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A natural or forced convection problem can be determined by using the Richardson number which is defined as the ratio of natural convection to forced convection: Ri=gβΔTL/(U_0)^2 where U_0 is the velocity of the flow [m/s] ΔT is the maximum temperature difference within the fluid region L is the representative length [m] g is the gravitational acceleration [m/s2] β=1/ρ*(∂ρ/∂T) is the volume expansion coefficient [1/K] When Ri=1, both natural and forced convection need to be considered. When Ri<<1, natural convection can be neglected When Ri>>1, forced convection can be neglected