Experimental results on the turbulent, strongly swirling flow field in a reverse flow gas cyclone separator are presented, and used to evaluate the performance of three turbulence closure models Mean and fluctuating velocity components were measured for gas cyclones with different geometric swirl numbers by means of laser-Doppler velocimetry. The experimental data show the strong effect of the geometric swirl number on mean flow characteristics, in particular with respect to vortex core size and the magnitude of the maximum tangential velocity. It is shown that the forced vortex region of the flow is dominated by the so-called precessing vortex core. Numerical calculation of the cyclonic flow shows that turbulence models based on the eddy-viscosity approach fail to predict the combined vortex observed experimentally. Predictions with the Reynolds stress transport model are in reasonable agreement with measured profiles for all three swirl numbers, though the turbulent normal stresses are generally overpredicted.