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In general relativity, a fluid solution is an exact solution of the Einstein field equation in which the gravitational field is produced entirely by the mass, momentum, and stress density of a fluid. In astrophysics, fluid solutions are often employed as stellar models. (It might help to think of a perfect gas as a special case of a perfect fluid.) In cosmology, fluid solutions are often used as cosmological models. ==Mathematical definition== The stress–energy tensor of a relativistic fluid can be written in the form : Here * the world lines of the fluid elements are the integral curves of the velocity vector , * the projection tensor projects other tensors onto hyperplane elements orthogonal to , * the matter density is given by the scalar function , * the pressure is given by the scalar function , * the heat flux vector is given by , * the viscous shear tensor is given by . The heat flux vector and viscous shear tensor are ''transverse'' to the world lines, in the sense that : This means that they are effectively three-dimensional quantities, and since the viscous stress tensor is symmetric and traceless, they have respectively 3 and 5 linearly independent components. Together with the density and pressure, this makes a total of 10 linearly independent components, which is the number of linearly independent components in a four-dimensional symmetric rank two tensor. 抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)』 ■ウィキペディアで「fluid solution」の詳細全文を読む スポンサード リンク
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