Settling-driven gravitational instabilities (SDGIs) can form at the base of buoyant particle-laden suspensions, modulating particle sedimentation in various settings such as meteorological and volcanic clouds, fluvial plumes, magma chambers, submarine hydrothermal plumes, or industrial emissions. These instabilities result in the formation of rapidly descending currents called ‘fingers’ within which fine particles settle faster collectively than individually. This study investigates SDGI triggering conditions underneath volcanic ash clouds through analogue experiments considering sedimentation from aqueous particle suspensions. We confirm that the conditions for which SDGIs develop are controlled by two dimensionless numbers: B_f (ratio of the characteristic finger velocity to the individual particle settling velocity); and B_i (ratio of timescale for individual particle settling to that for collective settling controlled by inertial drag). SDGIs are triggered for values of B_f and B_i > 1 for which particles are fully coupled with the flow within fingers. Using these parameters, we produce a regime diagram for the 2010 eruption of Eyjafjallajökull (Iceland) that describes particle settling as a function of particle concentration and size. More studies are needed to produce a general regime diagram accounting for the evolution of SDGIs properties with eruption and atmospheric parameters. Nonetheless, our study confirms that fingers affect sedimentation from volcanic clouds with high ash volume fractions above 10⁻⁶ vol.%. Our validation of criteria predicting the onset of fingers due to SDGIs constitutes a step forward toward the incorporation of these collective settling processes in volcanic ash transport and dispersion models.

中文翻译：

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沉降驱动引力不稳定性开始的粒子浓度和大小：实验验证及其在火山灰云中的应用


沉降驱动的引力不稳定性 （SDGI） 可以在充满浮力的颗粒悬浮液的底部形成，在各种环境中调节颗粒沉积，例如气象和火山云、河流羽流、岩浆室、海底热液羽流或工业排放。这些不稳定性导致形成快速下降的电流，称为“手指”，其中细小颗粒集体沉降的速度比单个颗粒更快。本研究通过考虑水颗粒悬浮液沉积的模拟实验，调查了火山灰云下 SDGI 的触发条件。我们确认 SDGI 发展的条件由两个无量纲数字控制：BF_（特征指速度与单个粒子沉降速度的比率）;和 B（单个粒子沉降的时间尺度与由惯性阻力控制的集体沉降的时间尺度之比）。当 B_、f 和 B > 1 的值时触发 SDGI，对于这些值，粒子与手指内的流动完全耦合。使用这些参数，我们生成了 2010 年 Eyjafjallajökull（冰岛）喷发的状况图，该图将颗粒沉降描述为颗粒浓度和大小的函数。需要更多的研究来生成一个通用的制度图，说明SDGIs特性随喷发和大气参数的演变。尽管如此，我们的研究证实，手指会影响火山灰体积分数高于 ^10-6 vol.% 的火山云的沉积。 我们对预测 SDGI 引起的手指开始的标准进行了验证，这是将这些集体沉降过程纳入火山灰传输和扩散模型向前迈出的一步。