An accurate two-dimensional methodology, the COIN Lambda Fast Solver for the computation of the flow field about a vertical axis wind turbine is presented. Such a technique determines the smooth flow field by integrating the compatibility conditions along the bicharacteristic lines. The time-averaged wind turbine effects are introduced by means of an actuator porous cylinder having the same radius of the wind turbine. A time-averaged flow field is determined The computed vertical axis wind turbine performances are compared with other numerical results. A qualitative comparison with experimental data is also shown. Such comparisons prove the merits of the proposed methodology.