How to interpret advection (v.del) v->v.(del v)

  • Thread starter Monty Hall
  • Start date
In summary, the conversation discusses the convective acceleration term in the Navier-Stokes equation and how it can be written as a vector dot product. The confusion arises due to the notation and terminology, but it is clarified that the quantity is a matrix. The index notation is also explained, along with the omission of the summation symbol. The conversation ends with a question about deriving the same result starting with a different notation.
  • #1
Monty Hall
6
0
I'm looking @ convective accerlation term in http://en.wikipedia.org/wiki/Navier_stokes_equation#Convective_acceleration. I don't understand the terminology. If v is a vector, it says that [tex](\mathbf{v}\cdot\nabla)\mathbf{v}[/tex] can be written as [tex]\mathbf{v}\cdot\nabla \mathbf{v}[/tex]. I thought that [tex]\nabla \mathbf{v}[/tex] is the transpose of the Jacobian matrix for [tex]\mathbf{v}[/tex]. As I'm not familiar with the terminology it almost looks like ([tex]\mathbf{v} \cdot \nabla \mathbf{v}[/tex] = vector . matrix), which can't be right. However, it appears [tex](v\cdot\nabla)v[/tex] is a vector. Can somebody shed some light on how [tex]v\cdot\nabla v[/tex] is a vector? If [tex](\mathbf{v}\cdot\nabla)\mathbf{v}=\mathbf{v}\cdot\nabla \mathbf{v}[/tex] & [tex]\nabla \mathbf{v} = (\mathbf{J}\mathbf{v})^T[/tex]. I'm sure I'm mutilating the terminology, if anybody could shed light on this, much appreciated.
 
Last edited:
Mathematics news on Phys.org
  • #2
In index notation, we have:
[tex]\vec{v}\cdot\nabla=\sum_{j=1}^{3}v_{j}\frac{\partial}{\partial{x}_{j}}[/tex]
where the indices ought to be fairly self-evident.
Usually, we just omit the summation symbol, writing [tex]v_{j}\frac{\partial}{\partial{x}_{j}}[/tex] instead.

Setting this alongside a vector [itex]v_{i}[/itex] then, we have:
[tex](\vec{v}\cdot\nabla)\vec{v}=(v_{j}\frac{\partial}{\partial{x}_{j}})v_{i}=v_{j}\frac{\partial{v}_{i}}{\partial{x}_{j}}[/tex]

The quantity [tex]\frac{\partial{v}_{i}}{\partial{x}_{j}}[/tex] is a matrix
 
  • #3
Thanks for your reply, I've seen that as well the (v.del) makes a lot more sense. I know (v.del)v should equal v.del(v), but I'm curious how can I come to same result starting with v.del(v) which to me looks like a vector.matrix.
 

FAQ: How to interpret advection (v.del) v->v.(del v)

What is advection?

Advection is a process in which a fluid or gas is moved by the bulk motion of another substance, such as wind or ocean currents.

What is v.del?

v.del is a mathematical notation used to indicate the dot product, or scalar product, between two vectors v and del.

What does v->v.(del v) mean?

v->v.(del v) is a mathematical expression that represents the advection of a vector v by the gradient of another vector v. It is often used to describe the transport of a physical property, such as temperature or velocity, by a fluid or gas.

How do you interpret advection using v.del?

To interpret advection using v.del, you first calculate the dot product between the two vectors v and del. This gives you a scalar value, which represents the magnitude and direction of the transport of a physical property by the fluid or gas.

What are some real-world applications of advection?

Advection is an important process in many natural phenomena, including weather patterns, ocean currents, and air pollution. It is also used in engineering and design, such as in the cooling of electronic devices and in the transport of materials in industrial processes.

Similar threads

Replies
21
Views
2K
Replies
3
Views
2K
Replies
1
Views
4K
Replies
1
Views
2K
Replies
9
Views
70K
Replies
6
Views
2K
Back
Top