Biologically generated turbulent energy cascade in shear flow depends on tensor geometry

TL;DR

This study reveals that the interaction geometry between biological agitation and background shear flow critically influences the energy cascade in biologically generated turbulence, affecting ocean mixing processes.

Contribution

It demonstrates that the tensor geometry between biological agitation and shear flow determines turbulence intensity and direction, a factor previously overlooked in the field.

Findings

Geometry affects energy transfer direction in turbulence

Different geometries can intensify or attenuate large-scale shear

Experimental and analogue results confirm the impact of flow-agitation interaction

Abstract

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 non-negligible mixing by moderate Reynolds number swimmers in quiescent water, such as zooplankton, 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 cascade. Measuring the migration of a centimeter-scale swimmer-as represented by the brine shrimp \textit{Artemia salina}-in a shear flow and verifying through an analogue 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.