Near free-fall oscillatory velocities in liquid metal rotating convection

The geomagnetic field is induced by liquid metal flow inside Earth´s outer core as a self-excited dynamo. Buoyancy drives the liquid metal flow because the iron rich core is cooling from its primordial state through heat loss to the mantle. The rotation of the Earth and Lorentz forces alter the resulting convective flow. However, the detailed flow topology is largely unknown. Here we will investigate the effect of rotation on a low Prandtl number thermal convection by means of laboratory experiments and DNS. Therefore, we consider a rotating Rayleigh-B\'enard convection setup in an upright cylindrical vessel of aspect ratio $\Gamma = D / H = 2$. We investigate supercriticalities in the range of $1 \leq \widetilde{Ra} < 20$ and Ekman numbers $4 \times 10^{-5} \leq Ek \leq 5 \times 10^{-6}$ in liquid gallium at $Pr = 0.03$. By means of ultrasound-Doppler velocity measurements, we find that oscillatory convection generates velocities approaching the freefall velocity. Multi-modal bulk oscillations dominate the vertical velocity field over the whole range of supercriticalities investigated. Additionally, coherent mean zonal flows and time-mean helicity suggesting that these oscillatory flows can be relevant for dynamo action.