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About Compressive Forces on Control Volumes

  1. Jan 12, 2014 #1
    Why is it that forces (for changes in pressure across a fluid) on the control volume are generally considered compressive? Even in the derivation of the Navier-Stokes equation, it is assumed that forces from fluid pressure will be compressive? Why?
  2. jcsd
  3. Jan 13, 2014 #2
    Hi Masshakar. Welcome to Physics Forums!!!

    Pressure represents the isotropic part of the overall stress tensor. In the overall stress tensor [itex]\vec{σ}[/itex] for an incompressible fluid, the stress tensor is represented by:
    where [itex]\vec{σ_v}[/itex] represents the viscous part of the stress tensor and [itex]\vec{I}[/itex] is the identity tensor. If we use the Cauchy stress relationship to determine the stress on a surface of arbitrary orientation (by dotting the stress tensor with a unit normal), the contribution of the pressure term to the overall traction vector is compressive.

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