In this experiment, a small scale vertical-axis wind-turbine (VAWT) is immersed in a boundary-layer in a wind tunnel and stereo particle image velocimetry is employed to quantify the 3D characteristics of the wake. The measurements show that the wake is strongest behind the sector of the rotor which turns into the wind. Two counter-rotating vortex pairs in the wake induce crosswind motion which reintroduces streamwise momentum into the wake. Terms of the mean kinetic energy budget are computed and demonstrate that this crosswind flow has a significant influence on the redistribution of momentum in the wake. A similar analysis of the turbulence kinetic energy budget identifies shearing at the boundary of the wake as the primary contributor to the production of turbulence. An analytical model is developed in order to obtain a theoretical basis from which to understand how the aerodynamic behavior of VAWTs induces crosswind motion consistent with the production of counter-rotating vortex pairs.