Asymptotic homogenization

In mathematics and physics, homogenization is a method of studying partial differential equations with rapidly oscillating coefficients, such as In mathematics and physics, homogenization is a method of studying partial differential equations with rapidly oscillating coefficients, such as where ϵ {displaystyle epsilon } is a very small parameter and A ( y → ) {displaystyle Aleft({vec {y}} ight)} is a 1-periodic coefficient: A ( y → + e → i ) = A ( y → ) {displaystyle Aleft({vec {y}}+{vec {e}}_{i} ight)=Aleft({vec {y}} ight)} , i = 1 , … , n {displaystyle i=1,dots ,n} . 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 lengthscale which is far bigger than the characteristic lengthscale of the microstructure. In this situation, one can often replace the equation above with an equation of the form where A ∗ {displaystyle A^{*}} is a constant tensor coefficient and is known as the effective property associated with the material in question. It can be explicitly computed as from 1-periodic functions w j {displaystyle w_{j}} satisfying: 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 u ϵ ≈ u {displaystyle u_{epsilon }approx u} for small enough ϵ {displaystyle epsilon } , provided u ϵ → u {displaystyle u_{epsilon } o u} in some appropriate norm as ϵ → 0 {displaystyle epsilon o 0} . 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' 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 A ∗ {displaystyle A^{*}} above.