THERMAL-CONVECTION IN SMALL ENCLOSURES - AN ATYPICAL BIFURCATION SEQUENCE

The present numerical study examines bifurcation sequences in Rayleigh -Benard convection for small aspect ratio enclosures. The three-dimens ional rectangular enclosure has insulated sidewalls. The top wall is c ooled and the bottom wall is heated, both isothermally. The Boussinesq approximation is invoked with the exception of temperature dependent viscosity of the fluid. The numerical simulations closely model specif ic experiments. Accordingly, the mean Prandtl number is set to 5 and t he aspect ratios are set to 2.42 and 1.23. The computations exactly ma tch the bifurcation sequence observed in the experiments while increas ing the Rayleigh number, which is steady state --> periodic --> quasi- periodic --> steady state. It is established that the counter-intuitiv e transition from quasi-periodic to steady dynamical behavior with an increase in Rayleigh number is due to spatial changes in the mean velo city and temperature fields that accompany the bifurcation. The comput ations span a range of Rayleigh numbers from 2.5 x 10(3) to 1.3 x 10(5 ). Both unsteady and steady thermal convection are examined in detail.