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| Titel |
Efficient random walk particle tracking algorithm for modeling advection-dispersion transport in highly heterogeneous porous media |
| VerfasserIn |
Michel Bechtold, Jan Vanderborght, Olaf Ippisch, Harry Vereecken |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250057254
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| Zusammenfassung |
| The random walk particle tracking (RWPT) method is a well established alternative to
grid-based Eulerian approaches when simulating the advection-dispersion transport problem
in highly heterogeneous porous media. A difficulty of the method is the loss of accuracy of
the dispersive displacements when the dispersion tensor or the water content is spatially
discontinuous, which is common in heterogeneous unsaturated soils. The interpolation
method is mostly used to smooth the variables in the vicinity of the discontinuity (e.g.,
velocity, dispersion tensor, water content), and to apply the smoothed variables to the
calculation of the dispersive terms. Convergence is achieved by refining the interpolation grid
and the time step size simultaneously, which is computational inefficient. An alternative
method which does not need the refinement of the interpolation grid is the partially reflecting
barrier method. However, this method has been outperformed by other RWPT methods in
previous studies. In this contribution, we present three corrections to the partially
reflecting barrier method that improve its accuracy and efficiency so that, especially for
large grid sizes, it outperforms other methods: i) The systematic overestimation
of the second dispersion displacement across an element interface when a linear
time splitting is used was corrected for using a non-linear time splitting. ii) The
two-sided reflection coefficient was replaced by a one-sided reflection coefficient that
represents the effect of discontinuous dispersion coefficients correctly for Δt–>0 but
eliminates redundant reflections and thereby reduces the error for discrete Δt. iii) The
transformation of the dispersive displacement across the element boundary for complex
multidimensional transport problems. The proposed corrections are verified numerically
by comparison with analytical solutions and by a detailed comparison with the
interpolation method for a three-dimensional test scenario. We demonstrate the general
applicability of the reflection barrier method to complex multidimensional transport
problems. The results indicate that the improved RWPT algorithm is a very efficient
approach for transport problems with highly heterogeneous dispersion tensors and
water contents. Because the new algorithm efficiently simulates both advection- and
dispersion-dominated transport conditions, it enhances the applicability of RWPT to
scenarios in which both conditions occur, as for example in the highly-transient unsaturated
zone. |
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