Derivation of the Euler equation for a streamline

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Discussion Overview

The discussion revolves around the derivation of the Euler equation for a streamline, specifically focusing on the application of Newton's second law to a gas flowing through a streamline. Participants explore the concept of pressure variation within a defined volume element and the implications of such variations in the context of fluid dynamics.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested
  • Homework-related

Main Points Raised

  • One participant questions the origin of the term '##+dp##' in the context of pressure variation across a box defined by dimensions ##dx##, ##dy##, and ##dz##.
  • Another participant explains that if pressure is not uniform along the x-axis, it will change with distance, leading to an infinitesimal change in pressure described by the function p(x).
  • A participant suggests that various factors such as gravity, temperature differences, or dynamic situations can produce pressure differences, emphasizing the need to account for these variations in the derivation.
  • Further clarification is provided that if pressure does not vary in a given direction, the associated derivative term will be zero, but if it does vary, it is an important parameter in the derivation.
  • Another participant adds that flow acceleration due to interactions with objects can also lead to pressure variations.

Areas of Agreement / Disagreement

Participants generally agree on the importance of considering pressure variations in the derivation, but there are differing views on the specific causes and implications of these variations. The discussion remains unresolved regarding the exact nature of pressure changes along the streamline.

Contextual Notes

Participants express uncertainty about the assumptions underlying pressure variation and the conditions under which certain terms may drop out of the equations. There is also a mention of the relevance of flow dynamics in understanding these variations.

greg_rack
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Hi guys, in the derivation of the Euler equation we apply Newton's 2nd law to a gas flowing through a streamline. To do so, we consider a "box" with sides ##dx## ##dy## and ##dz##:

Schermata 2020-12-30 alle 16.39.18.png as such;

Here, with reference to the image, I can't understand where does that '##+dp##' comes from, and hence why the pressure would vary on the right side of the box compared to that acting on the left side.
 
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If the pressure is not uniform along the x axis, then it will change with distance along x. For an infinitesimal change dx there will be an infinitesimal change in pressure. Given the function p(x), then the change of pressure is as described in the picture.
 
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hutchphd said:
If the pressure is not uniform along the x axis, then it will change with distance along x. For an infinitesimal change dx there will be an infinitesimal change in pressure. Given the function p(x), then the change of pressure is as described in the picture.
And why would the pressure vary along x?

PS: I'm sorry for asking these probably silly questions, but I'm taking a really cool online course on aerodynamics for fun, without any expertise :)
 
For instance there could gravity in the x direction. There could be a Temperature difference. Or any dynamic situation (stuff in motion) can produce pressure differences. It might be zero but the theory requires us to be able to deal with such a difference.
 
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hutchphd said:
For instance there could gravity in the x direction. There could be a Temperature difference. Or any dynamic situation (stuff in motion) can produce pressure differences. It might be zero but the theory requires us to be able to deal with such a difference.
Got it, thanks a lot for the brilliant clarification!
 
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greg_rack said:
Hi guys, in the derivation of the Euler equation we apply Newton's 2nd law to a gas flowing through a streamline. To do so, we consider a "box" with sides ##dx## ##dy## and ##dz##:

View attachment 275385 as such;

Here, with reference to the image, I can't understand where does that '##+dp##' comes from, and hence why the pressure would vary on the right side of the box compared to that acting on the left side.

You have no reason to assume that it wouldn't vary, so you account for that in the derivation. If the variation isn't important to the final result, it would drop out. It doesn't, so obviously it's an important parameter. If you have a flow in which the pressure does not vary in a given direction, then the associated ##\partial p/\partial ?## term will just be zero.

greg_rack said:
And why would the pressure vary along x?

PS: I'm sorry for asking these probably silly questions, but I'm taking a really cool online course on aerodynamics for fun, without any expertise :)

To add to some of the things that @hutchphd suggested, it could also be due to any sort of flow acceleration as a result of interaction with an object (e.g. flow over an airfoil or into a corner).
 

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