Understanding Transient Analysis and the Role of Velocity's Direction

[Mikealvarado100]

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]

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.

 

[CWatters]

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.

 

[Mikealvarado100]

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]

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]

I believe that changing direction of velocity (with constant amount) DOS NOT generate Water Hammer (and it produces Force only).

The term "hammer" indicates a very high momentary force and a corresponding abrupt change in velocity of an entire flow. A flow that moves continuously around an elbow results in no such momentary force.

We can, however, have a water hammer even without a 180 degree flow reversal. Consider a vertical water pipe on a moving train. The train is moving at 10 meters per second forward along the track. Water is flowing at 10 meters per second down the pipe and out from a valve at the bottom. From the point of view of a ground observer, the water movement is at a 45 degree angle down and forward. If the valve is closed there is an abrupt upward force. The entire flow stops moving vertically and is now moving at a 0 degree angle forward.