Fluids- hydrodynamic interactions

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Discussion Overview

The discussion revolves around hydrodynamic interactions in a system of suspended particles diffusing through a narrow channel. Participants explore the implications of neglecting these interactions and question how particles can diffuse without the influence of flow.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions the behavior of suspended particles when hydrodynamic interactions are neglected, asking if diffusion can occur solely due to flow.
  • Another participant clarifies that neglecting hydrodynamic interactions typically refers to a dilute limit where the motion of one particle does not influence others.
  • A third participant explains that in a suspension, the motion of one particle creates a flow field affecting other particles, suggesting that without these interactions, particles may not move or only move due to external flow or other effects.
  • A later reply interprets the absence of hydrodynamic interactions as each particle interacting with a flow field that remains unchanged by other particles, referencing literature on coupling effects in fluid dynamics.
  • One participant introduces the concept of settling as a potential example of particle movement in the absence of hydrodynamic interactions.

Areas of Agreement / Disagreement

Participants express differing views on the implications of neglecting hydrodynamic interactions, with no consensus reached on how particles would behave in such a scenario.

Contextual Notes

The discussion highlights assumptions regarding the definitions of hydrodynamic interactions and the conditions under which particles are considered to be in a dilute suspension. The implications of flow fields and particle interactions remain unresolved.

riverboat
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If we consider a system of N suspended particles diffusing through a narrow channel. What happened if the hydrodynamic interactions are neglected? Is there only flow? How can the particles diffusse without flow?
 
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Are you asking what happens in the limit of dilute particles- in other words, that the motion of one particle does not affect the motion of neighboring particles? That's usually what is meant by 'hydrodynamic interaction'.
 
It is know, that when one consider a suspension of particles. The motion of a given particle induces a flow field in the solvent, which will be felt by all the other particles. As a result these particles experience a force which is said to result from hydrodynamic interaction with the original particle.

Then, in the case that there is not hydrodynamic interactions, then the particles cannot move, or they move due to the flow or due to other effects?
or maybe in other words, if there is not flow field, then there is no hydrodynamic interactions, and then why the particles move?
 
If I understand you, if there is no hydrodynamic interaction, then each particle interacts with a flow field that is unaltered by the addition of other particles. Deiter Langbein has a couple very nice articles showing coupling effects for two spheres moving in a fluid, for various configurations of the spheres (parallel, series, etc).

As for your last sentence, think of settling.
 

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