Loss due to contraction vs loss due to expansion

[foo9008]

Homework Statement

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 can't be formula : (Va^2)-(Vb^2)/2g ? ,where Va= velocity at inlet , Vb = velocity at outlet ?

Homework Equations

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 ?
https://www.quora.com/Why-is-the-formula-for-the-loss-of-head-in-a-sudden-contraction-of-a-pipe-presented-in-terms-of-velocity-head-in-the-smaller-pipe
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[Simon Bridge]

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

 

[foo9008]

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

When the diamter small, velocity high, pressure low

 

[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 ?