Alexander Renz

When using the "steady flow/transient transport"-option, you have to make sure that the flow field is already in equilibrium, i.e. by running it first as a steady state flow-only model. FEFLOW will perfom a single iteration on the flow calculation and then proceed with the transient transport model. If the flow-field is not in steady-state by this time, you will get obstruse results (see also FEFLOW help system).

Does your model as well incorporate mass-density relationships? This can also affect your flow-field if "steady flow" is chosen, since FEFLOW still calculates density driven flow in its calculation (The term of the so called Boussinesq-approximation is still active, see Reference-manual ch. 1.10 for more explanation).

short answer: fixed!

Hi Michael and Thomas,

The difference between the confined model and unconfined ("phreatic"-option) model is quite simple:

In a confined simulation, FEFLOW calculates only the Darcy-equation.

The phreatic model is quite similar, just that if the cells pressure (not hydraulic head) falls below zero - meaning that it becomes dry - its conductivity is reduced linearly with its saturation. To avoid that the cell becomes absolutely impermeable, the water depth in the cell is not allowed to fall below a certain residual water depth (this can be adjusted in specific option settings).

The "free+movable" approach is essentially a confined approach, whereas the top slice is constantly shifted to match the water table. It can yields excellent results for homogenous aquifers, but also has a lot of pit-falls and should be handeled with care, especially if having heterogenous materials, 2nd, 3rd or 4th kind BC (which are shifted as well) or if the aquifer is falling partially dry.

In these cases, "phreatic" is more convenient and robust. It also gives the possibilities to simulate several free water table on top of each other.

The mathematics are described in detail in the reference manual chapter 2 and Whitepaper Vol I, chapter 2

give it a try!

Alex.

Hi Mac,

1.) That's right, making the inner side of the well low-permeable will mostly prohibit well-internal flow. Just keep in mind that a too high parameter contrast can make numerical problems; I usually use a parameter contrast of 100 or 1000.

2.) The flux-value on an internal boundary is applied twice, but no direction is given for the flux (meaning that the water does not have to flow with equal velocity in both direction). This applies for vertical BC (within a layer) as well as for horizontal BC (between two layers). In your case, the water will most probably flow in one direction only, you should therefore apply half of the calculated value to the BC. Don't forget to check the budget afterwards.

There is no in-built function for this in FEFLOW. Set an observation point on all slices and interpolate the result between these nodes with an external software.

Export your table (x,y,value) as formatted text (space-seperated) and rename the file to .trp

The coordinates in the coordinate box do refer to the mouse coursers position, not to the cell that is currently inspected, therefore it will in most cases not be the exact (x,y)-value of the node. Zoom in to increase accuracy.

When creating a multi-layer well in the flow boundary dialogue, FEFLOW sets a 4th kind (well) condition on the specified node on all specified layers. The total pumping rate is then applied to the bottommost node (all other nodes have a flux rate of zero), and the nodes are connected similar to a 1D discrete feature element.

Switching off the well-bore-condition in specific options setting would disable these connections (In some cases, this might become necessary). However, in the common case, it shall be activated.

In numerical models, the drawdown at point sources (like the well) is always dependent on the mesh discretization, whereas a finer mesh leads to a stronger drawdown.
The mathematical background is that, taking a single well with a constant pumping rate as an example, the well is defined as a point in space. Now, the constant flux has to flow through a zero area.
As a consequence, near this point, the velocity - and the pressure gradient - becomes infinite.

A very fine discretization will approximate this solution (showing a strong drawdown), while a very rough one will flatten this effect (lower drawdown)

--> In reality this is not happening, since the well in nature is not a point, but has a diameter. If you use an element size eqal to the well diameter, you get an approximation to the real drawdown. Note that this is no exact solution, since the element size does never meet the diameter.