Somanchi, S. S., Senyukov, V. V., Denman, C. J., & Lee, D. A. (2011). Expansion, purification, and functional assessment of human peripheral blood NK cells. JoVE (Journal of Visualized Experiments), (48), e2540-e2540.

I follow protocol from this paper. I use IL 2 along with RPMI and 10% FBS. I keep NK cells suspended in T75 flask in standing position.

Hope this helps.