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I encountered the following problem which I cannot solve with P/Thermal (it uses only similarity correlations for free convections while buoyant flow needs to be encountered). Is there a way to solve the following problem with FLUENT? Annulus (cylinder located concentrically inside a tube) starts with a uniform temperature. It is in horizontal position and ambient temperature is 70 deg.F. There is air closed in the annulus (at ambient pressure). Annulus is giving heat away to the surrounding but due to free convection buoyant flow top to bottom temperature delta starts to occur. What I need to calculate is the shape of cooling curve. Actually I need two of them, one for the most top and another for the most bottom location. My feeling is that top-bottom DT should start with 0 deg. reach maximum at some time point and then it should decrease to finally reach zero when whole annulus reaches ambient temperature. Is it possible to compute using FLUENT? If yes please help me I'm waiting for any sugestions