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Old   June 1, 2017, 04:45
Default Turbo Machine
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Umer Sohail
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Hi CFX Turbo Experts. I am doing Transient simulation of rotor 67 but i am not getting almost similar result of steady state. Kindly help me out in boundary conditions.
My Boundary conditions are as follows:
1) Total time 1sec and time steps 0.0000125
2) Inflow/Outflow boundary template: P total inlet/ P static Outlet (Compressor rotating)
3) for choking conditions : Inlet and outlet 1atm
4) Flow direction: normal to boundary
5) interfaces: Periodic (as i have only ratating blades, No stator blades)
6) Domain Initialization: Frame type : stationary
Velocity type: cartesian (0,0,0)m/s
Static Pressure: 100000 Pa
Temperature: 288 K
Turbulence: Medium (Intensity=5%)
plz need help. How may i get choking mass flow rate 34kg/s like steady condition.
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Old   June 1, 2017, 06:10
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Glenn Horrocks
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Please post the results you are getting.
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Old   June 1, 2017, 08:34
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Umer Sohail
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choking mass flow rate 18 kg/s and polytropic efficiency 92%. But it doesnot keep constant. When i keep running it goes down. I want to stop simulation automatically when it becomes steady and at maximum mass flow rate i.e. 34kg/s or near to that
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Old   June 1, 2017, 10:58
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May I ask what are you trying to achieve ?

If you have already run the steady case, and you have a concern of not being a stable steady solution (not sure why), I would have restarted the transient simulation from the steady state results. If steady, it should remain steady with minor oscillations if any.

In addition, I would have used the Exit Corrected Mass Flow option instead.
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Old   June 1, 2017, 11:14
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actually i just want to run transient simulation at different rpm. To check my transient simulation working properly i run transient simulation at same rpm of steady state. Its results should be near to steady state. But i noticed that mass flow rate first increases and than drops. I intend when rpm transient state achieve steady state rpm, simulation should stop. Or when it achieve maximum mass flow rate i.e. choking condition it should stop. So how may i run this
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Old   June 1, 2017, 13:22
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Perhaps it is better to clear out some misconception between steady state and transient simulations for non-linear problems.

If a transient simulation converges to a steady state solution, you should be able to obtain the steady state solution w/o solving for the transient regime using appropriate steady state solvers. Hopefully to a unique stable steady state solution (recall non-linear problems might have multiple solutions)

However, the opposite is not true. The fact that you obtained a steady state solution does not completely guarantee (in particular for non-linear problems) that a transient simulation will converge to the same steady state solution. The steady solution if not mesh independent may have falsely converged to a steady level due to high numerical diffusion (and a large false timestep). A transient simulation may (in some cases) show an unsteady feature thanks to the use of an accurate time integration. The typical example used is the vortex shedding behind a cylinder, or separation on the trailing edge of an airfoil.

In the case of a turbomachine, the flow conditions away from the design point can be highly unsteady (at least on the surge/stall regime) and the solution between both approaches may be quite different.

I did not understand the statement

"I intend when rpm transient state achieve steady state rpm, simulation should stop. Or when it achieve maximum mass flow rate i.e. choking condition it should stop."

Hope the above helps,
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