In collcated grid storage all flow variables are stored at the same locations.
Here we should write more about odd-even decoupling, Rhie-Chow interpolation etc.
Rhie, C. M. and Chow, W. L. (1983), "Numerical study of the turbulent flow past an airfoil with trailing edge separation", AIAA Journal, vol. 21, pp. 1525-1532, 1983.