forked from 1b-agent/fieldcraft-upstream
Christian Svensson
939b64e660
A D3Q7 passive scalar rides on the D3Q19 flow: it is advected by the cell velocity, diffuses with the fluid's thermal diffusivity (the relaxation time follows from k / (rho c)), and feeds back into the momentum through a per-cell Boussinesq force when the domain has an expansion coefficient and gravity. Walls and obstacle faces hold a temperature (anti-bounce-back), a heat flux (added to the bounced population) or are adiabatic (bounce-back); velocity inlets carry the held temperature in, pressure outlets copy their neighbour. The heat crossing each face group is accumulated during streaming and reported in physical units (W) in the result's heat histories, next to the force histories; frames gain a temperature field and the stats report the thermal relaxation time. Closed-form checks: linear conduction between held plates, a flux wall against q H / k with the wall's heat balance, an inlet temperature carried through a plug flow, and a periodic blob that conserves its total and returns after one lap. The de Vahl Davis cavity (Ra 1e3 and 1e4 against the published Nusselt numbers) is there as an ignored test because it takes minutes. The GPU backend declines domains with heat for now and asks for the CPU; its kernels follow. Part of #157.