Investigation of thermal boundary layer dynamics in turbulent liquid metal convection by ultrasound localization microscopy of near-wall velocity fields and temperature measurements

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The dynamics and interaction of thermal and viscous boundary layers (BL’s) will be studied experimentally in highly turbulent liquid metal convection at small Prandtl numbers (Pr) using the ternary alloy GaInSn (Pr = 0.03). Rayleigh-Bénard convection at large Rayleigh numbers (Ra) of up to Ra ≅ 5x10e9 is characterized by a fully turbulent flow field, with the temperature field exhibiting significantly more coherence than the velocity field due to the high thermal diffusivity. A crucial role for heat transport in turbulent convection is played by the BL’s. Here, a special feature of liquid metals becomes apparent, which has hardly been researched so far: The much thinner viscous boundary layer is embedded in the thermal BL. Therefore, the thermal BL and thus the convective heat transport are strongly influenced by the turbulent large-scale convection (LSC). The parameter range considered here has so far been inaccessible by direct numerical simulations. To investigate the interaction between BL’s and LSC in detail for the first time in liquid metal laboratory experiments, the study involves dedicated investigations on novel measurement methods. Near-wall velocities shall be measured by means of Ultrasound Localization Microscopy (ULM). To apply this super-resolution method, investigations on adaptive nonlinear beamforming techniques to locate particle movements in the micrometer-range will be conducted. Furthermore, a compound imaging by two opposing phased array probes to maintain sufficient spatial resolution throughout the full measurement depth will be investigated. Both methods are novel applications in ultrasound imaging. For obtaining spatially resolved temperature data in the melt, fiber optic sensor measurements will be applied. The application of fiber optic bragg grating is a novel method to obtain temperature fields in liquid metal flows. The experiments, in which near-wall temperatures and flow velocities are measured in turbulent convection with high resolution, set a new milestone for the understanding of convective transport processes in fluids at small Pr with their numerous applications in geo- and astrophysical flows as well as in engineering systems.