From the swept area ( not diameter) you can calculate the power from the energy in the wind. A Darrius will probably extract no more than half of that.
Having got power then use your intended tsr to derive working rotational speed.From power and speed calculate torque.
Energy in the wind is 1/2 ( rho) A.V^3 where Rho is air density, A is swept area and v is wind speed.
Flux