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Finite volume method for unsteady flow : ウィキペディア英語版
Finite volume method for unsteady flow
Unsteady flows are characterized as flows in which the properties of the fluid are time dependent. It gets reflected in the governing equations as the time derivative of the properties are absent.
For Studying Finite-volume method for unsteady flow there is some governing equations
〔http://books.google.co.in/books+finite+volume+method+for+unsteady+flows〕>
==Governing Equation==
The conservation equation for the transport of a scalar in unsteady flow has the general form as 〔An Introduction to Computational Fluid Dynamics H. K. Versteeg and W Malalasekra Chapter 8 page 168〕
\frac + \operatorname(\rho \phi \upsilon) = \operatorname(\Gamma \operatorname \phi) + S_\phi
\rho is density and \phi is conservative form of all fluid flow,

\Gamma is the Diffusion coefficient and S is the Source term.
\operatorname(\rho \phi \upsilon) is Net rate of flow of \phi out of fluid element(convection),

\operatorname(\Gamma \operatorname \phi) is Rate of increase of \phi due to diffusion,

S_\phi is Rate of increase of \phi due to sources.
\frac is Rate of increase of \phi of fluid element(transient),
The first term of the equation reflects the unsteadiness of the flow and is absent in case of steady flows. The finite volume integration of the governing equation is carried out over a control volume and also over a finite time step ∆t.
\int\limits_ \!\!\!\int_t^ (\frac \,dt)\,dV + \int_t^ \!\!\!\int\limits_A (n. \,dA)\,dt = \int_t^ \!\!\!\int\limits_A (n.(\Gamma \operatorname \phi)\,dA)\,dt +\int_t^ \!\!\!\int\limits_ S_\phi\,dV\,dt
The control volume integration of the steady part of the equation is similar to the steady state governing equation’s integration. We need to focus on the integration of the unsteady component of the equation. To get a feel of the integration technique, we refer to the one-dimensional unsteady heat conduction equation.〔An Introduction to COmputational Fluid Dynamics H. K. Versteeg and W Malalasekera Chapter 8 page 169〕
\rho c \frac = \frac} + S
\int_t^ \!\!\!\int\limits_ \rho c \frac \,dV\,dt = \int_t^ \!\!\!\int\limits_ \frac} \,dV\,dt + \int_t^ \!\!\!\int\limits_ S\,dV\,dt
\int_e^w \!\!\!\int_t^ (\rho c \frac \,dt)\,dV = \int_t^ ((k A \frac )_e - (k A \frac )_w )\,dt + \int_t^ \bar S\Delta V \,dt
Now, holding the assumption of the temperature at the node being prevalent in the entire control volume, the left side of the equation can be written as 〔(【引用サイトリンク】title=A Second-Order Time-Accurate Finite Volume Method for Unsteady Incompressible Flow on Hybrid Unstructured Grids )
\int\limits_ \!\!\!\int_t^ (\rho c \frac \,dt)\,dV = \rho c(T_P - ^O) \Delta V
By using a first order backward differencing scheme, we can write the right hand side of the equation as
\rho c (T_P - ^0) \Delta V = \int_t^ (K_e A \frac \bar S\Delta V \,dt
Now to evaluate the right hand side of the equation we use a weighting parameter \theta between 0 and 1, and we write the integration of T_P
I_T = \int_t^ T_P \,dt = (\theta T_P - (1 - \theta ) ^0 ) \Delta t
Now, the exact form of the final discretised equation depends on the value of \Theta . As the variance of \Theta is 0< \Theta <1, the scheme to be used to calculate T_P depends on the value of the \Theta

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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