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From the following site: Ref:https://www.me.psu.edu/cimbala/learning/fluid/CV_Energy/home.htm

I read that the the net work done by viscous force at a stationary wall is zero. enter image description here

I understand that the wall does not move due to the viscous force acting on it. But the wall exerts a force on the fluid too which causes it to slow down at the boundary and develops into a boundary layer. Doesn't this mean that the kinetic energy of the fluid in the boundary layer is changing, so the wall is doing work on the fluid due to viscosity?

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    $\begingroup$ In a fully developed boundary layer nothing is slowing down. Think pipe flow - mass flow is conserved. There is a pressure drop along the pipe that offsets the shear force. That allows a no-work approach to the problem (for incompressible fluids). This is choice you get to make in terms of how you set the problem up. You don't have to do it this way, but it reduces the problem to a linear system. Systems with work tend to not be linear. $\endgroup$
    – Phil Sweet
    Commented Feb 26, 2023 at 13:48
  • $\begingroup$ No slip condition means the fluid at the wall has zero velocity. This should be ok in laminar flow but it will no longer work in turbulent flow. $\endgroup$
    – Tomáš Létal
    Commented Mar 2, 2023 at 21:10

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