Question on partial derivative

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SUMMARY

The discussion centers on the assumption that the partial derivative ∂iuj∂jui equals zero in the context of fluid dynamics, specifically regarding the rate of strain. The user questions the validity of this assumption while attempting to prove that the rate of strain is always positive. The term in question is part of a larger expression involving velocity components and the Kronecker delta, δij. The conclusion drawn is that the assumption may not hold universally without specific conditions on the vector field u.

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Samoht
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I just handed in a homework where I used the assumption below

iujjui=0 ?

but when I start thinking about it I'm not so sure, could someone prove to me that it is zero? Or is that assumption totally off?

Regards
 
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Is there summation over i and j?
Is u something special, or just an arbitrary vector?
 
This partial derivative came up in my proof that the rate of strain in fluid dynamics is always positive. u is the velocity and I believe that there is summation but I am very new at this.

The term I tried to prove being positive is:

iuj((∂iuj+∂jui)-2/3 ∂kuk δij)

When I multiply ∂iuj into the outer bracket then I get

(∂iuj(∂iuj+∂jui)-2/3 ∂kuk δijiuj)

which in my mind becomes

(∂iujiuj+∂iujjui)-2/3 ∂kuk δijiuj

the right side of the minus sign can be simplified to 2/3 (∂kuk)2

My question relates to the second term in the remaining bracket. Can the term ∂iujjui be set equal to 0? AND, if so why?

Regards
 

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