Space- and time-resolved dissipation measurement in wall-bounded turbulence*

Dissipation is a key quantity of turbulent flow since, for instance, it sets the energy consumption of vehicles and pressure losses in pipes. At geophysical scale, it plays a role in energy balance and transport efficiency in climate models. At a fundamental level, the dissipative structures might drive the intermittency. Nevertheless, it is very difficult to measure it experimentally. Indeed, it necessitates the full spatial derivative of the entire velocity components. This requires a resolution beyond the reach of current velocimetry techniques. Direct Numerical Simulations fail also to estimate such quantity over long time at large Reynolds number because of numerical cost, and thus may underestimate rare events. Here we present an experimental technics allowing us to measure directly the norm of the strain-rate tensor, i.e the root mean square of the dissipation. The method is based on the diffusing wave spectroscopy (DWS) applied on a turbid fluid. With the use of a fast camera focused on the fluid interface, we are able to get a spatio-temporal map of the dissipation.

We first validate the method on the well-known Taylor-Couette flow [1], showing that the method is quantitative. Then we apply the DWS at the wall of a container enclosing a turbulent flow generated by an impeller. We resolve the surprisingly large fluctuations of the dissipative structures up to Reynold Re= 4×10⁵ near the wall container. We focus on the statistical properties of these fluctuations and their scaling with the Reynolds number, opening a new window on the near wall turbulence.

 

 

[1] Enzo Francisco, Vincent Bouillaut, Tong Wu & Sébastien Aumaître Experiments in Fluids (2023) 64:156