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Material Derivative (Convective Derivative Operator)

  1. Mar 15, 2015 #1
    Hi,

    I've learned that material derivative is equal to local derivative + convective derivative, but can't seem to find out which way the convective derivative acts, like for example in velocity fields:

    The equation my teacher gave us was (with a and v all/both vectors):
    Acceleration = material derivative of velocity = local acceleration + convective acceleration
    ∴ a = Dv/Dt = dv/dt + v⋅∇v

    My question is whether the convective acceleration term (v⋅∇v) works like:
    1. (v⋅∇)v, which in my understanding is the (v⋅∇) operator working on the vector v
    2. v⋅(∇v), which I take as the grad operator working on the vector v, dotted with the vector v outside the brackets
    3. Or is it that Options 1 and 2 are the same thing anyway?

    [Side note: Sorry, I'm new to PF and don't know how to use the equation symbols or LaTeX.]

    Thank you in advance for your help! :)
    - TheHappyPenguin
     
  2. jcsd
  3. Mar 16, 2015 #2

    joshmccraney

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    Hi thehappypeguin!

    1) and 2) are equivalent expressions, so technically 3) is correct. I'm sure someone else here can easily send you to a link where you can figure out latex, if that's what you're trying to figure out.
     
  4. Mar 16, 2015 #3

    dextercioby

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    Even if you can't write in LaTex, you can still use a writing to distinguish vectors from scalars. So write v for the velocity vector and v for its modulus (or projection onto a coordinate axis). :)
     
  5. Mar 22, 2015 #4
    Thank you! I'll look up latex when I have time :)

    About the Material Derivative, I read some more about it online and found that in Pgs 1-3 of http://www.chem.mtu.edu/~fmorriso/cm4650/lecture_6.pdf, ∇v is a tensor which would make 2) different from 1) as 1) is a vector. Is this accurate?

    Thank you again!
    - TheHappyPenguin :)
     
  6. Mar 22, 2015 #5

    stevendaryl

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    Yes, [itex]\nabla \vec{v}[/itex] is a tensor, but no, the two expressions are not different. The dot product of a vector with a tensor (of the right type) produces a vector. So both

    [itex](\vec{v} \cdot \nabla) \vec{v}[/itex] and [itex]\vec{v} \cdot (\nabla \vec{v})[/itex] produce vectors.
     
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