New User (Graduate Engineering Student) Modeling/Software Issues with Real Time Experiment

Use this forum to post Vensim related questions.
Post Reply
apl1001
Junior Member
Posts: 3
Joined: Wed Aug 07, 2019 6:18 pm
Vensim version: PRO

New User (Graduate Engineering Student) Modeling/Software Issues with Real Time Experiment

Post by apl1001 »

Hello,

I am a new engineering graduate student at the University of New Hampshire. My research involves using a system dynamic approach to analyze the effect of flooding in the soil of pavements, and we have chosen to use Vensim as the software. I am somewhat new to the software, but I have read through the online manual, the forum, and as well as completing some tutorials to get acclimated with the software. Attached is the current model I am using as an introduction to play around with the software myself. I am having a bit of a problem figuring out exactly how to do what I want in the software.

What the model is, represents an experiment where force is applied to an object, with an initial area and stiffness, over a set amount of time. The way I have the model right now is that over the time frame I want, there is a linear increase in the amount the object strains (deforms) as the force increases over time. However, in reality the area in the experiment would decrease as the force is increase. Basically, the object is being squished by the force and the area is getting smaller.

So, the problem that I am having is how to program my model so that after every time-step the "initial area" is decreased by some amount. This would allow for a more real life simulation of what would happen during the experiment. However, I can't seem to find anything similar to this type of model online, and without any reference I am not sure how to do this on my own.

Any help or recommendations would be much appreciated. Or if you know when a model similar would be that would also be appreciated. But, really verbal specific help but be much appreciated. If you have any questions or need anymore information from me please let me know. Thank you for taking the time to read this and giving me a hand.

Sincerely,

Alex Lefebvre
Attachments
Compression Test Model V3 (2).mdl
(1.84 KiB) Downloaded 149 times
Administrator
Super Administrator
Posts: 4573
Joined: Wed Mar 05, 2003 3:10 am

Re: New User (Graduate Engineering Student) Modeling/Software Issues with Real Time Experiment

Post by Administrator »

Instead of having "INITIAL AREA", call it AREA, make it a level, and drain it over time.

It might also be a good idea to visit
http://www.public.asu.edu/~kirkwood/sysdyn/SDRes.htm
On this page you will find a really good Vensim PLE tutorial which will cover basics like this.
Advice to posters seeking help (it really helps us to help you)
http://www.ventanasystems.co.uk/forum/v ... f=2&t=4391

Units are important!
http://www.bbc.co.uk/news/magazine-27509559
tomfid
Administrator
Posts: 3804
Joined: Wed May 24, 2006 4:54 am

Re: New User (Graduate Engineering Student) Modeling/Software Issues with Real Time Experiment

Post by tomfid »

I don't think you want to make the area a stock. It's a function of the elongation of the object.
tomfid
Administrator
Posts: 3804
Joined: Wed May 24, 2006 4:54 am

Re: New User (Graduate Engineering Student) Modeling/Software Issues with Real Time Experiment

Post by tomfid »

I think what you have is correct, but you're violating the SD stock-flow conventions by introducing the stiffness parameter into the stock via an information arrow.

If you want to follow conventions, Strain is a stock (as is) with a flow of Elongation, and elongation = force/area/stiffness.

I don't know what the "*100" represents, but you don't want parameters buried in your equations, especially if they have dimensions.

It's been a long time since I took materials science, but it seems like the first order effect would be for the area to be inversely proportional to strain (follows from conservation of mass, assuming density doesn't change)?
apl1001
Junior Member
Posts: 3
Joined: Wed Aug 07, 2019 6:18 pm
Vensim version: PRO

Re: New User (Graduate Engineering Student) Modeling/Software Issues with Real Time Experiment

Post by apl1001 »

So I no longer made "Area" a stock and changed it to a level. That makes more sense that it should be drained over time. The problem is that I need to know the Area in order to calculate the stress. So the variable must be referenced twice and two arrows must be used.The new model is attached please let me know if this makes sense.

I am a little confused on the stiffness variable however. It is a constant so it should have any effect on the problem of the time steps when being integrated correct?

Also, I will look at the reference link thanks for that.

Thanks again for the responses.
tomfid
Administrator
Posts: 3804
Joined: Wed May 24, 2006 4:54 am

Re: New User (Graduate Engineering Student) Modeling/Software Issues with Real Time Experiment

Post by tomfid »

Attachment?
apl1001
Junior Member
Posts: 3
Joined: Wed Aug 07, 2019 6:18 pm
Vensim version: PRO

Re: New User (Graduate Engineering Student) Modeling/Software Issues with Real Time Experiment

Post by apl1001 »

Sorry, attached here thanks.
Attachments
Compression Test Model V3 .mdl
(2.01 KiB) Downloaded 131 times
tomfid
Administrator
Posts: 3804
Joined: Wed May 24, 2006 4:54 am

Re: New User (Graduate Engineering Student) Modeling/Software Issues with Real Time Experiment

Post by tomfid »

I think there are still several problems here.

1. Units don't balance. This is an automatic disqualifier in my view.
2. I don't think area and strain can be independent. As long as density is constant, the volume of the sample will be conserved, so increasing strain decreases area, without integration in between.
3. The "force" variable isn't a force if it has units of N/minute.

I don't think I understand enough about the experimental setup this describes to comment further.
Post Reply