# Calculating damping force

1. May 17, 2017

### Tommtb

1. The problem statement, all variables and given/known data
I need to complete a mathematical model of a shock absorber, not in the frequency sense but by calculating the damping force due to the resistance in the valves.

For an orifice there seems to be two ways of calculating the force/pressure drop
1). Discharge coefficient, this results a value for pressure drop.
I am having a hard time understanding whether pressure drop is the pressure difference required to accelerate the liquid to a higher velocity (in the smaller area) or whether it is due to the friction in the orifice.

For example if i wanted to calculate the damper force and had two orifices in series, P1 being pressure before orifice 1, P2 being the pressure in between the two orifices and P3 being the pressure after third orifice. Would I have to work backwards from P3 to find the pressure at the piston face (P1)? by calculating pressure drop from p2 - p3 then p1 - p2. Then sum all the pressure drops to find the total piston face pressure?

2). Piple loss mehod

Is pressure drop from 1). the same as 'pipe friction losses' or head loss? Do i need to sum head loss with the pressure required to accelerate the fluid through the orifice ?

Thank you in advance!

2. Relevant equations

3. The attempt at a solution
Many thesis' analyse and I get more and more confused about the concept.

2. May 17, 2017

### Staff: Mentor

Welcome to the PF.

Please show us the work you have done, so we can help you with this problem.

3. May 17, 2017

### Tommtb

Hi there, well it's not a specific text book question, it's more about the conceptual understanding of it, I don't have any workings to show because it's all reading based.

Perhaps I didn't post this in the right section?

4. May 17, 2017

### haruspex

If the pipe cross section is the same each side of the orifice then any pressure difference is from frictional losses. Apart from that, there is a pressure drop entering the orifice to accelerate the fluid, but a corresponding pressure rise to decelerate it as it leaves.