Learning Forum

[luis.pusey-alvarado]

I would like to simulate the bladder inflation stage and determine the response time.

 

1. Inputs – Mass flow rate vs. pressure difference (density and temperature will remain constant—steady state over a longer duration, ideally with minimal thermodynamic effects).

 

2. Output – How long it will take to inflate.

 

 

I am currently encountering two major issues while using the WANG_NEFSKE airbag card in conjunction with *INTERACTION for our simulations:

 

1.Mass Flow Rate Input: We are providing the mass flow rate per unit time to the pump, but we actually need to specify the mass flow rate per unit area to accurately simulate the filling of the pump from the environment and maintain that supply.

 

2.Density and Temperature Variations: As mass enters the bladder, we are observing significant changes in density and temperature, which result in large discrepancies in displacement. Since our scenario is intended to be steady state, we aim to minimize these effects on displacement and volume. 

 

Please suggest how to simulate this field problem.

[Reno Genest]

Hello Luis,

There is an airbag course on the Ansys Learning Hub:

Airbag Modeling in Ansys LS-DYNA - Airbag Modeling in Ansys LS-DYNA

The mass flow rate depends on the pressure difference correct? So, mass flow rate is not constant. Pressure difference between what? 

Have you looked at *AIRBAG_INTERACTION? This keyword allows you to define mass flow rate vs pressure difference. Here is more information from the LS-DYNA User Manual Vol I:

LCID Load curve ID defining mass flow rate as a function of pressure
difference, see *DEFINE_CURVE. If LCID is defined, AREA, SF
and PID are ignored.

 

Also, have a look at *AIRBAG_PARTICLE.

 

Let me know how it goes.

 

Reno.

[luis.pusey-alvarado]

Thank you Reno for update,

 

We are working on filling a bladder from a pump based on mass flow rate and pressure difference. The pressure difference is calculated as atmospheric pressure minus pump pressure, which we are controlling. The goal is for the pump to automatically adjust to maintain the desired mass flow rate based on this pressure difference. The pump should focus on maintaining mass flow rate through the outlet area rather than time.

 

We tried AIRBAG_INTERACTION_CARD with *AIRBAG_WANG_NEFSKE based on mass flow rate vs pressure difference however we are seeing inconsistencies in results as bladder not fully inflating with mass flow rate with specified parameters. As density and temperature have significant impact on bladder inflation which we don’t want to consider.

The only way to achieve the desired outcome is by modifying the gas temperature from 300 K to 470 K, which does not accurately reflect our scenario of steady-state cooled air.

 

I would appreciate your guidance on how to get the fully inflated bladder with room temperature and second how to maintain constant mass flow in pump cavity based on area not based on time.

[Reno Genest]

Hello Luis,

Could you describe the problem with *AIRBAG_INTERACTION in more details? What do you mean by "inconsistencies in results"? Could you post a picture of the results? Without a model it is difficult to troubleshoot.

If you are a commercial customer, please create a support case on the Ansys Customer Support Space (ACSS) and we will have a look at your model:

customer.ansys.com

 

Note that Ansys employees are not allowed to upload and download models from the Ansys Forum. You can still upload your model and maybe other users will be able to help you.

 

Reno.

[luis.pusey-alvarado]

Thank you Reno for the update.

We are not getting the expected results, so we simplified the model using *AIRBAG_LINEAR_FLUID to check if it meets our requirements, avoiding thermodynamic effects since our model is steady-state and long-duration.

We used a *AIRBAG_LINEAR_FLUID and applied pressure for the baseline run to calculate the bladder volume and derive the mass flow rate. We then used this mass flow rate as input for RUN1, while keeping all other parameters the same.

However, the displacement we are getting is not matching the baseline of 40 mm, and the pressure also is not reaching the baseline level. Ideally, if I use the output from the baseline run as input for another run, I should achieve the same results. Please find the attached report for details.

We also tested other airbag cards, like SIMPLE_AIRBAG_MODEL and WANG_NEFSKE, but the results are still similar.

Looking forward to your feedback.