Extreme events of out-of-plane vorticity in Rayleigh-B\’enard convection from PIV experiments *

We present a study on extreme events of out-of-plane vorticity in Rayleigh-B\'{e}nard convection from stereoscopic PIV measurements. The range of Rayleigh numbers (Ra) studied goes from low to moderate values (Ra = 1.5\times10^{4}; 2\times10^{4}; 1\times10^{5}; 2\times10^{5}; 5\times10^{5}). The three largest Rayleigh numbers are obtained pressurising the whole set-up including cameras and objective lenses, up to 4.5 bars. The working fluid is air and therefore the Prandtl number of the experiments is 0.7. The measurements are performed in a Rayleigh B\'enard convection cell with aspect ratio 10, and vertical distance between the two horizontal plates of 3 cm.  At these experimental parameters, a transition from Gaussian to intermittent statistics of the velocity derivatives was observed from DNS data only. The main goal of this study is to reproduce DNS data that are acquired at the same Rayleigh numbers to study far-tail events of the out-of-plane vorticity component. We show that the experimental results are able to reproduce the statistics of DNS data of the same flow well, and present a detailed study of extreme events of the out-of-plane vorticity both from their spatial structure and time evolution.