将在第一年用于校准和验证OTS。OTS可以在短至1ms的时间窗内测量温度耗散谱。这样可以在非平稳条件下测量湍流，并可以解决波相依赖性。接下来的实验将量化当非破碎波在最初静止的流体上传播时湍流发生的时间和空间尺度。额外的仪器将用于观测平均流量、波高和波坡。最后的实验将量化湍流的产生和放大在更现实的条件下，当有显著的预先存在的湍流。首先，单色机械波将在含有网格产生的湍流的水面上传播。然后包括能量分布在~0.2 ~ 2hz的波。最后，波场将受到可变风的强迫，并将进行更长时间的采样，以研究剪切平均流存在时湍流的产生。本研究将解决预先存在的湍流的作用，解决湍流是否存在与波相关的阈值，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, 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 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.