Observation of Coexisting Upflow and Downflow Hexagons in Boussinesq Rayleigh-Bénard Convection

Abstract

We experimentally observed patterns consisting of domains of upflow hexagons coexisting with domains of downflow hexagons in Boussinesq Rayleigh-B\'enard convection. The sequence of patterns, as the control parameter $\ensuremath{\epsilon}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}\ensuremath{\Delta}T/\ensuremath{\Delta}{T}_{c}\ensuremath{-}1$ is increased, is from ordered or disordered rolls at onset to extended targets and spirals at $\ensuremath{\epsilon}\ensuremath{\sim}1$, to coexisting hexagons at $\ensuremath{\epsilon}\ensuremath{\sim}3$. Hexagons occur for all values of the Prandtl number $2.8\ensuremath{\le}P\ensuremath{\le}28$ investigated. Surprisingly, they appear in a range where only rolls were known to be stable, and their wave number differs substantially from the roll wave number they coexist with.