  The effects of insulating lids on the convection beneath were investigated
experimentally using rectangular convection cells in the flux Rayleigh number
range $2.3\times10^{9}\leq Ra_F \leq 1.8\times10^{11}$ and cylindrical cells in
the range $1.4\times10^{10}\leq Ra_F \leq 1.2\times10^{12}$ with the Prandtl
number Pr fixed at 4.3. It is found that the presence of the insulating lids
leads to reduction of the global heat transfer efficiency as expected, which
primarily depends on the insulating area but is insensitive to the detailed
insulating patterns. At the leading order level, the magnitude of temperature
fluctuation in the bulk fluid is, again, found to be insensitive to the
insulating pattern and mainly depends on the insulating area; while the
temperature probability density function (PDF) in the bulk is essentially
invariant with respect to both insulating area and the spatial pattern of the
lids. The flow dynamics, on the other hand, is sensitive to both the covering
area and the spatial distribution of the lids. At fixed $Ra_F$, the flow
strength is found to increase with increasing insulating area so as to transfer
the same amount of heat through a smaller cooling area. Moreover, for a
constant insulating area, a symmetric insulating pattern results in a symmetric
flow pattern, i.e. double-roll structure; whereas asymmetric insulating pattern
leads to asymmetric flow, i.e. single-roll structure. It is further found that
the symmetry breaking of the insulating pattern leads to a stronger flow that
enhances the horizontal velocity more than the vertical one.
