Abstract
We study an integro-differential equation that has important applications to problems of anomalous transport in highly disordered media. In one application, the equation is the continuum limit of a continuous time random walk used to quantify non-Fickian (anomalous) contaminant transport. The finite element method is used for the spatial discretization of this equation, with an implicit scheme for its time discretization. To avoid storage of the entire history, an efficient sum-of-exponential approximation of the kernel function is constructed that allows a simple recurrence relation. A 1D formulation with a linear element is implemented to demonstrate this approach, by comparison with available experiments and with an exact solution in the Laplace domain, transformed numerically to the time domain. The proposed scheme convergence assessment is briefly addressed. Future extensions of this implementation are then outlined.
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Abbreviations
- \(a_{p}\) :
-
Prony pre-exponential coefficient (–)
- \(A_{ IJ}\) :
-
Mass matrix (\(\hbox {m}^{3}\))
- \(b_{p}\) :
-
Prony exponent coefficient (1/min)
- C :
-
Concentration (\(\hbox {kg}/\hbox {m}^{3}\))
- \(D_{ij}\) :
-
Mean dispersion tensor (\(\hbox {m}^{2}/\hbox {min}\))
- h :
-
Element length (m)
- I :
-
Memory-concentration convolution (kg/m\(^{3}\))
- \(j_{i}\) :
-
Flux vector (kg/m\(^{2}\).min)
- \(L_{ IJ}\) :
-
Advection matrix (\(\hbox {m}^{3}/\hbox {min}\))
- M :
-
Memory function (1/min)
- \(P_{ IJ}\) :
-
Dispersion matrix (\(\hbox {m}^{3}/\hbox {min}\))
- \(Q_{I}\) :
-
Load vector (kg/min)
- \(q_{i}\) :
-
Mass flux (kg/m\(^{2}\).min)
- \(n_{i}\) :
-
Outwards unit normal vector (–)
- \(N_{I}\) :
-
Shape function (–)
- s :
-
Element cross-sectional area (\(\hbox {m}^{2}\))
- S :
-
Source (kg/m\(^{3}\).min)
- t :
-
Time (min)
- \(t_{1}\), \(t_{2}\) :
-
TPL parameters (min)
- u :
-
Laplace variable (1/min)
- \(v_{i}\) :
-
Mean velocity vector (m/min)
- V :
-
Domain
- \(x_{i}\) :
-
Coordinate vector (m)
- \(\beta \) :
-
TPL parameter (–)
- \(\varGamma \) :
-
Domain boundary
- \(\delta \) :
-
Dirac delta function (–)
- \(\varDelta \) :
-
Difference (–)
- \(\phi \) :
-
Upwind parameter (–)
- \(\theta \) :
-
Implicitness parameter (–)
- \(\xi \) :
-
Normalized element coordinate (–)
- \(\omega \) :
-
Time step interval parameter (–)
- i, j :
-
Coordinates indices
- I, J, K :
-
Nodal indices
- p :
-
Prony term index
- \(\psi \) :
-
Transition time PDF
- 0:
-
Initial
- 1, 2:
-
Local nodal indices
- , :
-
Covariant derivative
- c :
-
Convective (or advective)
- d :
-
Dispersive
- D :
-
Dirichlet
- N :
-
Neumann
- n :
-
Time step index
- R :
-
Robin
- \(\sim \) :
-
Transformed
- \(\cdot \) :
-
Time derivative
- −:
-
Prescribed
- \(\otimes \) :
-
Convolution
- ADE:
-
Advection–dispersion equation
- BC:
-
Boundary conditions
- BTC:
-
Breakthrough curve
- CTRW:
-
Continuous time random walk
- DOF:
-
Degree of freedom
- FEM:
-
Finite element method
- GME:
-
Generalized master equation
- IC:
-
Initial conditions
- LT:
-
Laplace transform
- PDE:
-
Partial differential equation
- PDF:
-
Probability density function
- TPL:
-
Truncated power law
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Acknowledgments
B.B. gratefully acknowledges support by the Minerva Foundation, with funding from the Federal German Ministry for Education and Research. B.B. holds the Sam Zuckerberg Professorial Chair in Hydrology. S.J. was supported by NSF under grant DMS-1418918.
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Appendices
Appendix 1
Applying the Galerkin method to the PDE (13) and its BCs (20)–(22) (see Zienkiewicz and Taylor 2000), with the weight functions \(N_{I}\) for (13), arbitrary \(N_{I}^{D}\) for (20), arbitrary \(N_{I}^{N}\) for (21) and arbitrary \(N_{I}^{R}\)for (22), we obtain
By choosing C such that (20) is satisfied, the integral on \(\varGamma ^{D}\) vanishes. We may also choose \(N_{I}^{N} =-N_{I}\) and \(N_{I}^{R} =-N_{I}\), because they are arbitrary. Applying integration by parts and the divergence theorem to the volume integral and substituting (24), (26) and (27) in (73), yields
We may also choose \(N_{I}\) such that it is 0 along \(\varGamma ^{D}\) so that the integral on \(\varGamma ^{D}\) vanishes. Substituting (14)–(16), (19) and (24) into (74), we finally obtain
or
where \(A_{IK}\), \(L_{IK}\), \(P_{IK}\) and \(Q_{I}\) are the discrete mass, advection, dispersion and load matrices and vector, respectively, defined by
and \(I_{J}\) is defined by
In the following, we assume that \(v_{i}\) and \(D_{ij}\) are constant within each element and in time. Therefore, \(A_{ IJ}\), \(L_{ IJ}\), \(P_{ IJ}\) and \(B_{ IJ}\) are constant for given geometry, grid, velocity, dispersion and the specific type of elements and schemes chosen. Note also that \(A_{ IJ}\) and \(P_{ IJ}\) are symmetric, whereas the advection matrix, \(L_{ IJ}\) and \(B_{ IJ}\) are not.
The semi-discrete equation in the time domain is obtained by substituting (17) in the inverse Laplace transform of (75) and rearranging, yielding
where \(T_{I}\) is defined by
For the ADE, substituting \(\tilde{M}(u)=1\) in (76) yields the same form, but with \(\tilde{I}_{J}\) replaced by \(\tilde{C}_{J}\). Substituting \(\tilde{M}(u)=1\) in (81) gives the simplified form
Let us now discretize (83) in time using (28) and (29):
where \(0\le \theta \le 1\) is an implicitness parameter (0 for fully explicit, 1 for fully implicit and 0.5 for Crank–Nicolson method).
For the Prony series model, we may further simplify (86) with \(I_{J}^{n+1}\) given by (40) and \(K_{J}^{n+1}\) given by (34). Rearranging, we finally obtain the following system of linear equations:
with
Note that for the ADE (87) remains unaltered, but (88)–(90) are degenerated to
The source term is presently not implemented.
Appendix 2
A typical memory function (for the first validation case) is presented in Fig. 6 in both the Laplace (Fig. 6a) and time (Fig. 6b, c) domains. At \(t\rightarrow 0\), \(\tilde{M}\left( u \right) \rightarrow N-1\) and at \(t\rightarrow \infty \), \(\tilde{M}\left( u \right) \rightarrow N\), where N is the normalization constant defined in (11), which is very close to the TPL parameter \(\beta \) defined in (10). Because the sum-of-exponentials approaches zero at infinity, it is used to approximate \(\tilde{M}( u )-N\), so that in the time domain, the \(a_{0} \delta ( t )\) term is added [see (30)], with \(a_{0} =N\) . As \(t\rightarrow \infty \), M(t) approaches 0, while it becomes a nearly constant negative value at \(t\rightarrow 0^{+}\) . At \(t=0\), there is a small increase in M(t) due to the \(a_{0} \delta ( t )\) term, but it is still negative. Both \(\tilde{M}( u )\) and M(t) are smooth, except the M(t) singularity at \(t=0\).
There are two ways of computing the convolution (82).
The first method evaluates M(t) via some representation and then computes the convolution (28), \(I_{J}^{n} =\int \limits _{0}^{{t}^{n}} {M(t^{n}-\tau )} C_{J} (\tau )d\tau \), directly by splitting the integration domain into n subintervals and approximating the integral on each subinterval via the trapezoidal rule. This direct method requires storage of \(C_{J} ( 0 )\), \(C_{J} (\varDelta t)\), ..., \(C_{J} (n\,\varDelta t)\) and O(n) work at the nth step. Thus, the overall computational cost is \(O( N_{t}^{2} )\) and the storage requirement is \(O( N_{t} )\), where \(N_{t}\) is the total number of time steps.
The second method to evaluate the convolution (82) first seeks an efficient sum-of-exponential approximation for M(t), i.e., \(M(t)\cong a_{0} \delta (t)+\sum \limits _{p=1}^{P} a_{p} e^{{-b}_{p} t}\) [see (30)]. Here, P is the number of terms needed in the sum-of-exponential approximation. We then observe that the convolution with an exponential function can be calculated efficiently via a simple recurrence relation. The computational cost of this scheme is \(O(P\cdot N_{t} )\) to evaluate \(I_{J}^{1} ,I_{J}^{2} ,\ldots ,I_{J}^{{N}_{t} }\); and the storage requirement is only O(P), i.e., one only needs to store the history part for each exponential mode. In practice, as in the present study, P is very often a small number and is independent of \(N_{t}\) for our particular problem. Hence, both the computational cost and the storage requirement of the second scheme are optimal.
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Ben-Zvi, R., Scher, H., Jiang, S. et al. One-Dimensional Finite Element Method Solution of a Class of Integro-Differential Equations: Application to Non-Fickian Transport in Disordered Media. Transp Porous Med 115, 239–263 (2016). https://doi.org/10.1007/s11242-016-0712-0
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DOI: https://doi.org/10.1007/s11242-016-0712-0