CFD Online Logo CFD Online URL
Home > Forums > CFX

Pulse inlet

Register Blogs Members List Search Today's Posts Mark Forums Read

LinkBack Thread Tools Display Modes
Old   September 5, 2007, 08:59
Default Pulse inlet
Posts: n/a
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.

  Reply With Quote

Old   September 5, 2007, 10:09
Default Re: Pulse inlet
Anderson T F
Posts: n/a
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


  Reply With Quote

Old   September 5, 2007, 10:37
Default Re: Pulse inlet
Posts: n/a
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.
  Reply With Quote

Old   September 5, 2007, 12:13
Default Re: Pulse inlet
Posts: n/a
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!

  Reply With Quote

Old   September 6, 2007, 17:24
Default Re: Pulse inlet
Posts: n/a
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?
  Reply With Quote


Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On

Similar Threads
Thread Thread Starter Forum Replies Last Post
Species mass flow inlet lorenz FLUENT 3 March 15, 2012 08:26
Inlet Velocity in CFX aeroman CFX 12 August 6, 2009 18:42
Pulsating flow with non-reflective inlet boundary cc1000 CFX 6 April 27, 2009 08:10
Inlet table in STAR-CD Sachin Siemens 1 March 26, 2008 11:22
what the result is negatif pressure at inlet chong chee nan FLUENT 0 December 29, 2001 06:13

All times are GMT -4. The time now is 11:28.