The continuity equation and the divergence

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Discussion Overview

The discussion revolves around the continuity equation in fluid dynamics, particularly in the context of a scenario where water spreads out from the center of a sphere with constant density. Participants explore the implications of the equation and the behavior of divergence in relation to the density at the center and elsewhere.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant states the continuity equation as (∂ρ/∂t) + divergence J = 0 and describes a scenario where water spreads from a center with constant density ρ, questioning the implications at the center.
  • Another participant requests clarification due to the complexity of the initial post, indicating a need for clearer expression of the problem.
  • A participant reiterates the continuity equation and suggests that if density is constant, the divergence should equal a constant C at the center and zero elsewhere, seeking confirmation on this interpretation.
  • Another participant agrees with the previous statement but challenges the idea that divergence can be zero at the center unless a point source is present, suggesting that it would instead be represented by a spherical delta function.

Areas of Agreement / Disagreement

Participants express differing views on the behavior of divergence at the center of the sphere, with some asserting it should be zero and others proposing it may not be, depending on the presence of a point source. The discussion remains unresolved.

Contextual Notes

There are assumptions regarding the nature of the fluid flow and the definition of divergence that are not fully explored, particularly concerning the treatment of the center point and the implications of constant density.

wuwei
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according to continuity equation (partial ρ)/(partial t) +divergence J = 0 . there is such a situation that there is continuous water spreads out from the center of a sphere with unchanged density ρ, and at the center dm/dt = C(a constant), divergence of J = ρv should be 0 anywhere except the center, but if I think that at the origin the density of water is unchanged and so the first term of continuity equation is 0 so divergence J is 0, too. but apparently it's wrong. what's the problem?
 
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It's really tough to say, exactly, because most of your post was an indecipherable run-on sentence. Would you mind starting over and restating the problem a bit more clearly?
 
Your case says
\frac{\partial \rho}{\partial t}+\nabla\cdot\mathbf{j}=C at the center, otherwise
\frac{\partial \rho}{\partial t}+\nabla\cdot\mathbf{j}=0

If density is constant
\nabla\cdot\mathbf{j}=C at the center, otherwise
\nabla\cdot\mathbf{j}=0

Anything wrong with it ?
 
sweet springs said:
Your case says
\frac{\partial \rho}{\partial t}+\nabla\cdot\mathbf{j}=C at the center, otherwise
\frac{\partial \rho}{\partial t}+\nabla\cdot\mathbf{j}=0

If density is constant
\nabla\cdot\mathbf{j}=C at the center, otherwise
\nabla\cdot\mathbf{j}=0

Anything wrong with it ?
Yes. The divergence is zero at the center also, unless you have a point source at the center, in which case, the divergence is a spherical delta function.
 

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