boundary and initial conditions

Boundary conditions are those which depends on space while initial conditions are depends on time. So, in boundary condition space coordinate will be varied and in initial conditions time will be varied.

while dealing with boundary conditions we can have multiple solutions while initial conditions problem gives unique solution.

For illustration,same Navier Stokes Equations are used to solve for the airflow past an aircraft,cars, kite flying and even to solve the pipe flow problems. We can\'t say all flows are dynamically similar. All these are different and unique in nature but governed by same Navier-Stokes equations.