Internal flow vs external flow

In fluid mechanics, for ease of studying we classify flows in many categories.

In brief

Internal flow is flow through a confined solid boundary (e.g. flow through pipe).

External flow is flow over the solid (e.g. flow over a ball).

internal flow	external flow
flow through a confined solid boundary	flow over a solid
no free surface	there is free surface
viscosity affects all flow field	viscosity affect flow field near surfaces and boundary layer

Not all flow through a confined solid boundary is internal flow.

example:

Consider a situation where water flows through pipe.

When tank is full, water flow fully through the pipe boundaries. (It will attach to the pipe inner surface.)

But when the tank is very low, it will flow partially.

Not all water is attached to pipe inner surface.

Water flow with a free surface in pipe.

This kind of flow is called open channel flow.

Not all flow over a surface is external flow.

example:

First, we give a brief definition. Then, we introduce the contradicting part of the brief definition.

Finally, students may develop the actual definition of those terms. (Answer Question 1)

Activities

1. Search for internal flow meaning in internet.
2. Search for external flow meaning in internet.
3. Do the below questions.

Questions

1. Define internal and external flows.
2. what is the difference between internal flow and open channel flow?
3. do water flow in pipe always internal flow?
4. do air flow over a ball always external flow?