If a forced flow interacts with buoyancy forces the wall region and, in part, the conductive layer will be destabilized by the velocity gradients. In the turbulent region (see fig. ) the turbulent diffusivity will increase rapidly with the distance from the surface, with a consequent reduction of the temperature fluctuations. The profile of above the surface will be such that the maximum is at the interface between the viscous-conductive layer and the overlying turbulent layer but will decrease much more rapidly than in the free convection case.
Figure: Temperature and velocity fluctuations in presence of forced flow over a horizontal plane
All the experimental and anedoptical evidence shows that very little seeing will be produced if a forced convection regime is fully established. A more significant case is the intermediate situation in which a weak air flow interacts with free convection producing a mixed convection regime, characterized by the Froude number:
where U is the mean flow speed.
When the Froude number is large, forced convection is dominant and therefore mirror seeing should disappear. When the Froude number is small, free convection is dominant. In-between one has a mixed convection case, in which case identity of the Froude number is required for any similarity between tests at different scales.
In section we will show that the rate of seeing produced by a given surface-air temperature difference is indeed a function of the Froude number.