# Berkeley Fluids Seminar

## University of California, Berkeley

### Bring your lunch and enjoy learning about fluids!

Wednesday, September 21, 2016

3110, Etcheverry Hall, 12:00-13:00

** Prof. Nicholas Ouellette** (Civil and Environmental Engineering, Stanford)

** Geometry and Ordering in the Turbulent Cascade **

** Abstract: ** Turbulent flows are inherently multi-scale. The mechanism that drives motion on many scales arises from the nonlinearity in the Navier-Stokes equations, which expresses the interaction of wavenumber triads that couple dynamics on different length scales. In turbulence, these triads self-organize to produce a net transfer of energy from the scales at which it is injected into the flow to the scales at which it is dissipated. In two dimensions, this cascade drives energy from the forcing scale to larger length scales, where large-scale friction damps the motion. Turbulent flows also famously self-organize in space into so-called coherent structures such as strong vortices. It is therefore natural to ask the question of whether these two types of turbulence-driven organization are related. Typically, we think about the transfer of energy between scales in Fourier space; but in doing so, we lose all connection to the spatial structure of the flow field. I will discuss results from experiments in a quasi-two-dimensional turbulent flow where we use a filtering technique to localize the flux of energy between scales in space and in time. This method allows us to study the spatial structure of the scale-to-scale energy transfer, which displays intriguing patterns. In particular, I will discuss the relative alignment of the turbulent stress tensor that is a manifestation of the Navier-Stokes nonlinearity and the large-scale rate of strain, which together determine the direction and strength of the energy cascade.