A dimensionless number used in heat transfer in general and free convection calculations in particular. Proportional to (Buoyancy force) / (Viscous force).

Gr = (L3 rho2 g beta (T2-T1)) / mu2

Gr = (L3 g beta (T2-T1)) / vu2

beta: coefficient of expansion, (T2-T1): temperature difference, g: gravitational acceleration, L: characteristic length, mu: viscosity, nu: kinematic viscosity, rho: density

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