Is Pressure Constant Along the Horizontal Axis in a Fluid Element?

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Discussion Overview

The discussion revolves around the behavior of pressure in a fluid element, particularly whether pressure remains constant along the horizontal axis while varying with height in the vertical direction. The conversation touches on concepts from fluid mechanics, including incompressibility and density fields.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant asserts that pressure is constant along the horizontal x-axis but varies with height along the z-axis, proposing a relationship involving density and gravitational acceleration.
  • Another participant challenges the initial claim, stating that the assertion holds only if density is constant and advises caution regarding the signs in the equations presented.
  • A subsequent contribution assumes the fluid is a liquid and therefore practically incompressible, suggesting implications for density behavior.
  • Another participant counters that incompressibility does not necessarily imply a constant density field, introducing the concept of density fields in continuum mechanics.
  • One participant explains that while incompressibility means the density of a fluid particle remains constant, the density field can vary over time due to different fluid particles occupying the same spatial point.
  • A clarification is made that if all fluid particles have the same density, then the density field will remain constant.

Areas of Agreement / Disagreement

Participants express differing views on the implications of incompressibility and the nature of density fields, indicating that multiple competing perspectives remain without consensus on the relationship between pressure and density in this context.

Contextual Notes

The discussion highlights potential limitations in assumptions regarding density constancy and the definitions of incompressibility, as well as the implications of using density fields versus particle densities.

Fritz
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If you have a rectangular fluid element vertical z-axis and a horizontal x-axis, pressure is constant along the x=axis, but p=f(z) along the z-axis.

If [tex]\frac{dP}{dz}=-density.g[/tex]

is the integral of [tex]\frac{dP}{dz}[/tex] equal to p(z) - p(z1) = -density.g(z-z1)?
 
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That would be true only if the denisity is constant. Also, be careful with your signs on the right hand side.
 
Assume it is a liquid and therefore (practically) incompressible.
 
Incompressibility does not entail that the density field is constant.
 
Density = mass / volume.

If the volume doesn't change (incompressible), surely the mass wouldn't change?
 
In continuum mechanics, we use density FIELDS, rather than particle densities.

Incompressibility entails that the density of A FLUID PARTICLE remains constant; but because the individual fluid particles may jump around in space, it does not follow that the density FIELD is constant.

The field measures the density of whatever particle happens to be AT A FIXED POINT IN SPACE; since it may be different fluid particles which occupy that point at DIFFERENT TIMES, the density field, evaluated at that point may change in time.

If you suppose in addition that all particles has the SAME density, then the density field will be constant.
Get it?
 

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