# Why does pressure during a hydraulic transient phenomenon like fluid hammer goes below the line pressure?

I am trying to understand why the pressure fluctuations during a hydraulic hammer phenomenon would go below the line pressure (Pmin < Pi) I thought there will just be a rise in pressure and then the pressure would came back to the settling pressure but thats not the case as the pressure seems to go well below the settling pressure too (which could cause cavitation if the pressure drops below vapor pressure). I know there are multiple peaks because of the pressure wake being reflected back and fourth in the hydraulic line (and that the pressure at any point in the pipe at any given time is a superposition of all the wave, incident and reflected, at the point). I am not able to understand how the pressure can go below the line pressure as surge is suppose to be a compression wave. The Pmin suggest that a expansion wave is travelling behind the compression wave. Any material to understand this better would also be helpful.

• pressure pulse cause compression and when it is not there... Dec 19, 2018 at 8:02
• @SolarMike Sorry about the question. It wasn't displayed fully for some reason. Dec 19, 2018 at 16:46
• @SolarMike why the passing of the compression wave is followed by an expansion wave? Dec 19, 2018 at 16:47

The negative pressure excursions occur because the moving fluid in the system possesses inertia. This means it wants to keep moving beyond its equilibrium position even after the driving force has gone to zero. Here is an easy way to visualize how this creates negative excursions:

Imagine we have a very, very long gun barrel loaded with an explosive charge and a tight-fitting projectile. We set off the explosive and the pressure in the barrel behind the projectile goes positive, and the projectile starts to move.

As it moves, the hot gas behind it expands and its pressure diminishes. Now we have set the projectile into motion and even though the positive driving pressure will eventually fall to atmospheric, the projectile remains in motion at that point and begins pulling a vacuum behind it where there once was positive pressure. This vacuum now acts to slow down the projectile. Eventually the projectile is brought to a halt, but as there is now a hard vacuum behind it, its direction of travel reverses and it slides back towards its starting position.

The mass of the projectile causes it to overshoot its equilibrium point on each cycle and it exhibits harmonic oscillation back and forth until friction bleeds off the kinetic energy and the projectile stops. This is how a transient impulse can create negative pressure excursions.

• Imagine this...there is a long pipe ..and inside the pipe is a tight fitting cork which can slide. from one side the cork is driven by fluid which is pumped into the pipe by piston pump. Now if the cork is suddenly stopped and tends to move in the reverse direction....will the shock that is generated at he cork (and travelling towards the pump) still have negative excursion( as the cork is trying to push the fluid towards the pump)? Dec 20, 2018 at 2:59
• Your explanation here about the generation of hard vacuum behind the projectile seems to be about the shock on the tank side of a quick closing valve...where the inertia of the fluid will create a vacuum cavity. What about on the pump side of the valve? ( ie considering on one side of the valve there is a pump and on the other side there is a tank.) Dec 20, 2018 at 3:06
• anytime you drive an underdamped system containing inertance and compliance with an impulse you will get oscillations that go both positive and negative. Have you taken a college-level systems modeling class? Dec 20, 2018 at 4:12
• Thank you. That's around what I expected the answer to be....still do you know any study material to understand this better? No I haven't taken a systems modelling class. Dec 20, 2018 at 4:56
• you may be able to find papers written on this effect by googling on "water hammer". For system modeling I recommend the book System Dynamics: a Unified Approach by Karnopp & Rosenberg Dec 20, 2018 at 5:03