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Thread: What is the expanded form of...

  1. #1
    May 2010

    What is the expanded form of...

    What is the expanded form of (v ∙ grad)w?
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  2. #2
    MHF Contributor

    Apr 2005
    Quote Originally Posted by thefrog View Post
    What is the expanded form of (v ∙ grad)w?
    Since we have a dot product involving v, I assume that v is a vector but then (v- grad) evaluates to a scalar operator and w could be either a vector or a scalar.

    Let $\displaystyle v= v_x\vec{i}+ v_y\vec{j}+ v_z\vec{k}$ and $\displaystyle v\cdot\nabla= v_x\frac{\partial}{\partial x}+ v_y\frac{\partial}{\partial y}+ v_z\frac{\partial}{\partial z}$

    If w is a vector, $\displaystyle w= w_x\vec{i}+ w_y\vec{j}+ w_z\vec{k}$ then this is a scalar product- the scalar multiplying each part of the vector:
    $\displaystyle (v_x\frac{\partial w_x}{\partial x}+ v_y\frac{\partial w_x}{\partial y}+ v_z\frac{\partial w_x}{\partial z})\vec{i}$$\displaystyle + ( v_x\frac{\partial w_y}{\partial x}+ v_y\frac{\partial w_y}{\partial y}+ v_z\frac{\partial w_y}{\partial z})\vec{j}$$\displaystyle + ( v_x\frac{\partial w_z}{\partial x}+ v_y\frac{\partial w_z}{\partial y}+ v_z\frac{\partial w_z}{\partial z})\vec{k}$

    If w is a scalar function then this is a product of two scalars:
    $\displaystyle v_x\frac{\partial w}{\partial x}+ v_y\frac{\partial w}{\partial y}+ v_z\frac{\partial w}{\partial z}$
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