Restoration of Axisymmetric Flow Structure in Turbulent Thermal Convection by Polymer Additives

We present an experimental study of turbulent Rayleigh-Bénard convection in a cylindrical cell. With the addition of a tiny amount of long-chain polymers, we find, surprisingly, that the large-scale flow structure possesses axisymmetric topologies, rather than the well-known single-roll large-scale circulation (LSC), itself a result of spontaneous symmetry breaking. This symmetry restoration is found to arise from anisotropic suppression of velocity fluctuations, i.e., horizontal rms velocity becomes much less than the vertical one, as compared to the nearly isotropic bulk turbulence in the Newtonian case. This mechanism of symmetry restoration resulting from reduced fluctuations contrasts sharply with known symmetry restorations occurring in nature that usually result from increased thermal fluctuations. We further find that the net energy transfer between mean flow and turbulent fluctuations vanishes for axisymmetric topologies, whereas there is a positive net energy transfer from mean flow to turbulence for the lower-symmetry LSC topology (with or without polymers). A comparison of measured heat transfer efficiency and the large-scale flow topology points to a possible explanation for the original spontaneous symmetry breaking, in that the lower-symmetry LSC provides a higher transport efficiency. The present study provides an example of manipulating large-scale flow structures and their symmetries through small-scale properties, leading ultimately to the manipulation of turbulent transport.