The following relates to a conversation I had in the comments of an earlier post.
When considering an expression like $(u \times \nabla) \times v$, it is supposedly possible to apply the vector triple product identity in an unjustified (though "formally correct") way to get a correct simplification such as $(u\times \nabla) \times v = \nabla(u\cdot v) - (v\cdot \nabla) u$. In particular, the vector triple product formula states that for vectors $a,b,c \in \Bbb R^3$, we have $$ (a \times b) \times c = (a \cdot c)b - (b \cdot c)a = b(a \cdot c) - (c \cdot b)a. $$ If we treat $\nabla$ as a vector, then "plugging in" to the above yields $(u\times \nabla) \times v = \nabla(u\cdot v) - (v\cdot \nabla) u$.
Is there a rigorous approach to simplifying $(u \times \nabla) \times v$ (and other such expressions) that makes use of vector identities in a similar fashion?
Edit: Having derived a correct simplification in my answer below, it seems that the supposed identity I give above is incorrect (or at least, it seems to be since I don't see how the two expressions would be equal; I have not gone through the trouble of finding a $u,v$ that disprove the identity, though).
In some sense, the solution $(u \times \nabla) \times v = u \cdot (\nabla v) - u(\nabla \cdot v)$ fits the form of $(a \times b) \times c = (a \cdot c)b - a(b \cdot c)$, though it is perhaps counterintuitive that the first term should be $u \cdot (\nabla v)$ which, if the triple product rule can be directly applied in this context, should be our interpretation of $(u \cdot v)\nabla$.
Interestingly, we find that $$ u \times (\nabla \times v) = u \cdot (\nabla v) - (u \cdot \nabla)v $$ The associated vector identity is $a \times (b \times c) = (a \cdot c)b - (a \cdot b) c$. Again, the second term seems to "behave as expected".