28 novembre 2019

Seminaire Jiangang CHEN

Jiangang CHEN a soutenu sa thèse de doctorat à Harbin Institute of Technology, Shenzhen· Institute for Turbulence-Noise Vibration Interactions and Control- et est maintenant un chercheur PostDocorant au LFML depuis Septembre 2019
Large-and small-scales interaction in the wake of a circular cylinder

When a fluid flows past a stationary bluff body, like a circular cylinder, the shear layers shed from the cylinder and form the well-known Kármán vortex street. The large-scale organized vortices in the street have significant influence on the momentum and heat transport as well as the behavior of the small scales. The present talk focuses on the effect of these coherent vortices on the statistics of the small scales in the near wake of the cylinder, mainly based on our recent studies (Chen et al. 2017 JFM, 2018 JFM, and 2019 JFM).

In the work, a multi-wire probe composed of 4 X-wires and 4 cold wires was used to measure the velocity vector, vorticity vector, temperature fluctuation and the temperature gradient vector simultaneously at nominally the same point in the plane of the mean shear. Measurements are made at x/d = 10, 20 and 40, where x is the streamwise distance from the cylinder axis and d is the cylinder diameter, at a Reynolds number of 2.5 × 103 based on d and free-stream velocity.

In the talk, we start with the spatial distribution of the turbulent kinetic energy dissipation rate with respect to the organized Kármán vortex, which reveals that the dominant energy dissipation mainly takes place within the Kármán vortex. A more complete picture for the flow mechanism is proposed by incorporating this information into Hussain & Hayakawa’s (1987 JFM) model. The properties of the fluctuating velocity, vorticity, temperature and temperature derivative vector within the Kármán vortex are then explored in detail. The study at last compares the velocity and temperature fields in terms of their spectra and physical structures. It is found that in the small scales, the analogy between the spectra of enstrophy and the temperature dissipation rate is well satisfied when x/d ≥ 20, particularly where the Kármán vortex is very weak.

28 novembre 2019, 11h0012h00
Bâtiment M6, salle de réunion