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Old   September 5, 2007, 08:59
Default Pulse inlet
  #1
jakjak
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I am looking for suggestion on implementing a pulse input at one of the inlets in a mixer.

The transient case I am studying is similar to the static mixer from one of CFX tutorials. Suppose that one of the inlets has some flow rate, while the other inlet is blocked. At some point in time, the blocked inlet is opened up for a short period of time, say 1 second, and allow some flow to come in.

How would one define such inlet?

thanks in advance.

jak
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Old   September 5, 2007, 10:09
Default Re: Pulse inlet
  #2
Anderson T F
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Hi Jak,

I am quite sure this question has already been discussed several times, but letīs see if I have already learned the answer.

You should create an expression and use it instead of a constant numerical value of your inlet velocity.

one possibility is:

- Click in create a CEL Expression - create your inlet velocity expression (myinlet), such as:

(step(( t - Tini)/1 [s] ) * step(( Tfin - t )/1 [s] )) * myFlow - Then create other expressions for the values of initial time (Tini), final time (Tfin), and your maximum flow rate (myFlow). Make sure to type them all with respective dimensions.

Then, in your inlet velocity, instead of writing a fixed value, write "myinlet".

Not really difficult, indead.

If you need more info, I am quite sure there is an explanation about this kind of expression in the tutorial with the free surface flowing over a bump.

Have fun

Anderson

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Old   September 5, 2007, 10:37
Default Re: Pulse inlet
  #3
jakjak
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I imagine you can use velocity and mass flow rate to simulate a blocked inlet (prior to and after the pulse input) by setting them to zero. Would you be able to use a pressure inlet to do this?

p/s: Anderson, thanks for the response.
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Old   September 5, 2007, 12:13
Default Re: Pulse inlet
  #4
CycLone
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Hi Jakjak,

You can do this with any inlet type. If it is a pressure inlet, you will need the boundary condition pressure to drop below the domain pressure to block it off. An inlet in CFX only allows flow to enter the domain. If the inlet pressure is low enough to draw fluid through the inlet, an artificial wall will be put up to prevent reversed flow.

An alternative to Anderson's suggestion (which is a reasonable approach) would be to create a 1-D profile function with time as the input and specify the pressure (or whatever you choose) at different points in time. Or you could combine the two approaches by making your profile function go from 0 to 1 and back to zero at specific times and use this to "turn on" another function by using it as a multiplier.

There are loads of other CEL functions that may be of use to use, including min(), max(), int(0), nint(). If you decide to get creative, use the plot feature to check your expression before running!

-CycLone
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Old   September 6, 2007, 17:24
Default Re: Pulse inlet
  #5
jakjak
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Thanks CycLone!

I managed to get the CEL setup and checked it with the plot feature. However, I am having problem by using pressure inlet. I have used 'Total Pressure' and set it to a relative pressure slightly lower (~1 psi) than the main domain for the inlet. I can see that the solver is putting a wall at that inlet, but the solver has problem converging and failed before getting through one time-step.

What could be the problem here? Is it because the pressure is not low enough?
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