Vertical axis turbines have been studied a lot over the last decade with a growing interest caused by the development of offshore, urban and tidal turbines. These specific types of turbines may benefit from the characteristics of vertical axis rotors (lower center of gravity, insensitivity to the flow direction, etc...). For a while, studies focused on the rotor itself, improving the performances of isolated turbines and making simulations more accurate. Nowadays, industrial projects are being studied and farm configurations need to be considered.

The present work studies the effect of mesh density in 2D CFD simulations on the wake of a blade-resolved vertical axis turbine. Mesh sensitivity is generally assessed by checking the effect of grid refinement on power, moment and drag coefficients but rarely by paying attention to its effect on the wake development. When considering turbine-to-turbine interactions within a farm, simulating accurately the rotors' wakes becomes a major concern.

In this paper, we present a mesh sensitivity study for the 2D simulation of a small scale isolated turbine and compare the results with experimental data recently obtained at LEGI (streamwise velocity profiles at several locations downstream the turbine as well as power and thrust coefficients). We will give some recommendations about which mesh size should be used in the wake when turbine-to-turbine interactions are to be considered. The comparison to experimental data will give information about what level of accuracy can be expected from 2D RANS CFD simulations in the wake.

Finally, a configuration where two turbines interact with each other (one is placed downstream the other one) will be studied in term of mesh sensitivity. The effect of mesh density in the wake on the output of the downstream turbine (average power coefficient and instantaneous torque coefficient) will provide further insight on the kind of mesh refinement to use for such applications.