MATH
feflow_user_manual_classic.pdf

# Note that for axisymmetric problems the axis of

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be performed for such kind of models. Note that for axisymmetric problems the axis of rotation corre- sponds to the vertical y -axis (where the x -coordinate is zero). For horizontal problem projection, optional toggle buttons are available to switch between confined and unconfined aquifers. Unconfined aquifers can either have a ’free and movable’ top slice or be marked as Figure 5.2 Chemical species definition.

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QS ö rëÉêÛë j~åì~ä J m~êí f RK mêçÄäÉã bÇáíçê ’phreatic’ where unsaturated flow is computed for dry areas using a simplified approach. Note that an uncon- fined, free surface aquifer model leads to a nonlinear problem formulation. For three-dimensional saturated-flow problems with free surfaces, an additional Free surface definition menu is available for defining multiple free surfaces and movable slices. For more detailed information please see the FEFLOW online Help and the Tutorial (Section 10.3.2.1). The model can be transformed from a transient to a steady-state model by changing the Type of simulation in the Problem Class menu . If time-dependent bound- aries, constraints or flow material exist, a special win- dow appears where you can insert the steady-state time step. The boundaries and parameters are interpolated and then set constant. RKP qÉãéçê~ä C `çåíêçä a~í~ jÉåì Essential parameters that control the stability and accuracy of the simulation are defined here. The parameters include: number of time steps, type of time stepping, final time, level to switch to Crank-Nicholson (CN) higher-order scheme, error tolerance and conver- gence criteria, types of upwinding, and nature of the time-varying functions. Figure 5.3 Temporal & control data.
cbcilt SKM `ä~ëëáÅ ö QT RKP qÉãéçê~ä C `çåíêçä a~í~ jÉåì qáãÉ ëíÉéë Transient simulations require temporal discretiza- tion. You can choose between Constant time steps , Varying time steps , and Automatic time step control . This last choice also allows to you select between dif- ferent adaptive schemes. For constant time steps, the default setting is 1 day. Note that time steps are not applicable for steady-state problems. Constant time steps usually reduce the numerical effort during the simulation because the simulator can use efficient resolution techniques. This avoids updat- ing the matrix system for linear problems in a time-step marching process. Varying time steps can be entered directly in a Time-step editor , or may be imported from a database. The Automatic time-step control can be per- formed using either the Forward Euler/backward Euler (FE/BE) time-integration scheme, which is 1st order in time (often appropriate for density-driven and unsaturated problems), or the Forward Adams-Bash- forth/backward trapezoid (AB/TR) time-integration scheme, which is 2nd order in time. For specific kinds of problems the Aggressive target-based time- marching scheme , either fully implicit or semi- implicit, may provide an effective solution strategy.

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