[fayazsaheer]

I gave inlet pressure as an expression , but the outlet pressure is not varying and giving very low outlet pressure as shown in the image

[MKuhn]

What do you expect? It looks quite feasible, if the outlet area is greater, then the inlet and even if you have an ideal gas flow.

[fayazsaheer]

see here the fluid is water but the outlet pressure is not varying with what the inlet value is given. It is almost the same value as the gauge pressure I gave to the outlet. I want to see the variation in outlet pressure, when the inlet pressure expression is IF(0[s]<=t<=1e-3[s], 34e7[Pa]-68e9[Pa/s]*abs(t-5e-4[s]), 0[Pa])
The flow is through a simple pipe
what should I do to get the pressure variation at outlet?
Thank you

[MKuhn]

Did you try to decrease the time step and to increase the iterations per time step?

[fayazsaheer]

the time step is 1e-4 and iterations per time step 250 ,and should I further increase it?

[MKuhn]

Yes your pressure variation is only in 1e-3 s, so you have only 10 time steps to map the flow behavior. Try with 1e-5 s and take only 100 iterations or less (depends on the convergance)

[fayazsaheer]

ok thank you so much I'll try it

[MKuhn]

There you go and let me know if it helps.

[fayazsaheer]

hi ,i changed the time step to 1e-5 but still there is no change , now the outlet pressure is negative

[MKuhn]

I would say, that is a big change, with negative outlet pressure something is completly wrong. Refining the time step should always lead to better results.

[fayazsaheer]

What would you say could be wrong here?

[fayazsaheer]

The PS is to reduce the effect of high pulse pressure by putting in baffles, how would you recommend me to proceed from models onwards?
the inlet is of type pressure and outlet is massflow-outlet type , the model i used is k omega realizable with the timestep as you recommended.
the input pressure expression is IF(0[s]<=t<2e-3[s],25e4[Pa],IF(2e-3[s]<=t<3e-3[s],33.75e7[Pa]-68e9[Pa/s]*abs(t-5e-4[s]),IF(3e-3[s]<=t<=5e-3[s],25e4[Pa], 0[Pa]))) and oulet massflow is 0.06kg/s.
Thank you very much!!

[MKuhn]

I would recommend to use a pressure outlet instead of masslfow outlet. What about the residuals?

[fayazsaheer]

And what to give gauge pressure at outlet?

[MKuhn]

Take 0, and for the inlet the pressure drop for the flow without the pressure pulse.

[fayazsaheer]

The objective is to reduce the pulse
Yea this is how i did initially(outlet pressure=0[Pa]) and then i got the first graph attached above in the first post which seems too low and practically to achieve that low pressure at outlet is not possible.
when i did with outlet as outflow i got the inlet and outlet pressures coincident which i am attaching below.

[MKuhn]

With the mass flow outlet, you set a constant pressure drop between in- and outlet. Is this realistic? With variable pressure inlet and costant pressure outlet you will have a variable massflow depeding on the pressure difference between inlet and outlet. These are completely different things. Does the solution converge?

[fayazsaheer]

yes it does converge after 100 iterations, what changes would you suggest for the outlet to be approximately 1 order below inlet. thank you very much

[MKuhn]

Set to pressure outlet (0 gauge pressure) and massflow inlet. Run the calculation in steady state. Calculate the pressure drop (via surface report). Change the inlet boudary to pressure inlet (set inlet pressure 0 + calculated pressure drop). Run the simulation in steady state. Save the data, change to transient and set your pulse pressure inlet function (you have to change the function by adding the pressure drop for the flow without pulse). It is always a good advise to start a transient calculation from a steady state solution.