# Generating Water Hammer

Hi
Transient is result of changing in momentum and momentum is result of changing in velocity. Velocity is a vector parameter. It means that Velocity's change is due to changing in velocity's quantity or velocity's direction. All discussions about transient Analysis speak about Transient as result of changing in AMOUNT of velocity not changing in DIRECTION of it.
Assume a pipe with no operations or branches, valves or ... but there is a Tee along the pipe. Changing in velocity's direction makes momentum and therefore it makes a transient. Am I right or I make mistake and Transient is due to JUST velocity's quantity?

russ_watters
Mentor
Changing a direction is the same as changing an amount if you work in vectors. For piping it is easy since the flow is 1-dimensional: a change in direction changes the velocity from x to -x; a change in speed of 2x.

mheslep
CWatters
Homework Helper
Gold Member
Mike.. You are correct.

You have acceleration if either the magnitude or direction component of velocity changes.

However your question is about water hammer... let's say you have a pipe with a 90 degree bend in it. As water flows around the bend it accelerates (because the direction changes). This does not cause a transient force (hammer) because the water is constantly flowing. A constant acceleration means a constant force. You only get a transient force when the water is turned on or off.

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russ_watters & CWatters
Hi
I believe that changing direction of velocity (with constant amount) DOS NOT generate Water Hammer (and it produces Force only), but not because of what you (CWatters) believe. You say Hammer will be generated if velocity becomes to zero or becomes more that zero (from zero). It is not right. Hammer occurs if velocity CHANGES (not just becomes zero). My justification is according to Jakowski's equation [dH= (a/g)dv]. In this equation dh is not vector, and dv can not be vector. because if dv be vector, then dH becomes vector.
But I am not sure about my reason.
Do you have another idea?
Thank you.

CWatters
Homework Helper
Gold Member
You say Hammer will be generated if velocity becomes to zero or becomes more that zero (from zero). It is not right. Hammer occurs if velocity CHANGES (not just becomes zero).
I agree. I was wrong to say it only occurs when the water is turned on or off.

jbriggs444