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Simultaneous velocity–density measurements of downslope density clouds
Advances in Water Resources ( IF 4.7 ) Pub Date : 2022-05-07 , DOI: 10.1016/j.advwatres.2022.104215
M.E. Negretti , A. Martin , F. Naaim-Bouvet

We present results from laboratory experiments of the downslope propagation of a finite volume released gravity current by means of combined PIV/PLIF measurements. The experimental data were used to estimate the global characteristics of the current, such as the propagation speed, the lateral surface and the buoyancy, revealing that thermal theory is a robust model that can predict such quantities properly, especially the models of Beghin et al. (1981), Maxworthy (2010) and the theoretical model of Dai (2013a). The collected data allowed also for the determination of the entrainment rates based on the turbulent fluxes instead using the variation of the volume flux, and the regions of entrainment/detrainment have been identified. The results support previous observations of the mechanism responsible for the buoyancy loss of the cloud and its consequent deceleration down the slope, as a large scale recirculation vortex at the back of the cloud. A very complex interior structure of the cloud is observed, with a large variety of turbulent processes taking place, such as the large-scale convectively unstable recirculation vortex at the scale of the current itself, the small-scale convectively unstable motions inside the head of the cloud and close to the bottom boundary and shear (Kelvin–Helmholtz) instabilities at the boundary between the current and the ambient water. The synoptic velocity and density measurements allowed also to test existent parametrizations of turbulent fluxes, that have been quantified in 2D fields with a high spatial resolution. Results confirm that parametrization laws based on the assumption of a constant turbulent diffusivity or mixing length do not apply for buoyancy clouds due to their high spatial heterogeneity. Hence, the parametrization of the turbulent diffusivities in such flows should be based on scalar quantities that avoid the problem of spatial heterogeneity and takes into account the different sources of turbulence production, e.g. using energetic considerations that compare the terms having a definite exchange of energy and acts as a source or a sink.



中文翻译:

下坡密度云的同时速度-密度测量

我们通过组合 PIV/PLIF 测量,展示了有限体积释放重力流下坡传播的实验室实验结果。实验数据用于估计电流的全局特性,如传播速度、侧表面和浮力,表明热理论是一个可以正确预测这些量的稳健模型,尤其是 Beghin 等人的模型。(1981)、Maxworthy (2010) 和 Dai (2013a) 的理论模型。收集的数据还允许基于湍流通量而不是使用体积通量的变化来确定夹带率,并且已经识别了夹带/分离区域。这些结果支持了先前对导致云浮力损失及其沿斜坡减速的机制的观察,即云后部的大规模再循环涡旋。观测到云的内部结构非常复杂,发生了各种各样的湍流过程,例如在海流本身尺度上的大尺度对流不稳定再循环涡旋,云头内部小尺度的对流不稳定运动。云和靠近底部边界以及当前和环境水之间的边界处的切变(开尔文-亥姆霍兹)不稳定性。天气速度和密度测量还允许测试湍流的现有参数化,这些参数化已在具有高空间分辨率的二维场中量化。结果证实,基于恒定湍流扩散率或混合长度假设的参数化定律不适用于浮力云,因为它们具有高度的空间异质性。因此,此类流动中湍流扩散率的参数化应基于避免空间异质性问题的标量,并考虑湍流产生的不同来源,例如使用能量考虑来比较具有确定的能量交换和充当源或汇。

更新日期:2022-05-07
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