Thermal convection in ﬂuid layers heated from below are usually realized experimentally as well as treated theoretically with ﬁxed boundaries on which conditions for the\ud temperature and the velocity ﬁeld are prescribed. The thermal and velocity boundary\ud layers attached to the upper and lower boundaries determine to a large extent the properties of turbulent convection at high Rayleigh numbers. Fixed boundaries are often absent\ud in natural realizations of thermal convection. This paper studies the properties of convection driven by a planar heat source below a cooling source of equal size immersed in an\ud otherwise stably stratiﬁed ﬂuid layer are studied in this paper. Unavoidable boundaries\ud do not inﬂuence the convection ﬂow since they are separated from the active convection\ud layer by nearly motionless stably stratiﬁed regions. The onset of convection occurs in an\ud inner unstably stratiﬁed region where the mean temperature gradient is reversed. But\ud the region of a reversed horizontally averaged temperature gradient disappears at higher\ud amplitudes of convection such that the vertical derivative of the mean temperature no\ud longer changes its sig
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