I have changed the boundary condition and parameter value, which works here:
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real Ra= 10^4;
Boundary conditions:
on(1,2,3,4,5,6, u1=0, u2=0, u3=0)
+ on(1, T=sin(pi*y))+ on(3, T=1) ;
======= Results=======
However, could you please comment on identifying the orientation of a mesh as mentioned in [ 3d labels questions]?