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Old   July 29, 2004, 11:15
Default pulsating inlet boundary condition
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Jason
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Hi, all,

I am simulation a pulsejet using CFX5.7. I have a question about the inlet boundary condition.

The phisical problem: The pulsejet is simplified as a duct. Air comes into the combustion chember through several ports in the inlet. There is a valve behind these ports controlling the port openning/closing. (When Pressure in the chember is higher than outside pressure, port closed, no air comes in, combustion stopped; then when the pressure in the chember is lower, port open and combustion started.) This process will repeat. Fuel is injected continiously into the chember.

How do I specify this inlet boundry condition(air injection) based on the pressure difference?

Like the technique tip on the CFX website, I am now trying to specify a period value for the air flow using User CEL Function. But is this method based on the fact that we already know the scalar period? I want to know whether I can model this air injection process by pressure difference and how.

Any suggestion?
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Old   July 29, 2004, 11:47
Default Re: pulsating inlet boundary condition
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Robin
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Hi Jason,

You can make your CEL expression a function of the domain pressure. Exactly how you do that depends on where you measure the pressure. Supppose it is based on the pressure at the inlet named "Inlet 1", then you might do something like this. Define the following expressions:
EXPRESSIONS:
Pcritical = 1 [atm]
Pinlet = areaAve(Pressure)@Inlet 1
END



Where you specify the velocity at the inlet, say 500 [m/s], enter the following:
500 [m/s] * step((Pcritical-Pinlet)/1[atm]))
Now when Pinlet is greater than Pcritical, the expression inside the step function will be less than 1 and the step function will return zero. Note that the expression inside the step function must be unitless, thus dividing by a pressure.

Regards,
Robin
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