Quelltext der Seite Benutzer:Larsks13/Asymptotische Homogenisierung

{{Importartikel}}
{{distinguish|Homogeneous polynomial#Homogenization{{!}}Homogenization of a polynomial}}

In [[mathematics]] and [[physics]], '''homogenization''' is a method of studying [[partial differential equations]] with rapidly oscillating coefficients,<ref name="S-P">{{cite book | last=Sanchez-Palencia | first=E. | title=Non-homogeneous media and vibration theory | volume=127 | publisher=Springer Verlag | date=1980 | series=Lecture Notes in Physics | isbn=978-3-540-10000-3 | doi=10.1007/3-540-10000-8}}</ref><ref name="B-P">{{cite book | author-link1=Nikolai Sergeevich Bakhvalov | last1=Bakhvalov | first1=N. | last2=Panasenko | first2=G. | title=Homogenisation: Averaging Processes in Periodic Media | publisher=Kluwer | location=Dordrecht | date=1989 | series=Mathematics and its Applications | doi=10.1007/978-94-009-2247-1 | isbn=978-94-010-7506-0}}</ref><ref name="BLP">{{cite book | last1=Bensoussan | first1=A. | author-link2=Jacques-Louis Lions | last2=Lions | first2=J.L. | last3=Papanicolaou | first3=G. | title=Asymptotic Analysis for Periodic Structures | publisher=North-Holland | location=Amsterdam | date=1978 | series=Studies in Mathematics and its Applications | isbn=0-444-85172-0}}</ref> such as

:<math>
\nabla\cdot\left(A\left(\frac{\vec x}{\epsilon}\right)\nabla u_{\epsilon}\right) = f
</math>

where <math>\epsilon</math> is a very small parameter and  
<math>A\left(\vec y\right)</math> is a 1-periodic coefficient:
<math>A\left(\vec y+\vec e_i\right)=A\left(\vec y\right)</math>, 
<math>i=1,\dots, n</math>.

It turns out that the study of these equations is also of great importance in physics and engineering, since equations of this type govern the physics of inhomogeneous or heterogeneous materials. Of course, all matter is inhomogeneous at some scale, but frequently it is convenient to treat it as homogeneous. A good example is the continuum concept which is used in [[continuum mechanics]]. Under this assumption, materials such as [[fluids]], [[solids]], etc. can be treated as homogeneous materials and associated with these materials are material properties such as [[shear modulus]], [[elastic moduli]], etc.

Frequently, inhomogeneous materials (such as [[composite materials]]) possess [[microstructure]] and therefore they are subjected to loads or forcings which vary on a length scale which is far bigger than the characteristic length scale of the microstructure. In this situation, one can often replace the equation above with an equation of the form

:<math>\nabla\cdot\left(A^*\nabla u\right) = f</math>

where <math>A^*</math> is a constant tensor coefficient and is known as the effective property associated with the material in question. It can be explicitly computed as
:<math> A^*_{ij}=\int_{(0,1)^n} A(\vec y)\left(
\nabla w_j(\vec y)+\vec e_j\right)
\cdot\vec e_i\, dy_1\dots dy_n , \qquad i,j=1,\dots,n
</math>
from 1-periodic functions <math>w_j</math>  satisfying:
:<math>
\nabla_y\cdot\left(A(\vec y)\nabla w_j\right)=
-\nabla_y\cdot\left(A(\vec y)\vec e_j\right). 
</math>

This process of replacing an equation with a highly oscillatory coefficient with one with a homogeneous (uniform) coefficient is known as ''homogenization''. This subject is inextricably linked with the subject of [[micromechanics]] for this very reason.

In homogenization one equation is replaced by another if <math>u_\epsilon\approx u</math> 
for small enough <math>\epsilon</math>, provided
<math>u_\epsilon\to u</math> in some appropriate norm as <math>\epsilon\to 0</math>.

As a result of the above, homogenization can therefore be viewed as an extension of the continuum concept to materials which possess microstructure. The analogue of the differential element in the continuum concept (which contains enough atom, or molecular structure to be representative of that material), is known as the "[[representative volume element|Representative Volume Element]]"<ref>{{cite book | last=Ostoja-Starzewski | first=M. | title=Microstructural randomness and scaling in materials | publisher=Chapman and Hall/CRC Press | date=2007 | isbn=9781584884170 | series=Modern Mechanics and Mathematics}}</ref> in homogenization and micromechanics. This element contains enough statistical information about the inhomogeneous medium in order to be representative of the material. Therefore averaging over this element gives an effective property such as <math>A^*</math> above.

Classical results of homogenization theory<ref name="S-P"/><ref name="B-P"/><ref name="BLP"/> were obtained for media with periodic microstructure modeled by partial differential equations with periodic coefficients. These results were later generalized to spatially homogeneous random media modeled by differential equations with random coefficients which statistical properties are the same at every point in space.<ref>{{cite journal | first1=S.M. | last1=Kozlov | title=Homogenization of Random Operators. | journal=Mat. Sbornik | date=1979 | volume=109 | issue=151 | pages=188–202}} (English transl.: Math. USSR, Sb. 37:2, 1980, pp. 167-180)</ref><ref>{{cite journal | first1=G. C. | last1=Papanicolaou | first2=S.R. | last2=Varadhan | title=Boundary Value Problems with Rapidly Oscillating Coefficients | journal=Seria Colloq. Math. Society Janos Bolyai | volume=27 | pages=835–873 | location=Amsterdam | date=1981 | url=http://math.stanford.edu/~papanico/pubftp/pubs_old/pap_vara_79.pdf}}</ref> In practice, many applications require a more general way of modeling that is neither periodic nor statistically homogeneous. For this end the methods of the homogenization theory have been extended to partial differential equations, which coefficients are neither periodic nor statistically homogeneous (so-called arbitrarily rough coefficients).<ref>{{cite journal | author-link1=Leonid Berlyand | first1=L. | last1=Berlyand | first2=H. | last2=Owhadi | title=Flux Norm Approach to Finite Dimensional Homogenization Approximations with Non-Separated Scales and High Contrast | journal=Archive for Rational Mechanics and Analysis | date=November 2010 | volume=198 | issue=2 | pages=677–721| doi=10.1007/s00205-010-0302-1 | bibcode=2010ArRMA.198..677B | arxiv=0901.1463 }}</ref><ref>{{cite journal | first1=A. | last1=Målqvist | first2=D. | last2=Peterseim | title=Localization of elliptic multiscale problems | journal=Mathematics of Computation | date=2014 | volume=83 | issue=290 | pages=2583–2603| doi=10.1090/S0025-5718-2014-02868-8 | doi-access=free }}</ref>

== The method of asymptotic homogenization ==
Mathematical homogenization theory dates back to the French, Russian and Italian schools.<ref name="S-P"/><ref name="B-P"/><ref name="BLP"/><ref>{{cite book | last=Dal Maso | first=G. | title=An Introduction to Γ-Convergence | publisher=Birkhauser | date=1993 | isbn=9780817636791 | doi=10.1007/978-1-4612-0327-8 | series=Progress in Nonlinear Differential Equations and Their Applications}}</ref> The method of asymptotic homogenization proceeds by introducing the fast variable <math>\vec y=\vec x/\epsilon</math> and posing a formal expansion in <math>\epsilon</math>:

:<math>
u_\epsilon(\vec x) = u(\vec x,\vec y) = u_0(\vec x,\vec y)+
\epsilon u_1(\vec x,\vec y)+\epsilon^2 u_2(\vec x,\vec y)+O(\epsilon^3)\,
</math>

which generates a hierarchy of problems. The homogenized equation is obtained and the effective coefficients are determined by solving the so-called "cell problems" for the function <math>u_1(\vec x,\vec x/\epsilon)</math>.

==See also==
*[[Asymptotic analysis]]
*[[Γ-convergence]]
*[[Mosco convergence]]
*[[Effective medium approximations]]

==Notes==
{{Reflist}}

==References==
*{{Citation
| last = Kozlov
| first = S.M.
| last2 = Oleinik
| first2 = O.A.
| author2-link = Olga Oleinik
| last3 = Zhikov
| first3 = V.V.
| title = Homogenization of differential operators and integral functionals
| place = [[Berlin]]-[[Heidelberg]]-[[New York City]]
| publisher = [[Springer-Verlag]]
| year = 1994
| zbl = 0838.35001
| isbn = 3-540-54809-2}}
*{{Citation
| last = Oleinik
| first = O.A.
| author-link = Olga Oleinik
| last2 = Shamaev
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| last3 = Yosifian
| first3 = G.A.
| title = Mathematical problems in elasticity and homogenization
| place = [[Amsterdam]] - [[London]] - [[New York City]] - [[Tokyo]]
| publisher = [[Elsevier#Imprints|North-Holland]]
| year = 1991
| series = Studies in Mathematics and its Applications 
| volume = 26
| zbl = 0768.73003
| isbn = 0-444-88441-6}}
*{{Citation
| last = Hornung, Ulrich (Ed.).
| title = Homogenization and Porous Media
| year = 1997
| series = Interdisciplinary Applied Mathematics
| volume = 6
| publisher = [[Springer-Verlag]]
| isbn = 978-1-4612-7339-4
| doi = 10.1007/978-1-4612-1920-0
}}
*{{Citation
| last = Bakhvalov 
| first = N. S.
| last2 = Panasenko
| first2 = G. P.
| author-link = Nikolai Sergeevich Bakhvalov
| title = Averaging of Processes in Periodic Media (English translation: Kluwer,1989)
| place = [[Moscow]]
| publisher = [[Nauka (publisher)|Nauka]]
| year = 1984
| zbl =  0607.73009
}}
*{{Citation
| last = Braides 
| first = A.
| last2 = Defranceschi
| first2 = A.
| title = Homogenization of Multiple Integrals
| series = Oxford Lecture Series in Mathematics and Its Applications
| place = [[Oxford]]
| publisher = [[Clarendon Press]]
| year = 1998
| isbn = 978-0-198-50246-3}}

<nowiki>[[Category:Asymptotic analysis]]
[[Category:Partial differential equations]]</nowiki>