This study is aimed at understanding the effects of circumferential groove casing treatment on the performance of a low speed contra-rotating fan. Three dimensional, transient simulations are carried out using the open source CFD library Open-FOAM. The numerical results are validated with experiments for the smooth casing, which show a good agreement. Three treated casing configurations are investigated: 1) grooves at the top of the first rotor, 2) grooves at the top of the second rotor, and 3) grooves at the top of both rotors. Two operating points are simulated for all configurations. Flow inside the grooves is highly dominated by the main and blade passage flows. Grooves increase fan performance at near-stall flow conditions by up to 3 % and reduce pressure fluctuations significantly. Furthermore, they have a larger impact on fan performance when placed on top of the first rotor compared to placing them on top of the second rotor.