I have tried to verify all the measurements but so far no succes in lowering the pressure in the simulation. I also found out that when I added the pressure sensor I got extra leakage which caused the difference between simulation and measurement. I have fixed that as good as possible.
This is...
The pressure spike is just as the BB passes the sensor and just before it exits the barrel. I went a step back and did a measurement with the barrel and no BB but I closed the barrel on the end. Same as I did with only the cylinder. But is pretty far off.
So this is with no barrel and closed...
Hi have been sick last couple of days, but have been able to measure the pressure in the chamber and in the barrel (add the exit of the nozzle) and the pressure in the barrel right at the end (when the bb exits).
It seems that the pressure in the chamber is a bit higher then what is measured...
I found a mistake with the friction that I forgot the sign when velocity is negative
Still had to tune the mass to 31gram instead of 28gram but that is acceptable, the friction coefficient is now 0.49 which is in the range of what I would expect.
For the second simulation I have now these...
Tried adding adiabatic temperature rise but seems it is not happening
Simulation with static tempeature
Simulation with adiabatic temperature rise
$$\text{Temp}(n) = T \left(\frac{P(n)}{P_0}\right)^{\frac{\gamma - 1}{\gamma}}$$
If I remove the friction force and increase the piston weight...
In this case due to the design of the piston most of normal force is dependent on the pressure, because when the piston is moving the air will push the oring to the cylinder wall. On the front of the piston there are small hole that channel the air to the inside of the oring. So when the piston...
@Chestermiller this was a very good tip thank you, I added the damping factor times velocity. Only the frequency seems a bit of.
If i tune the friction coefficient to 0.5 and increase the spring constant from 692N/mm (which i measured with force meter) and change it to 735N/mm even the...
Yes maybe that is a good idea then I can model the friction losses better probably, will do a measurement with the end port closed and try to model that
% Spring force (Hooke's Law with pre-load)
$$F_{\text{spring}}(n) = k \cdot (x_{\text{preload}} - x_p(n-1)) % \text{Spring force with...
Indeed the flow pressure is probably different due to compression but it will be pretty hard to exactly know the value. Therefor I would rather use only pressures and mass flow rate to calculate everything.
But assuming there is no exit tube would mean just compressing air and the piston will...
I'm bit struggling on which mass air flow rate formula to use for my model, i have been watching and searching a lot about the formulas and there seem to be a lot of different variants. Some use the differential pressure and some use the mach number and stagnation. But since i don't know the...
mass of the piston is 28 gram and the spring is 9 gram.
Already tried the accelerometer but data wasn't very useful or repetitive
So what I think happens the Cd value drops when the piston comes near the end and therefor the pressure rises
Below is a plot of the complete system
yeah could be that it isnt an measurement error in the rest of the measurement data you can see small oscillations in pressure due to the piston bouncing, I assume the Cd value drops probably very hard at the end. What pressure value did you use for the mass flow calculation also atmospheric...
I just found an error in the mass flow rate calculation 🙈, I updated the main post. There are small differences but that the big peak is not in de simulation. Could be a measurement error.
So lets assume the calculations are correct then I still am doubting about the high friction coefficient...
If the i do P1V1^1.4=P2V2^1.4 would be P1*V1^1.4/V2^1.4= 20MPa.
The part of the piston with regarding spring force and mass are easy to model but without friction of pressure it would be to fast.