8.7: The Equations of Hydrodynamics
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1$#1_$
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We now derive the equations governing fluid flow. The equations of mass and momentum balance are ∂ρ∂t+∇⋅(ρV)=0∂(ρV∗α)∂t+∂Π∗αβ∂xβ=0 , where \[\RPi\ns_{\alpha\beta}=\rho\, V\ns_\alpha V\ns_\beta + p\,\delta\ns_{\alpha\beta} \, - \, \stackrel
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{\overbrace{\left\{\eta\Bigg( {\pz V\ns_\alpha\over\pz x\ns_\beta} + {\pz V\ns_\beta\over\pz x\ns_\alpha}-\frac{2}{3}\,\bnabla\!\cdot\!\BV\,\delta\ns_{\alpha\beta}\bigg) +\zeta\,\bnabla\!\cdot\!\BV\,\delta\ns_{\alpha\beta}\right\}}}\ .\] Substituting the continuity equation into the momentum balance equation, one arrives at ρ∂V∂t+ρ(V⋅∇)V=−∇p+η∇2V+(ζ+13η)∇(∇⋅V) , which, together with continuity, are known as the Navier-Stokes equations. These equations are supplemented by an equation describing the conservation of energy, T∂s∂T+T∇⋅(sV)=˜σ∗αβ∂V∗α∂xβ+∇⋅(κ∇T) . Note that the LHS of Equation [NSB] is ρDV/Dt, where D/Dt is the convective derivative. Multiplying by a differential volume, this gives the mass times the acceleration of a differential local fluid element. The RHS, multiplied by the same differential volume, gives the differential force on this fluid element in a frame instantaneously moving with constant velocity V. Thus, this is Newton’s Second Law for the fluid.
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