An experiment was conducted to evaluate the initial wake expansion in scaled wind turbine tests as a means to guide future wake interference studies. Five scaled wind turbine rotors with different diameters were designed for testing in a closed-loop water channel to evaluate the effects of blockage on the initial wake expansion behind a wind turbine. The initial wake expansion was assessed by using quantitative dye visualization to identify the propagation of tip vortices downstream of the rotor. The thrust coefficient developed by the scaled models was recorded using a six-component balance and was correlated to the downstream wake expansion. The rotors used in the experiment were operated at a tip speed ratio of 6, a Reynolds number based on the tip speed and tip chord of approximately 23,000 and resulted in blockage values that ranged from 6% to 25%. Dye visualization indicated that the initial wake expansion downstream of a rotor was narrowed and that tip vortex pairing behaviour was modified because of increasing blockage. Blockage effects were significant and resulted in a wake that was more than 50% narrower when blockage was 25% compared with the observed expansion with 10% blockage. A computational simulation was conducted with the Generalized Unsteady Vortex Particle (GENUVP) discrete vortex method code using the rotor in freestream conditions and was compared with the experiments. The magnitude of the wake expansion in the freestream computations was similar to the wake expansion in the experiment when blockage was less than 10%. Copyright © 2013 John Wiley & Sons, Ltd.