High Reynolds turbulence : Study of the inertial intermittency in axisymmetric jet and von Kármán cryogenic flows

Overview & Synopsis

In this experimental thesis, we study very high Reynolds number (Reλ) turbulent flows with emphasis on the understanding of inertial scale dynamics and intermittency. Thanks to the low kinematic viscosity of helium at low temperatures (T = 1.8 - 2.4K) we were able to study jet and von Kármán flows for Reλ varying in a vast range between 1000 and 13000. The turbulent fluctuations of the velocity are measured thanks to home-made Wollaston hot-wires. The high values of Reynolds numbers reached in our experiments push the dissipative scales to the micron range, which unfortunately could not be resolved by our anemometers (300 µm in length), so that we limit our study to the inertial intermittency. A dedicated cryostat has been built and operated to calibrate the hot wires, based on the flying hot-wire technique. In order to study well established turbulent flows, we have modified the Hejet experiment already built in a previous thesis, in order to reduce the instability of the jet, by installing a new turbine better adapted to the Hejet flow. We have fully characterized this new turbine, and verified that its performance is in a good agreement with its design characteristics. However, the new turbine does not allow to solve the issue of the instability of the jet, which exhibits unusual characteristics: in particular, the fluctuations of the velocity are not Markovian, probably due to the instability of the jet induced by the lateral confinement. Most of the measurements presented in this thesis are therefore obtained in the SHREK von Kármán experiment with different kinds of flows. We verify that these very high Reynolds number flows fulfill usual scaling laws. From the study of inertial intermittency, it seems that intermittency tends to reduce as the Reynolds number increases. In order to move a step further, we apply a Fokker-Planck approach to investigate the evolution in scale of the whole PDF of the velocity increments. Indeed, this approach is valid in the SHREK flows, and the drift and diffusion coefficients derived are presented and analyzed. The Integral Fluctuation theorem (IFT) is fulfilled with a good accuracy. Our study confirms that the diffusion coefficient, strongly connected to intermittency, decreases as the Reynolds number increases, which tends to prove that K41 is a model well adapted to infinite Reynolds number flows.