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Ed Mueller January 12, 2009 17:27

Fully Developed Flow at an Inlet
Hey all,

I am doing an analysis on cavitation in a pump in CFX. My inlet BC is a static pressure and my outlet BC is a mass flow rate. My question is: how can I get a fully developed flow at the inlet?

I know one way would be to extend the inlet pipe, but since my Re = 10^6, I would need to extend it to 44 times my diameter, which is not feasible. Any ideas on how I could specify a fully developed flow and static pressure for my inlet Boundary Condition?


Ed Mueller

wan January 13, 2009 07:31

Re: Fully Developed Flow at an Inlet
1.normally for a flow in cavity, we use mass flow/velocity at inlet and static pressure at outlet. 2.If you are using water, you can refer to some paper that using analytical solution to know the developing region length, peak velocity and mean velocity.

LKF January 13, 2009 09:46

Re: Fully Developed Flow at an Inlet
If you were to set your outlet BC as static pressure, then you could use CEL to define a fully developed flow profile at the inlet.

Is there a particular reason why you want your mass flow defined as an outlet BC and your inlet BC as static pressure?

You could also study what the effect is of setting the Turbulence parameter for your inlet static pressure to Zero Gradient - this setting is recommended for fully developed turbulence conditions.

Ed Mueller January 13, 2009 10:02

Re: Fully Developed Flow at an Inlet
Because I am modeling the cavitation of the pump, static pressure at the inlet is very important. Is there anyway to keep the static pressure at the inlet as well as define the velocity profile?

LKF January 13, 2009 12:18

Re: Fully Developed Flow at an Inlet

You can define a Total Pressure profile at your inlet using a velocity profile function to calculate the dynamic pressure - basically a dynamic pressure profile plus your desired inlet static pressure.

To ensure that your simulation runs with the right flow rate, you'll need to accurately define the outlet static pressure w.r.t. the desired inlet static pressure (i.e. the static pressure change between inlet and outlet). This value could be iterativley determined (or perhaps you already have it from previous runs. You could adjust the outlet static pressure on the fly with mass flow as a monitor point that you watch in the solver manager.

It is likely that other more experienced people will have simpler and perhaps more accurate means of accomplishing your goal, but this approach should work . . .

LKF January 13, 2009 12:50

Re: Fully Developed Flow at an Inlet
In hindsight, I believe my previous post is no different than defining an inlet velocity profile and an outlet static pressure. You would just iteratively adjust your outlet static pressure until the inlet static pressure meets your desired value.

Of course you are relying on the CFD model to accurately predict the static pressure change across the domain so it would be wise to validate the result in some manner.

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