Implimenting Boundary Condition
 

Hi All

I am doing an internal simulation with a diffuser at the outlet. Due to the reverse flow at the outlet the simulation crashes as CFX generates a virtual wall to stop the reverse flow. And hence I am interested to know if there is any ' Pressure gradient ' boundary condition we can give at the outlet in CFX
As my application is like inlet a mass flow rate is available relevant to it there is an operating pressure and then out let is exposed to the atmosphere

So for Inlet : I know the operating pressure , mass flow rate, there by velocity
outlet : flow is going to the atmosphere so atmospherice pressure nothing else.

But when I do the simulation the pressure at the inlet is not relevant to the mass flow rate we gave.so bit confusing. Then at the outlet CFX says it creates wall due to reverse flow. So what can it be the reason. And hence I thought of giving some convective pressure gradient or something at the Boundary condition !! Can someone suggest me some ideas.

It would be good to know the following:

Fluid is compressible, or incompressible ?

Do you know the total pressure at the inlet ?

Assuming your fluid is compressible and steady state simulation. Why not run mass flow outlet, and total pressure inlet ?

Yes It is Incompressible and I wanted to try steady state first and then LES
There is only two options in cfx for inlet as pressure : Static Pressure or total pressure right ? but as per cfx manuel it says giving pressure boundary condition is not correct !!
Anyways I have confusion with this pressure the pump behind the pipe operates at 10 bar which corresponds to give a flow rate of 5 l/min so what is this pressure total or static pressure ? And I have reference Presseure as 1 atm on my intialization

Now, we know it is incompressible it should be easier to solve.

I do not recall where the manual says anything using pressure boundary as incorrect. I recall referring to ill-posed boundary conditions combinations. For example, static pressure inlet and outlet is a very ill-posed problem since the discretized problem may not be able to match a predetermined pressure change.

Back to your problem. Since the flow is incompressible the level of pressure is irrelevant; therefore, you can solve for any total pressure you want at the inlet, specified mass flow at the outlet.

Once you obtain a solution add any reference pressure level, say the operating pressure to the solution and you should be fine.

Dear Opaque

Could you please elaborate!!