Turbulence is a canonical example of multi scale phenomenon. This multi scale character is actually at the very center of the phenomenological theory of turbulence by Kolmogorov.
During this lab course, the trainees will be initiated to two measurement techniques used for the spatial mapping of a flow, that will be illustrated through the study of the wake development behind a NACA foil, a classical study case of turbulence.
This introduction to major experimental techniques in fluid mechanics (and to their limitations) will be augmented by an initiation to numerical techniques (and the issues associated to them) such as direct numerical simulations, RANS method, or Large Eddy Simulations.
This lab-course is well suited to L3 and Master students with background skills in fluid mechanics and turbulence
EXPERIMENTAL FLUID MECHANICS: observing turbulence using hot wire anemometry and particle image velocimetry (PIV)
4 hrs session
This module aims at initiating the participants to the principle and use of hot wire anemometry and PIV, two key techniques used in experimental fluid mechanics. The basics of measurement and signal processing are explained and applied to a classical study case in which the students observe the development of turbulence in the wake of a NACA airfoil placed in a laminar flow. On one hand, PIV measurements provide instantaneous pictures of the flow enabling to observe the unsteady separated flow behind the airfoil and calculate basic statistical quantities such as the mean velocity and turbulent quantities. On the other hand, hot wire anemometry provides time resolved quantities. Thanks to their spectral analysis, the frequencies characteristic of the shedding vortices in the wake (Strouhal number) and of the smaller scales of turbulence (Kolmogorov scale) can be obtained. Transversal scans also show the downstream development of the wake and illustrate the turbulent diffusion of momentum.
MODELLING TURBULENCE: a numerical study of a flow past a cylinder
4 hrs session
Due to the development of a large range of motion scales, the explicit simulation (by direct numerical simulation, DNS) of all these scales is not available with standard computational resources. Other approaches based on the modeling of a part of the flow fields have been developed to overcome this limitation.
RANS (Reynolds averaged Navier-Stokes) simulation consists in modelling all the turbulent fields and to explicitly solve only the mean fields. On the other hand, LES (large eddy simulation) consists in modelling only the smallest scales of the flow fields and to explicitly solve the large scales. In this lab course, the flow around a cylinder is simulated by using RANS and Unsteady-RANS approaches. The results are then compared with LES results and with experimental measurements to better understand the limitation of each numerical approach