Si Xinyu, Fang Lei
Department of Civil and Environmental Engineering, University of Pittsburgh, Pittsburgh, PA 15261, USA.
PNAS Nexus. 2024 Feb 8;3(2):pgae056. doi: 10.1093/pnasnexus/pgae056. eCollection 2024 Feb.
It has been proposed that biologically generated turbulence plays an important role in material transport and ocean mixing. Both experimental and numerical studies have reported evidence of the nonnegligible mixing by moderate Reynolds number swimmers, such as zooplankton, in quiescent water, especially at aggregation scales. However, the interaction between biologically generated agitation and the background flow, as a key factor in biologically generated turbulence that could reshape our previous knowledge of biologically generated turbulence, has long been ignored. Here, we show that the geometry between the biologically generated agitation and the background hydrodynamic shear can determine both the intensity and direction of biologically generated turbulent energy flux. Measuring the migration of a centimeter-scale swimmer-as represented by the brine shrimp -in a shear flow and verifying through an analog experiment with an artificial jet revealed that different geometries between the biologically generated agitation and the background shear can result in spectral energy transferring toward larger or smaller scales, which consequently intensifies or attenuates the large-scale hydrodynamic shear. Our results suggest that the long ignored geometry between the biologically generated agitation and the background flow field is an important factor that should be taken into consideration in future studies of biologically generated turbulence.
有人提出，生物产生的湍流在物质输运和海洋混合中起着重要作用。实验和数值研究均已报告，在静止水体中，中等雷诺数的游动者（如浮游动物）会产生不可忽视的混合，尤其是在聚集尺度上。然而，生物产生的搅动与背景流之间的相互作用，作为生物产生湍流的一个关键因素，可能会重塑我们之前对生物产生湍流的认识，却长期被忽视。在此，我们表明，生物产生的搅动与背景流体动力剪切之间的几何关系能够决定生物产生的湍能通量的强度和方向。通过测量厘米级游动者（以卤虫为代表）在剪切流中的迁移，并通过人工射流的模拟实验进行验证，结果表明，生物产生的搅动与背景剪切之间不同的几何关系会导致谱能量向更大或更小尺度转移，从而增强或减弱大尺度流体动力剪切。我们的结果表明，生物产生的搅动与背景流场之间长期被忽视的几何关系是一个重要因素，在未来生物产生湍流的研究中应予以考虑。
