psinton@aquageo.us

Phreatic mode doesn't work very well in cases where the water table must pass through several layers of the model, such as may happen in mountainous terrain.

Pete

AquaGeo, Ltd., invites FEFLOW users to try our free plug in called ZoneTable.  This plug in works with Version 6.2 in 3D flow models, and provides a different approach to specifying Kx, Ky, Kz, specific storage and unsaturated flow porosity (specific yield).  The plug in can simplify the process of model setup and modification by using a material-property lookup table and zonation array.

http://aquageo.com/software/FeFlow/IFM/ZoneTable/index.html

Cheers,
Pete Sinton & Bill Wingle

Alice, are you attempting to set the elevation of each slice to one value, and then deactivate elements to mimic elevation differences of geological layers? In 6.1, elements cannot be deactivated, but you can set the permeability and storage of the element to very small values so that the act like an impermeable barrier or block of material.  This won't work if the elements you want to "inactivate" are at the top of the model where non-zero recharge (in/out flow) is applied.

Pete

I've been testing AlgoMesh and figured out how to do this in 6.2: Open feflow, but do not open a fem or smh file. Click "File" and "Import Mesh...", and then add, for example, a shape file of the mesh. It will import as a 2d confined model. I don't know yet what it does if you have a model already open...

Pete

The initial temperature can also be set in all earlier versions of feflow

You can also define the wall using super mesh (SM) elements instead of lines.  Ahalo of SM elements that grow in width with distance from the wall to facilitate control mesh density.

Pete

select the nodes you want the data for, export the temperature for the selection, then use excel or some other program to compute the average

Pete

I haven't yet found a more efficient way other than close and careful inspection of the supermesh, and I agree, using feflow to do the supermesh is the most efficient editing method.

Pete

The 'ideal element size' is based on a 2D analytical solution applied to a 3D system? What analytical solution is used? The Book doesn't indicate for flow-only situations.  I agree that smaller element size (closer node spacing) may not result in more accuracy because at some point the numerical approximation will match the continuous PDE very closely (within the limits of the machine). 

What Peter is calling an "over estimate" is not the same thing as numerical error. When you add nodes (smaller elements), the simulation is indeed more accurate (or as accurate as the machine precision allows) but the computed drawdown values at nodes inside the "virtual radius" (the actual or physical radius of the pumped well) do not represent what actually happens inside the bore hole.

I assume the analytical solution Peter refers to is the typical one developed by CV Theis.  The Theis solution does not apply to what actually happens inside of the bore hole either. However, both the numerical and the analytical solutions can be used to compute drawdowns within the radius of the bore and, since the bore hole is not actually simulated by either, both will provide accurate results for a point sink  (one of the assumptions for the Theis equation is that the borehole is infinitely small).

I wonder if the virtual radius is really a good guideline for node spacing near the bore hole in cases where one wants to simulate drawdown near to, or at, the borehole wall. Maybe I will do some tests.

As for the artifact (numerical error), I agree with Peter that its not that important provided (1) you are not attempting to accurately simulate the drawdown near to, or at, the wall of the borehole and (2) the error doesn't overwhelm your mass balance. 

I think Mark did get more accurate results when he added nodes.

Pete

I see. I hadn't used this feature of FEFLOW before, but looking at the help file, the "ideal element size" seems fixed regardless of the pumping rate. When I change the well radius (which is the radius of the vertical pipe element feflow internally assigns), the "ideal" radius changes, but the rate has no effect on the "ideal" radius. 

However, the larger the pumping rate, the larger the hydraulic gradients at the well node and associated pipe elements, which to me means that elements have to be smaller (nodes closer together) to get an accurate simulation of heads in and near the well.

The help file states this: "Due to spatial discretization in numerical modelling, the hydraulic head resulting from the simulation at the well nodes themselves highly depends on the size of the elements around the well location. " 

So while I don't understand why the ideal radius isn't also a function of the pumping rate, it is clear to me that smaller elements are needed as the pumping rate increases. In any case, the way you describe your problem leads me to think your node spacing (and hence element size) is too large. 

You basically tested this when you put nodes closer to the pumping well. your result was more drawdown, which is exactly what I would expect with a more accurate simulation. The drawdown wasn't over-estimated in that simulation, it was more accurately calculated.  FEFLOW is simulating an ideal pumping well but your field data comes from a realistic (non-ideal) well.

Pete