i'm working on college project where i need to check if water is flowing through pipe or no accordingly execute specific commands. i'm using aurdino r3 board.

 

Welcome to AAC!
If you google "flowmeter" you will see there are several types, working on different principles. The best type would depend on the expected flow-rate (a trickle, a torrent?).

 

Do you need to detect flow, or just the presence of fluid? The latter can be done by additional, perhaps simpler methods.

If we're looking for flow, what linear velocity are we talking about discriminating? I mean, should 1 ft/sec register as a flow, while 0.1 ft/sec is not? Or is the "on" flow 100 ft/sec and anything less than 5ft/sec is "off"?

The industrial solution is usually a magmeter. No contact, moving parts, or flow restriction, and very accurate. But not cheap.

 

There are other ways to detect flow. What's in the water?

What are the characteristics of the water?

 

There are simple non-qualified flow meters hall effect based you can pick up for ~$5.00, if you just need a go-nogo test, they also output a pulse rate dependant on flow.
Max.