Tank discharge

Hi,

My goal is to simulate and observe hydraulic heads and out flows in a cubic tank discharge model with a double porosity continuum.
In order to illustrate the model, the problem would be a tank filled with 4 "towers" of rocky material (matrix) separated from 1) each other and 2) the tank border by an aperture of size X (fracture).
In this model, the discharge would take place through a hole located at the bottom of the tank, in the middle of one side.

I have some questions regarding with my FEFLOW (5.3) models 1) the boundary condition I should have at the outlet, 2) the free surface parameter, and 3) discrete feature elements and a 3D approach.

I tried to work step by step from a simple discharge of a homogeneous material through a node 2D model to a 3D model with higher conductivities areas.

Here are my questions :
- In a 2D horizontal model (*.fem file attached), considering a discharge in the case of 2 conductivities media. Top elevation of the aquifer is 0.30 [m], bottom is 0 [m], and a square mesh of 0.30x0.30 [m]. I change the "dry" parameter (residual water depth) to 0.000101 [m] in order to set a fixed head boundary (1st kind BC) on an outer border node at 0.001[m], because 0 isn't working right?

- I tried to use a discrete feature element to represent the fracture medium but it gets messy, is this changing a lot from a 2 conductivities approach?

- Would it improve a lot the first 2D results to switch to a 3D problem? I tried to, but I am not sure of how many slices I should use (3 or more?) and 1) which [i]"free surface"[/i] condition I should use in this case [i]"constrained"[/i] with a [i]"seepage face"[/i] ?, 2)Is [i]"phreatic"[/i] recommended for the top slice ?


Thanks for your answers!


Vivien

Hi Viven,  I recommend you read white_papers_vol1.pdf.  There are many examples of what to consider when setting up models, especially the type you are working on.  I think a 3d model would work much better than 2d, and I think you will need a dense mesh (both horizontal and vertical, and small time steps) to simulate the problem accurately.  I would use variably-saturated mode. The expected flow rate (or hydraulic gradient) are the primary consideration when setting up the mesh.  Large gradients and/or flows require denser mesh.  The tutorial has a fine example of this in which successively finer (denser) mesh is used to more accurately simulate head near a pumping well.  In your tank model, the largest flows & gradients will be at the spigot (outlet).  I would model the outlet with a fixed head set at elevation of the outlet, but I may also include material properties around the spigot to help account for turbulent flow near and in the spigot. I think 1d and 2d discrete features may be unnecessary if the mesh is dense enough.

Pete