连续性方程 0p+Y·p=0 at y·n P(x,y,z, tdx dy d-z= mass inthe volumed dM dm=-pv, dt ds=-py n ds dt v·ndS p下nS But it is also true that: M=Lpd dt jody pv·ndS pd=-V·pvd Fn△S=VF∥→「 p·ndS V·p 高斯散度定理 +V·pν=0 t ( x y z t dx dy dz massinthevolume dV , , , )  dm v dt dS v n dS dt n = − = −  v n dS dt dm = −   = −  S v n dS dt dM   = V M  dV   = −  V S dV v n dS dt d      =  S V F n dS F dV    =  S V  v n dS  v dV   = −  V V dV v dV dt d   +  = 0   v t   n v vn = v n dS But it is also true that: 连续性方程 +  = 0   v t   高斯散度定理