As a continuation of the previous article (Cooling telescope mirror) I report the results of some fluid dynamics simulations conducted on a control volume equal to that contained within the primary box.
These simulations were conducted to try to reproduce the movement of air above the primary mirror of a Dobsonian-type Newtonian telescope, in different conditions, in order to identify which of the three investigated solutions, presents the best performance from the point of view of cleaning the air column above the mirror. The solution that creates the least turbulence, is also what presumably, will ensure less disturbance to the incident wave front, thus favoring a better vision through the eyepiece right away, or already during that transitory, which gradually takes the mirror to go into thermal equilibrium with the ambient temperature.
In this period, as you know, the primary being warmer than the surrounding air, creates a turbulent upward convective motion, which greatly disturbs the image in the eyepiece. To minimize the acclimatization time of the mirror, it is usual to use fans that increase the heat exchange.
As already shown in the previous article, the most used solutions, basically they are divided into: fans that blow air towards the mirror and solutions that try to suck in the air from above the mirror, then unloading it from an opening in the lower part of the box.
In this regard, they have been simulated 3 different situations: natural convection, with blower fan against the rear face of the mirror, and with boundary layer suction, both with mirror pointing to the zenith and with the mirror inclined at 45 °.
NATURAL CONVECTION
The videos below refer to the simulation in natural convection, of a mirror of 420mm in diameter.
The mirror was assigned a suitably large temperature, 20° C higher than that of the ambient air, in order to bring out the convective motions in a better way.
BLOWERING FAN ON THE BACK OF THE MIRROR
In this simulation, in a square region of 80x80mm present on the bottom of the primary box, a flow rate equal to that of a PC fan of the same size has been set, powered at 12V.
In particular the fan used in the actual telescope in my possession, is the Artic AFACO-08000 from 52,7 m3 / h.
SUCTION OF THE BOUNDARY LAYER
In order to use this method, it is necessary to make some changes to the box that contains the primary mirror. In particular, it is necessary to introduce an additional "box" around the mirror, which obviously must have a circular opening to allow light to reach the reflecting surface, and mail 10/15 mm above it, in order to leave only an opening on the perimeter of the mirror, dedicated to the passage of air (area where the air will be sucked in). While at the bottom of this "box" there will be an opening, generally square, where the fan will be positioned which will suck the air from above the mirror and expel it from the bottom of the box.
The following video shows the 3D CAD, of the solution I adopted and then reproduced in the simulations.
Below are the results of the simulations:
CONCLUSIONS
It is important to note how the various simulations are consistent with what is shown in the real tests, posted in the previous article, as this consistency denotes the goodness of the results obtained from the simulations themselves.
As for the investigated solutions, also in this case, the air intake from above the mirror, seems to show greater cleanliness of the air column above, presumably ensuring a better vision right away.
It is therefore interesting, really experience the three solutions investigated so far, to understand the true potential of the various methods.
So for the next article, it might be interesting to be able to shoot a star, appropriately out of focus, so you can see the diffraction rings first, and evaluate the movement caused by air turbulence both in natural convection, both with the aspiration of the boundary layer.