# Boundary Conditions for Duct Flows

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May 30, 2024, 12:44
Boundary Conditions for Duct Flows
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Hi

I am working on internal flows like duct or intake flows in STARCCM+ and got handicap or stuck with boundary conditions. It would great help if someone can provide some suggestions.

I am simulating a body with an inlet duct. The body is placed in Pressure far-field domain (see picture below "inlet.jpg"). Flow can enter in duct on its own but there is a pressure outlet boundary condition at duct exit to control mass flow rate.
I have set reference pressure for simulation to 101325 Pa
For any pressure outlet value, there is a specific mass flow rate passing through pressure outlet.
For example, if 5000 pa static pressure is applied at duct outlet, the pressure is achieved and required mass-flow is also achieved.
Then pressure is increased 10000 the mass-flow reduces and required values are achieved.
Then static pressure is increased to 15000 and required static pressure and mass flow is achieved at duct exit.
But if pressure is increased from this point on-wards, no matter what, the specified pressure is not achieved and mass flow also got stuck to a fix point. The details can be seen below:

Press Val Spec  Press Val achvd   Mas Flo rate achvd     Status
5000 Pa           5000 Pa                  20 kg/s         Easily Converged
10000             10000                    18              Easily Converged
15000             15000                    16             Easily Converged
20000             18000                    14      Req. values not achieved
25000             18000                    14      Req. values not achieved
30000             18000                    14      Req. values not achieved

From the point onwards where pressure got stuck, I have tried few changes but not succeeded yet. Summary

Changed "Pressure outlet BC" with fixed pressure to "Pressure outlet BC" with target mass-flow rate option ............... result: specified mass flow rate, pressures and velocities are highly oscillatory in duct..... no steady solution achieved

Changed "Pressure outlet BC" with fixed pressure to "Outlet BC" with fixed mass-flow rate option ............... result: specified mass flow rate is fixed on specified value but pressures and velocities are highly oscillatory ..... no steady solution achieved

Changed "Pressure outlet BC" with fixed pressure to "Pressure outlet BC" with avg face pressure option ............... result: specified mass flow rate, pressures and velocities are highly oscillatory ..... no steady solution achieved

I also tried to Change "Pressure outlet BC" with fixed velocity option ............... result: velocity is fixed but mass flow rate and pressures are highly oscillatory ..... no steady solution achieved

Only little progress is made with tweaking with back pressure

Tweaked with Outlet "Back Pressure" from "Enviornmental" option to "Static Pressure" Option ............... result: specified mass flow rate, pressures and velocities achieved ..... but only for limited number of simulations .... summary below:

Press Val Spec   Press Val achvd    Mas Flo rate achvd          Status
5000 Pa              5000 Pa            20 kg/s           Easily Converged
10000               10000              18                Easily Converged
15000               15000              16                 Easily Converged
20000               20000              12                Easily Converged
25000               22000              11          Req. values not achieved
30000               22000              11          Req. values not achieved

I can't find any option or limiting factor that why my specified pressures or velocities or mass flow rates are not achieved in form of steady state solution after a certain limiting value of pressure.

Any possible solutions to this problem, Also see the setup picture attached ... "inlet.jpg".

Any help in this regard is much appreciated.
Attached Images
 Inlet.jpg (29.1 KB, 0 views)