87-2 Dynamic differential equation of viscosity liquid -Navier-Stokes equation 一、 stress in viscosity fluid In balanced or dynamic ideal fluid, surface force act on fluid micro-group only is compressive stress(pressure)that normal to surface, and compressive stress takes on a little isotropy. But in dynamic viscosity fluid, because of influence of viscosity, surface force act on fluid micelle is not only compressive stress p but also shear stress T And compressive stress at one point does not take on isotropy any longer. Shown as figure (7-1), because surface force have three component in each infinitesimal,so one point in real fluid, such as stress of point A in figure can be expressed with stress matrix composed of nine elements 77 §7-2 Dynamic differential equation of viscosity liquid ——Navier—Stokes equation In balanced or dynamic ideal fluid ,surface force act on fluid micro-group only is compressive stress(pressure) that normal to surface,and compressive stress takes on a little isotropy.But in dynamic viscosity fluid , because of influence of viscosity,surface force act on fluid micelle is not only compressive stress but also shear stress . And compressive stress at one point does not take on isotropy any longer.Shown as figure（7—1）,because surface force have three component in each infinitesimal,so one point in real fluid , such as stress of point A in figure can be expressed with stress matrix composed of nine elements. Pn  一、stress in viscosity fluid