Search for: All records

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 Artemia salina—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. 

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 Artemia salina—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. 

In turbulent flows, energy flux, the cornerstone of turbulence theory, refers to the transfer of kinetic energy across different scales of motion. The direction of net energy flux is prescribed by the dimensionality of the fluid system: Energy cascades to smaller scales in three-dimensional flows but to larger scales in two-dimensional (2D) flows. Manipulating energy flux is a formidable task because the energy at any scale is not localized in the physical space. Here, we report a theoretical framework that enables control over energy flux direction. On the basis of this framework, we conducted experiments and direct numerical simulations, producing a 2D turbulence with forward energy flux, contrary to classical expectations. Beyond theory, we discuss how our theoretical framework can have profound applications and implications in natural and engineered systems across length scale ranges from 10⁻³to 10⁶meters, including enhanced mixing of microfluidic devices, biologically generated turbulence, breaking persistent coastal transport barriers, and ocean energy budget.