Scaling of the Reynolds number in turbulent thermal convection

A riddle in turbulent thermal convection is the apparent dispersion from 0.42 to 0.5 in the value of the scaling exponent γ of experimentally measured Reynolds number Re∼Ra^γ, where Ra is the Rayleigh number. The measured Re may be divided into two groups: one based on the circulation frequency of the mean wind and the other based on a directly measured velocity. With new experimental results we show that in frequency measurements the dispersion in γ is a result of the evolution in the circulation path of the wind, and that in the velocity measurements it is caused by the inclusion of a counterflow in the mean velocity. When these factors are properly accounted for both groups give γ=0.5, which may imply that a single mechanism is driving the flow for both low and high values of Ra.