Very viscous horizontal convection

'Horizontal convection' arises when a temperature variation is imposed along a horizontal boundary of a finite fluid volume. Here we Study the Infinite-Prandtl-number limit relevant to very viscous fluids, motivated by the Study of convection in glass furnaces. We consider a rectangular domain with insulating conditions oil the sides and bottom. and a linear temperature gradient oil the top. We describe steady states for a large range of aspect ratio A and Rayleigh number Ra, and find universal scalings for the transition from small to large Rayleigh numbers. At large Rayleigh number, the top boundary-layer thickness scales as Ra-1/5, with the Circulation and heat flux scaling as Ra-1/5. These scalings hold for both rigid and shear-free boundary conditions oil the top or on the other boundaries, which IS Initially Surprising, but is because the return flow is dominated by a horizontal intrusion immediately beneath the top boundary layer. A downwelfing plume also forms oil one side, but because of strong stratification in the interior, the Volume flux it carries is much smaller than that of the horizontal intrusion. decaying as the inverse of the depth below the top boundary. The fluid in the plume detrains into the interior and then returns to the top boundary, thus forming a 'filling box'. We find analytic solutions For the interior temperature and stream function and match them to a similarity solution for the plume. At depths comparable to the length of the top boundary the streamfunction has O(1) values and the temperature variations scale as 1/Ra. Transient calculations with a large, but finite, Prandtl number, show how the steady state is reached from hot and cold initial conditions.