Simultaneous temperature and velocity Lagrangian measurements in turbulent thermal convection

Abstract

We report joint Lagrangian velocity and temperature measurements in turbulent thermal convection. Measurements are performed using an improved version (extended autonomy) of the neutrally buoyant instrumented particle (Shewet al.,Rev. Sci. Instrum., vol. 78, 2007, 065105) that was used by Gasteuilet al.(Phys. Rev. Lett., vol. 99, 2007, 234302) to performed experiments in a parallelepipedic Rayleigh–Bénard cell. The temperature signal is obtained from a radiofrequency transmitter. Simultaneously, we determine a particle’s position and velocity with one camera, which grants access to the Lagrangian heat flux. Due to the extended autonomy of the present particle, we obtain well-converged temperature and velocity statistics, as well as pseudo-Eulerian maps of velocity and heat flux. Present experimental results have also been compared with the results obtained by a corresponding campaign of direct numerical simulations and Lagrangian tracking of massless tracers. The comparison between experimental and numerical results shows the accuracy and reliability of our experimental measurements and points also out the finite-size effects of the particle. Finally, the analysis of Lagrangian velocity and temperature frequency spectra is shown and discussed. In particular, we observe that temperature spectra exhibit an anomalous$f^{-2.5}$frequency scaling, likely representing the ubiquitous passive and active scalar behaviour of temperature.