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NX Flow Dynamic Modelling - Time Varying?



I'm trying to figure out how to simulate a vehicle engine manifold in NX Flow. There are two outlet ports and one opening. The two outlet ports need to be timed according to the intake port opening and closing times. Is there somewhere to input an equation or 'volume flow VS time' values? I don't have an equation made yet, but I know the port opening and closing degrees with respect to output shaft, and I know the maximum theoretical volume flow rate in L/min.



Re: NX Flow Dynamic Modelling - Time Varying?


I should clarify: It is an intake manifold. The two "flow outlet" ports are the intake tubes into the engine, so the vacuum pressure will fluctuate as the intake port opens and closes.

Re: NX Flow Dynamic Modelling - Time Varying?

Siemens Experimenter Siemens Experimenter
Siemens Experimenter

Hi CS13B,


You can definitely define time-varying flow BCs. You want to define the Magnitude property in flow BCs using the "Field" option, as illustrated below. This will let you define a time-varying Field (or function).


Also note that you probably want to use a very small flow rate value when the port is closed (as opposed to zero). It may also be a challenge to get proper convergence for the dynamic switching on & off of ports. You'll probably need a fairly small time step to capture this behavior.


You can also simulate various configurations of open/closed ports through a number of steady state runs. It all depends what you're after.




Re: NX Flow Dynamic Modelling - Time Varying?


Hi Nicolas,

Thanks for your reply.
I figured out how to model it with the Field option, however I'm having trouble entering an equation. I made a spreadsheet in Excel instead, but it won't import properly using the "import from file" button, nor the "edit table in spreadsheet" button. So I entered the data points manually into the table field which was just time consuming to do for every individual runner. Seems to work okay, though.
I'm using NX8.5 from my school.


Re: NX Flow Dynamic Modelling - Time Varying?

Siemens Phenom Siemens Phenom
Siemens Phenom

What is the form of the equation? Is it simply time dependent? NX expression syntax is admittedly rigid. The attached image shows a time dependent pressure as an equation/formula. The first required step in the Formula Field dialog is to specify the independent domain as Time. Then you can define the equation as a function of ugvar("time").


The attached image is probably an overly complicated example, but it points out some expressions pitfalls and features. First notice that "time" is listed as a variable at the bottom of the dialog. Double-clicking a variable will insert it into an equation where the cursor is located. That prevents the user from having to type the entire "ug_var" string. Second notice the units where they are and are not defined. A number with no units will assume the dependent variable's units. Here 25 and 12 have units of N/mm^2. I defined the units for 10 as psi. I started to type the units string, then NX gave me a list of completed strings to choose. Last, notice that the formula/equation resolves to a pressure unit. Where I multiplied by time, I also divided by time (i.e. 1 sec) to cancel the units. Expressions always take units into account.





Mark Lamping

Simulation Product Management

Product Engineering Software


Siemens Industry Sector

Siemens Product Lifecycle Management Software Inc.