thermo sypon

The velocity of a Thermo Syphon system that describes which property of the system? Your need is yet to be clearly defined. Try restating it and may be you can be helped.

ok maby I should explane the aplication I am wanting to build a solar updraft tower and am trying to figure out what my air speed will be through the system so I can desighn the terbine acordingly

I must admit that you have a very interesting conceptual application of the Thermosyphon science on the Solar Updraft technology.

That said, I believe that the air motion is due to Bouyancy Effects of the Natural Convection due to the Collector Heating effects and not Pressure Differential as is the case with standard thermosyphon analysis. The very small upward slant of the Collector towards the tower means that you can solve the fluid dynamic equations with a first order approximation of radial flow and improve on the solution with Pertubation Analysis. The combination of the simplified first approximation flow equation and the heat/thermal equation should give you the mass flow into the tower; then solve the flow through the tower - hopefully circular - based on the mass flowrate and that should be the answer you want.

I think that you should be able to get a good Tower Updraft velocity to run a mini-turbine.

I hope that this is helpful; and Goodluck