Coherent structures form due to some sort of instability, e.g. the Kelvin–Helmholtz instability. Identifying an instability, and hence the initial formation of a coherent structure, requires the knowledge of initial conditions of the flow structure. Hence, documentation of the initial condition is essential for capturing the evolution and interactions of coherent structures, since initial conditions are quite variable. Overlooking the initial conditions was common in early studies due to researchers overlooking their significance. Initial conditions include the mean velocity profile, thickness, shape, the probability densities of velocity and momentum, the spectrum of Reynolds stress values, etc. These measures of initial flow conditions can be organized and grouped into three broad categories: laminar, highly disturbed, and fully turbulent.