Jump to content
GMS, SMS, and WMS User Forum
NoNoBoy

Some problem about the compressibility factor

Recommended Posts

I am running the flow in Femwater.

When I input the compressibility of the medium(the soil is gravel).

So I input the compressibility factor 1.0e-007.

But my model can’t run accuracy.

Then I find something in card type “MP2”.

(MP2 1 0.4058 0.4058 0.4058 0.0 0.0 0.0 2.0e-008 0.3543 1.0)

The 0.3543 is the value that I input about saturate moisture content (α).

Why this value appear here?

How do I input the right compressibility factor in the model?

Share this post


Link to post
Share on other sites

The 2.0E-008 is the compressibility, the 0.3543 is the porosity (=saturated moisture content), the 1.0 is the effective porosity ratio (which is supposed to be on the MP5 card, but GMS puts it in the wrong place). I am not sure why the compressibility is different from the value you entered (1.0E-007) though. Maybe there is another MP2 card?

Edited by woodward
  • Like 1

Share this post


Link to post
Share on other sites

You can use GMS to set up your femwater model files (fws, 3dm, 3bc), then go into the files, especially the 3bc file, and make any changes/corrections you want. Then you can go back to GMS and RUN the model and view the outputs (don't save though or you will overwrite your changes). Another approach is to run FEMWATER from the command line: copy the femwater3.exe into the same folder as the fws file, go to dos and type > femwater3 whatever.fws. I prefer this way. Once you have got your mesh and bcs set up you can make other changes directly in the 3bc file without using GMS. Then you can use the FEMWATER|Read Solution to load in the results and view them in GMS.

  • Like 1

Share this post


Link to post
Share on other sites

I am running the flow in Femwater.

When I input the compressibility of the mediumthe soil is gravel.

So I input the compressibility factor 1.0e-007.

But my model cant run accuracy.

Then I find something in card type MP2.

(MP2 1 0.4058 0.4058 0.4058 0.0 0.0 0.0 2.0e-008 0.3543 1.0)

The 0.3543 is the value that I input about saturate moisture content (α).

Why this value appear here?

How do I input the right compressibility factor in the model?

You enter the compressibility (alpha) of water (and materials for that matter) in the FEMWATER fluids and materials dialogue boxes. These are then converted to modified compressibility (beta) which is stored in the MP3 card. That is why the values in the card differ from the value you entered (see p 41 and 47 of the FEMWATER manual). Also remember that the value entered depends on the units used. A typical value quoted for water at 20 deg C is 4.6E-10 m^2/N(metres squared per Newton). To convert it to your correct input value, divide by the acceleration of gravity given in the required units in the FEMWATER Fluids properties dialogue box (if using metres and days it is 73206249984 (m/d^2)(should give a compressibility of 6.2E-21 md^2/kg) A similar conversion is required for material compressibility, if used.

Edited by Rob Virtue

Share this post


Link to post
Share on other sites

You enter the compressibility (alpha) of water (and materials for that matter) in the FEMWATER fluids and materials dialogue boxes. These are then converted to modified compressibility (beta) which is stored in the MP3 card. That is why the values in the card differ from the value you entered (see p 41 and 47 of the FEMWATER manual). Also remember that the value entered depends on the units used. A typical value quoted for water is 4.6E-10 m^2/N(metres squared per Newton) or (ms^2/kg) If your units are kg, days and metres, you will need to convert squared seconds to squared days (should be 6.2E-20 md^2/kg) A similar conversion is required for material compressibility, if used.

I don't think that is quite right. There are two compressibilities:

The value supplied to the MP3 card (beta) is the compressibility of water in units of L T^2/M. According to the manual (page 41) Femwater converts this to modified compressibility (betap, Equation A23, manual page 93). This occurs at line 5137 of the source code. So GMS should not (and I think does not) modify this. Freeze & Cherry (1979, page 529) suggest a value of 4.4E-10 m^2/N = 5.9E-20 m d^2/kg, similar to what Rob quoted.

The value supplied to the MP2 card (alp) is the compressibility of the medium, also in units of L T^2/M. According to the manual (page 47) Femwater converts this to modified compressibility (alp, Equation A22, manual page 93). This occurs at line 5109 of the source code. GMS should not (and I think does not) modify this. Freeze and Cherry (1979, page 55) suggest values of 1E-6 to 1E-11 m^2/N across a range of materials.

By the way, I am using GMS 6.5.6.

Edited by woodward
  • Like 1

Share this post


Link to post
Share on other sites

Thank for everybody's explanation very much.

I calculate compresses coefficient, again.

I think the value that controlled by time unit.

Because, the acceleration of gravity is not 9.8 m/s2.

I think to use value 1.27E+8 m/h2.

Share this post


Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...

×
×
  • Create New...