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

Gr = (L^{3} rho^{2} g beta (T_{2}-T_{1})) / mu^{2}

Gr = (L^{3} g beta (T_{2}-T_{1})) / vu^{2}

beta: coefficient of expansion, (T_{2}-T_{1}): temperature difference, g: gravitational acceleration, L: characteristic length, mu: viscosity, nu: kinematic viscosity, rho: density

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