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COMSOL giving me wrong Ks value!
Posted 30.06.2014, 03:22 GMT-4 Fluid & Heat, Studies & Solvers Version 4.3a 3 Replies
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I am using the Richards Equation interface and there is an equation relating kappa (permeability) to k (hydraulic conductivity) which is written in the documentation but I have calculated by hand what I should be getting and it seems COMSOL is making a mistake. (I have also googled my equation and found that my hand calculations are confirmed elsewhere.) The equation states that:
kappa = k*mu/(rho*g)
where mu is the fluid dynamic viscosity (in my case 1E-3 Pa*s) and rho is the fluid density (in my case 1000 kg/m^3) and g is the gravitational acceleration (~9.8 m/s^2). My ks (k at saturation) is 6.94E-7 m/s, so my kappa at saturation should be 7.07E-14 m^2. I set my kappa as such, and sure enough it is showing me that "kappas" (saturated permeability) is 7.07E-14, and then when I check the saturated hydraulic conductivity it says that "ks" = 2.94E-4, which is off by several orders of magnitude.
Does anyone know why COMSOL might be using a different equation than it should be? I am sure that my math is right and all of the units are as they should be - I have no idea what the problem is!
Thanks,
Shoshana
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I use my own PDE written Richard's equation, so cannot comment much.
It seems like the 2.94E-4 is the default Ks input into the permeability model by COMSOL. Try changing that to let us say 2.94E-12 and see if you now get 2.94E-12 m/s, no matter what your kappa value is. If this is the case, I suggest that you best ask the support.
Suresh
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You're totally right, thanks! I changed the value for kappa and got the same k as always, so it seems it doesn't care what the equation is, it's just keeping its own default value. I can change it manually but that doesn't change the kappa value - I have to set each one individually, as it appears that COMSOL is ignoring the equations and not making the connection between k and kappa. Glad I at least figured out the problem though!
Thanks for your help,
Shoshana
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Based on your observation, it is best that you contact COMSOL because you are using a predefined module and you may not know how it is dealt internally.
Suresh