There are two general types of flow, steady and unsteady flow.  For steady flow to occur the velocity at a given point must not vary with time.  In other words $\frac{∂v}{∂t}=0$.  The velocity for unsteady flow however will vary as time changes.  As a result it is normally much more difficult to analyze a problem where unsteady flow is occurring.  Generally in reality all flows are unsteady in some sense.

Normally if possible, it is best to try to analyze a problem as if it were in a steady state.  However, depending on how the flow is behaving this can yield inaccuracies in you result in comparison to what might really happen.  If the inaccuracies in your analytical results are large enough, than you may need to try to analyze the unsteady flow.  There are a few different types of unsteady flow. Just to list a few you could have nonperiodic flow, periodic flow, or completely random flow.  Sometimes one or possibly more than one of these may be occurring.  Generally however it isn’t immediately obvious.

As mention above one example of an unsteady flow is a nonperiodic flow.  An example of when this would occur is when you turn off a valve.  Generally,  this type of problem does not need to be analyzed.  However, if the valve were to be turned off really fast water hammer could result.  Water hammer is a force that travels through a fluid when it suddenly stops.  You have probably heard it if you’ve used an old shower and turned the water off fast.  You will hear the pipes bang.  This can be a real concern sometimes, since if the force is large enough the plumbing system could be damaged.