Loss due to contraction vs loss due to expansion

1. Mar 27, 2016

foo9008

1. The problem statement, all variables and given/known data
why the formula of fricitional pipe loss due to contraction is given by formula : kc(V^2)/2g ?
why the formula of fricitional pipe loss due to contraction cant be formula : (Va^2)-(Vb^2)/2g ? ,where Va= velocity at inlet , Vb = velocity at outlet ?

2. Relevant equations

3. The attempt at a solution
in this link , i was told that the pressure varies in an unknown and unpredictable way. but , for pipe loss due to expansion , the formula is
(Va^2)-(Vb^2)/2g ? ,where Va= velocity at inlet , Vb = velocity at outlet ?this make me confused... isn't the pressure also varies in an unknown and unpredictable way in enlarged pipe ?

Last edited by a moderator: May 7, 2017
2. Mar 28, 2016

Simon Bridge

Note: Quora is not a scientific source.
How would you go about modelling the pipe diameter change effect on pressure?

3. Mar 28, 2016

foo9008

When the diamter small, velocity high, pressure low

4. Apr 12, 2016

foo9008

I was told that the friction loss due to expansion is bigger than the friction loss due to contraction because of flow separation , is it true ? why ?