Massflow BC, natural frequency in pipi

CFXMUFFIN · January 28, 2016, 15:13

Hey guys,

what do you think about the following solution.

Task:

1.There is a large pressure resorvoir, where the pressure is kept constant with a regulator.

2. From the resorvoir leads a pipe (straight and elbow parts, same diameter)
to a valve

For the task, the flow through the valve is not important. What i want to investigate is what happens in the pipe after the valve closes. (natural frequency of the pressure waves)

Here the idea:

I modeled the tank as a big rectangular Volume, where the pipe enters.
The other end of the pipe (where the valve is positioned) is covered with an Outlet plane.

At the volume all BC Planes (except the one where the pipe enters) have an Opening BC, with fixed Inlet pressure and Temperature.

The pipe, of course Wall BC.

The Outlet gets a prescribed massflow BC.

The steady state solution for given massflow leads to the necessary Outlet pressure, to achieve the presrcibed massflow.
This solution is the Initilisation for the transient calculation.

In the transient calculation the massflow at Outlet is time dependent. It starts with the value of the SS calculation and decreases to 0 within 1ms (closing valve)

The pressure at the Outlet rises, drops, rises and so on. I want to investigate this pressure oscilation in the next step.

But what do you think about the setup? Is it legitim to use the massflow BC at the Outlet? Because, when the massflow decreases CFX puts a wall to parts of the Outlet plane to prevent the fluid from flowing out. Can i use this to "simulate" a closing valve?
Do you think the pressure oscilation is the right natural frequency?

Thanks for your opinions

Greetz
Muffin

ghorrocks · January 28, 2016, 17:16

Sounds like you need to use an opening, not an outlet.

The approach you describe sounds valid and should work.

CFXMUFFIN · February 1, 2016, 16:54

thx for the feedback

I thought about the Opening Outlet BC as well,
and maybe i will try it to compare the results.
But im not sure if the pressure will rise, if the massflow continues to flow out of the pipe.
There will be an Outflow and a Inflow at the BC, resulting in zero massflow, instead of zero massflow as a result of a wall (closed valve).

January 28, 2016, 15:13	Massflow BC, natural frequency in pipi	#1
CFXMUFFIN New Member Michael Join Date: Jul 2015 Posts: 5 Rep Power: 11	Hey guys, what do you think about the following solution. Task: 1.There is a large pressure resorvoir, where the pressure is kept constant with a regulator. 2. From the resorvoir leads a pipe (straight and elbow parts, same diameter) to a valve For the task, the flow through the valve is not important. What i want to investigate is what happens in the pipe after the valve closes. (natural frequency of the pressure waves) Here the idea: I modeled the tank as a big rectangular Volume, where the pipe enters. The other end of the pipe (where the valve is positioned) is covered with an Outlet plane. At the volume all BC Planes (except the one where the pipe enters) have an Opening BC, with fixed Inlet pressure and Temperature. The pipe, of course Wall BC. The Outlet gets a prescribed massflow BC. The steady state solution for given massflow leads to the necessary Outlet pressure, to achieve the presrcibed massflow. This solution is the Initilisation for the transient calculation. In the transient calculation the massflow at Outlet is time dependent. It starts with the value of the SS calculation and decreases to 0 within 1ms (closing valve) The pressure at the Outlet rises, drops, rises and so on. I want to investigate this pressure oscilation in the next step. But what do you think about the setup? Is it legitim to use the massflow BC at the Outlet? Because, when the massflow decreases CFX puts a wall to parts of the Outlet plane to prevent the fluid from flowing out. Can i use this to "simulate" a closing valve? Do you think the pressure oscilation is the right natural frequency? Thanks for your opinions Greetz Muffin

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January 28, 2016, 17:16		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	Sounds like you need to use an opening, not an outlet. The approach you describe sounds valid and should work.

February 1, 2016, 16:54		#3
CFXMUFFIN New Member Michael Join Date: Jul 2015 Posts: 5 Rep Power: 11	thx for the feedback I thought about the Opening Outlet BC as well, and maybe i will try it to compare the results. But im not sure if the pressure will rise, if the massflow continues to flow out of the pipe. There will be an Outflow and a Inflow at the BC, resulting in zero massflow, instead of zero massflow as a result of a wall (closed valve).