Suppression of free convection effects for spherical 1 kg mass prototype

We investigate the free convection processes in vicinity of a spherical 1 kg mass standard by two- and three-dimensional direct numerical simulations using spectral element method. Our focus is on determination suppression updraft forces high-precision comparator which are caused temperature differences between its environment millikelvin range – source systematic uncertainties high-precison determination. A two-dimensional model presented first, obtains good agreement with previous laboratory measurements for smaller up to 15 mK. The influence different boundary conditions side lengths square domain discussed positioned center chamber. complexity increased subsequently configurations additional built-ins counter heating form planar plates or hemispherical shells above standard. latter ones lead full compensation force. Three-dimensional closed cubic chamber confirm findings additionally reveal complex secondary flow patterns reduction heat transfer due also demonstrated comparison Nusselt numbers as function Rayleigh number chosen parameter range. suggest that such constructive measures can enhance precision related uncertainties.