Momentum transport in gases in 2d

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Discussion Overview

The discussion centers on the concept of momentum transport in gases in a two-dimensional context. Participants explore the assumptions made about the motion of gas molecules and seek clarification on the implications of these assumptions for momentum transport analysis.

Discussion Character

  • Exploratory, Conceptual clarification, Debate/contested

Main Points Raised

  • One participant questions the assumption that half of the gas molecules move in the positive y direction while the other half moves in the negative direction, seeking an explanation for this choice.
  • Another participant suggests that the assumption of a static gas mass implies that the average speed of all molecules is zero, but acknowledges that individual particles still have positive and negative velocities.
  • A participant expresses a desire for resources that explain momentum transport in two dimensions without relying on tensor mathematics, indicating a search for alternative explanations.

Areas of Agreement / Disagreement

Participants express differing views on the assumptions regarding the motion of gas molecules, particularly concerning the distribution of velocities and the implications of a static gas mass. The discussion remains unresolved regarding the justification for the assumption of half positive and half negative velocities.

Contextual Notes

Participants note the dependence on the assumption of a static gas mass and the implications this has for momentum transport analysis. There is also mention of the complexity introduced by different scenarios, such as an accelerating gas mass.

Mohankpvk
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I was trying to understand the momentum transport between gas molecules in 2d.In the image below, it is stated that half of the molecules move up(positive velocity in y direction) and half negative.But the author didnt explain why he assumed it.
IMG_20180914_172950.jpeg
 

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Because the whole gas mass is assumed to be static, not moving up, down, left, right.

You could do a different analysis for an upward accelerating gas mass, such as on a rocket, but the answers would change.
 
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anorlunda said:
Because the whole gas mass is assumed to be static, not moving up, down, left, right.

You could do a different analysis for an upward accelerating gas mass, such as on a rocket, but the answers would change.
But why half if the gas is static?Please explain.I referred a few books for momentum transport but all of them used tensors.Is there any book(or other source) in which transport is explained only in 2d?
 
Mohankpvk said:
But why half if the gas is static?

No, all the particles are moving, some + some -, the average speed of all is zero.
 
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