ENGINEERING AND CFD

A team of researchers led by TU Ilmenau Professor Jörg Schumacher have been using the JUWELS supercomputer at the Jülich Supercomputing Centre to run highly detailed direct numerical simulations (DNS) of turbulent flows at the so-called mesoscale—the intermediate range where both small-scale turbulent fluid interactions and large-scale fluid dynamics converge.

Turbulent convection is one essential process to transport heat in fluid flows. In many of the astrophysical or technological applications of convection the working fluid is characterized by a very low dimensionless Prandtl number which relates the kinematic viscosity of the fluid to its temperature diffusivity. Two important cases are turbulent convection in the Sun and turbulent heat transfer in the cooling blankets of nuclear fusion reactors. Massively parallel simulations of the simplest setting of a turbulent convection flow, Rayleigh-Bénard convection in a layer or a straight duct that is uniformly heated from below and cooled from above, help to understand the basic heat transfer mechanisms that these applications have in common.

Recent direct numerical simulations in closed slender Rayleigh-Bénard convection cells advanced to Rayleigh numbers of Ra = 10¹⁵ which were never obtained before and reveal a classical turbulent transport law for the heat transfer from the bottom to the top of the cell which is based on the concept of marginally stable boundary layers. Our simulations were able to resolve the complex dynamics inside the thin boundary layers at the top and bottom plates of the convection cell and to determine a steady increase of the turbulent fluctuations without an abrupt transition near the wall for a range of 8 orders of magnitude in Rayleigh number.

An international team of scientists conducted high-precision spectral element simulations which resolved the fine-scale structure of turbulent Rayleigh-Bénard convection, in particular the statistical fluctuations of the temperature and velocity gradients.