1. In general, the rate of change of the deformation over time ( viscous stress ). 2. This tensor may be expressed as the sum of the viscous stress tensor minus the hydrostatic pressure. 3. In very general terms, the fluid's viscosity is the relation between the strain rate and the viscous stress . 4. In the absence of such a coupling, the viscous stress tensor will have only two independent parameters and will be symmetric. 5. The calculated friction drag D _ f utilizes the x-projection of the viscous stress tensor evaluated on each discretized body surface. 6. However, if the deformation is changing with time, even in fluids there will usually be some viscous stress , opposing that change. 7. In these regions the inertia force becomes less important and the flow is determined by the balance of viscous stresses and the pressure gradient. 8. When the viscosity is negleted, the term containing the viscous stress tensor \ mathbf { \ tau } in the Navier Stokes equation vanishes. 9. In particular, the local strain rate is the only property of the velocity flow that directly affects the viscous stress at a given point. 10. In most fluids the viscous stress tensor too is symmetric, which further reduces the number of viscosity parameters to 6 ?6 = 36.
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