Picture water flowing in a duct. In these kinds of scenarios, the loading exerted by the moving fluid to the solid in the perpendicular direction is often taken as being equal to the pressure of the fluid at the wall. I agree that the above assumption is true for a stationary fluid. For a moving fluid, however, should not the perpendicular loading be equal to the corresponding normal stress component of the fluid at the wall? Since pressure is defined as the negative average of the normal stresses, surely the pressure and the aforementioned normal stress component are most often not equal for a moving fluid? Or is the difference just very small so that the assumption is justified?