Pulsating inlet boundary

Catho · March 5, 2020, 22:55

Hi, I have been trying to run a (in my view) simple task but seem not to get it work

.

For simplification, I have a straight tube with an inlet and outlet boundary. For the inlet (set up as "pressure outlet") I want to have pressure (sine wave) vs. time set up, basically a repeating pressure wave of pulsation.
For the pressure I specified a field function with ${Time}*sin(${AbsoluteTotalPressure}*$(TimeStep}) with:
Time = 50ms
TotalPressure = 1.0 to 1.1 bar
TimeStep = 5ms

For the outlet I used "stagnation inlet".
For model selection is used 3D, gas, implicit unsteady, segregated flow, gradients, turbulent, ideal gas, K-epsilon turbulence, Two-layer all y+, segretated fluid temperature.

The simulation itself is running but the pressure remains stationary and does not follow any wave like shape as if the field function is not considered although it was chosen. Any thoughts what could go wrong? Seems such a simply task to complete. Thanks a lot!!
(By the way, is it normal that the values in "field function" --> Definition --> preview are always back to zero after exiting that window?)

ashokac7 · March 6, 2020, 01:10

Can you plot velocity on cross-section and also mass flow monitors on inlet and outlet to see if any flow is happening in the domain. post the pics here.

Also I think giving Mach number is better way to give pulsating flow boundary condition.

And yes. For pressure range, it should show some profile in output window (previewing field function)

ping · March 28, 2020, 20:39

if you want pressure to be a function of time then you need time within the sin function

cwl · March 28, 2020, 22:21

First: ping is right, check your field function.
Second: Pressure Outlet for inlet, Stagnation Inlet for outlet .. why? 0_o

ping · March 30, 2020, 02:55

to expand on cwl's reply - a stagnation inlet is the one to use as a pressure inlet and it is not good practice to use pressure outlets as inlets

ring1 · April 8, 2020, 07:43

dear catho
i have the same question as you, I put the inlet as stagnation inlet boundary conditions and set the outflow conditions,the interesting is the flow is reverse, so I want to know why you choose the stagnation inlet boundary conditions as outlet ?

ping · April 11, 2020, 05:25

stagnation inlet is designed for cases when you are pretty confident there is a positive pressure drop across the domain and so reverse flow is not going to happen
so if you have a case where there is a likelihood of reverse flow then choose a pressure outlet - read the help on this boundary type for more details

March 5, 2020, 22:55	Pulsating inlet boundary	#1
Catho New Member Carl Join Date: Mar 2020 Posts: 3 Rep Power: 6	Hi, I have been trying to run a (in my view) simple task but seem not to get it work . For simplification, I have a straight tube with an inlet and outlet boundary. For the inlet (set up as "pressure outlet") I want to have pressure (sine wave) vs. time set up, basically a repeating pressure wave of pulsation. For the pressure I specified a field function with ${Time}sin(${AbsoluteTotalPressure}$(TimeStep}) with: Time = 50ms TotalPressure = 1.0 to 1.1 bar TimeStep = 5ms For the outlet I used "stagnation inlet". For model selection is used 3D, gas, implicit unsteady, segregated flow, gradients, turbulent, ideal gas, K-epsilon turbulence, Two-layer all y+, segretated fluid temperature. The simulation itself is running but the pressure remains stationary and does not follow any wave like shape as if the field function is not considered although it was chosen. Any thoughts what could go wrong? Seems such a simply task to complete. Thanks a lot!! (By the way, is it normal that the values in "field function" --> Definition --> preview are always back to zero after exiting that window?)

March 28, 2020, 20:39		#3
ping Senior Member Ping Join Date: Mar 2009 Posts: 556 Rep Power: 20	if you want pressure to be a function of time then you need time within the sin function cwl likes this.

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March 6, 2020, 01:10		#2
ashokac7 Senior Member Ashok Chaudhari Join Date: Aug 2016 Location: Pune, India Posts: 260 Rep Power: 10	Can you plot velocity on cross-section and also mass flow monitors on inlet and outlet to see if any flow is happening in the domain. post the pics here. Also I think giving Mach number is better way to give pulsating flow boundary condition. And yes. For pressure range, it should show some profile in output window (previewing field function)

March 28, 2020, 22:21		#4
cwl Senior Member Chaotic Water Join Date: Jul 2012 Location: Elgrin Fau Posts: 435 Rep Power: 17	First: ping is right, check your field function. Second: Pressure Outlet for inlet, Stagnation Inlet for outlet .. why? 0_o

March 30, 2020, 02:55		#5
ping Senior Member Ping Join Date: Mar 2009 Posts: 556 Rep Power: 20	to expand on cwl's reply - a stagnation inlet is the one to use as a pressure inlet and it is not good practice to use pressure outlets as inlets

April 8, 2020, 07:43		#6
ring1 New Member wangh Join Date: Jan 2020 Posts: 2 Rep Power: 0	dear catho i have the same question as you, I put the inlet as stagnation inlet boundary conditions and set the outflow conditions,the interesting is the flow is reverse, so I want to know why you choose the stagnation inlet boundary conditions as outlet ?

April 11, 2020, 05:25		#7
ping Senior Member Ping Join Date: Mar 2009 Posts: 556 Rep Power: 20	stagnation inlet is designed for cases when you are pretty confident there is a positive pressure drop across the domain and so reverse flow is not going to happen so if you have a case where there is a likelihood of reverse flow then choose a pressure outlet - read the help on this boundary type for more details