Analytic element method
The analytic element method (AEM) is a numerical method used for the solution of partial differential equations.[1][2][3] It was initially developed by O.D.L. Strack at the University of Minnesota. It is similar in nature to the boundary element method (BEM), as it does not rely upon discretization of volumes or areas in the modeled system; only internal and external boundaries are discretized. One of the primary distinctions between AEM and BEMs is that the boundary integrals are calculated analytically.
Mathematical basis
The basic premise of the analytic element method is that, for linear differential equations, elementary solutions may be superimposed to obtain more complex solutions. A suite of 2D and 3D analytic solutions ("elements") are available for different governing equations. These elements typically correspond to a discontinuity in the dependent variable or its gradient along a geometric boundary (e.g., point, line, ellipse, circle, sphere, etc.). This discontinuity has a specific functional form (usually a polynomial in 2D) and may be manipulated to satisfy Dirichlet, Neumann, or Robin (mixed) boundary conditions. Each analytic solution is infinite in space and/or time.
Commonly each analytic solution contains degrees of freedom (coefficients) that may be calculated to meet prescribed boundary conditions along the element's border. To obtain a global solution (i.e., the correct element coefficients), a system of equations is solved such that the boundary conditions are satisfied along all of the elements (using collocation, least-squares minimization, or a similar approach). Notably, the global solution provides a spatially continuous description of the dependent variable everywhere in the infinite domain, and the governing equation is satisfied everywhere exactly except along the border of the element, where the governing equation is not strictly applicable due to the discontinuity.
The ability to superpose numerous elements in a single solution means that analytical solutions can be realized for arbitrarily complex boundary conditions. That is, models that have complex geometries, straight or curved boundaries, multiple boundaries, transient boundary conditions, multiple aquifer layers, piecewise varying properties and continuously varying properties can be solved. Elements can be implemented using far-field expansions such that model containing many thousands of elements can be solved efficiently to high precision.
The analytic element method has been applied to problems of groundwater flow governed by a variety of linear partial differential equations including the Laplace, the Poisson equation, the modified Helmholtz equation, the heat equation, and the biharmonic equations. Often theses equations are solved using complex variables which enables using mathematical techniques available in complex variable theory. A useful technique to solve complex problems is using conformal mapping which maps the boundary of a geometry, e.g. an ellipse, onto the boundary of the unit circle where the solution is known.
A contemporary student of Strack's who is a proponent of the Analytic Element Method (AEM) in groundwater modeling applications is Dr. David Steward of Kansas State University.
Comparison to other methods
As mentioned the analytical element method thus not rely on discretization of volume or area in the model, as in the finite elements or finite different methods. Thus, it can model complex problem with an error on the order of machine precision. This is illustrated in a study which modeled a highly heterogeneous, isotropic aquifer by including 100,000 spherical heterogeneity with a random conductivity and tracing 40,000 particles.[4] The analytical element method can efficiently be used as verification or as a screening tool in larger projects as it may fast and accurately calculate the groundwater flow for many complex problems.[5][6]
See also
References
- Haitjema, H. M. (1995). Analytic element modeling of groundwater flow. San Diego, CA: Academic Press. ISBN 978-0-12-316550-3.
- Strack, O. D. L. (1989). Groundwater Mechanics. Englewood Cliffs, NJ: Prentice Hall.
- Fitts, C. R. (2012). Groundwater Science (2nd ed.). San Diego, CA: Elsevier/Academic Press. ISBN 9780123847058.
External links
- Analytic elements community wiki
- Fitts Geolsolutions, AnAqSim (analytic aquifer simulator) and AnAqSimEDU (free) web site
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- ^ Strack, Otto D. L., 1943- (1989). Groundwater mechanics. Englewood Cliffs, N.J.: Prentice Hall. ISBN 0-13-365412-5. OCLC 16276592.
{{cite book}}: CS1 maint: multiple names: authors list (link) CS1 maint: numeric names: authors list (link) - ^ Strack, Otto D. L. (2017/08). "Analytical Groundwater Mechanics". Cambridge Core. doi:10.1017/9781316563144. Retrieved 2020-04-20.
{{cite web}}: Check date values in:|date=(help) - ^ Haitjema, H. M. (Henk M.) (1995). Analytic element modeling of groundwater flow. San Diego: Academic Press. ISBN 978-0-08-049910-9. OCLC 162129095.
- ^ Janković, I.; Fiori, A.; Dagan, G. (2006). "Modeling flow and transport in highly heterogeneous three-dimensional aquifers: Ergodicity, Gaussianity, and anomalous behavior—1. Conceptual issues and numerical simulations". Water Resources Research. 42 (6). doi:10.1029/2005WR004734. ISSN 1944-7973.
- ^ Hunt, Randall J. (2006). "Ground Water Modeling Applications Using the Analytic Element Method". Groundwater. 44 (1): 5–15. doi:10.1111/j.1745-6584.2005.00143.x. ISSN 1745-6584.
- ^ Kraemer, Stephen R. (2007). "Analytic Element Ground Water Modeling as a Research Program (1980 to 2006)". Groundwater. 45 (4): 402–408. doi:10.1111/j.1745-6584.2007.00314.x. ISSN 1745-6584.