Verifying Fluid Dynamics Equation

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Discussion Overview

The discussion revolves around verifying a fluid dynamics equation involving vector notation and index notation. Participants explore the relationship between different terms in the equation and seek clarification on the notation used.

Discussion Character

  • Homework-related
  • Technical explanation

Main Points Raised

  • One participant expresses confusion about how to proceed with the verification of the equation and mentions decomposing ωk into Curl(v).
  • Another participant suggests that ∂i(1/2*v2) can be represented as vj∂ivj, providing a hint to assist with the problem.
  • A participant seeks clarification on the meaning of vj∂ivj in vector notation, indicating a lack of familiarity with index notation.
  • One participant explains the summation convention and how to derive the expression for ∂iv2, contributing to the understanding of the notation.
  • A later reply indicates that the explanation has clarified the participant's understanding, allowing them to proceed with the problem.

Areas of Agreement / Disagreement

There is no explicit consensus reached in the discussion, but participants generally agree on the validity of the hint provided regarding the notation. The initial confusion about the equation and notation remains a point of exploration.

Contextual Notes

Participants express varying levels of familiarity with index notation, which may affect their understanding of the problem. The discussion does not resolve all uncertainties regarding the application of the notation in the context of the fluid dynamics equation.

Who May Find This Useful

This discussion may be useful for students or individuals learning about fluid dynamics, vector calculus, and index notation, particularly those seeking clarification on related equations and concepts.

ICSunSpots
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1. Verify that vjjvi=∂i(1/2*v2)-εijkvjωk,where ωkklmlvm


Homework Equations





The Attempt at a Solution


I am confused on how to proceed with this problem up to this point I have decomposed ωk into Curl(v). Which leaves vj X Curl(v). Decomposing this leaves vmivm-vllvi. I am stuck on where to go from here. How do I get rid of the ]=∂i(1/2*v2) term? Any help is appreciated. Thanks.
 
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Welcome to PF!

ICSunSpots said:
1. Verify that vjjvi=∂i(1/2*v2)-εijkvjωk,where ωkklmlvm


Hi ICSunSpots! Welcome to PF! :smile:

Hint: ∂i(1/2*v2) = vjivj, isn't it? :wink:
 
i(1/2*v2) = vjivj


Thank you for your reply. That would certainly make my solution thus far work.
However,
I am fairly new to index notation. Can you explain to me in vector notation what vjivj means? I understand the ]∂ivj term, but what does the vector vj in front of it do? Thanks.
 
ICSunSpots said:
i(1/2*v2) = vjivj


Thank you for your reply. That would certainly make my solution thus far work.
However,
I am fairly new to index notation. Can you explain to me in vector notation what vjivj means? I understand the ]∂ivj term, but what does the vector vj in front of it do? Thanks.

I'm using the summation convention … repeated indices are added over all possible basis values.

So v2 = vjvj,

and so ∂iv2 = ∂i(vjvj) = (∂ivj)vj + vj(∂ivj) = 2vjivj :smile:
 
Great! It's clear as day now. This index notation is really neat stuff, it just takes a little bit for it to be intuitive. Thank you for your help, I can now finish the problem.
 

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