# Homework Help: Incompressible flow

1. Jun 17, 2010

### coverband

1. The problem statement, all variables and given/known data
A velocity field is given by
$$\vec {u} = f(r)\vec{x}, r = | \vec{x}| = \sqrt {x^2 + y^2 + z^2}$$ written in rectangular cartesian coordinates, where f(r) is a scalar function. Find the most general form of f(r) so that $$\vec {u}$$ represents an incompressible flow

2. Relevant equations
Incompressible flow implies $$\nabla . \vec {u} = 0$$.

3. The attempt at a solution
The solution is $$\nabla . \vec {u} = 3f + rf' so f(r) = A/r^3$$ (A is an arbitrary constant) but I don't see how it is arrived at. Thanks

2. Jun 17, 2010

If the norm of x is given, you can figure out its components, right?

3. Jun 17, 2010

### coverband

Right, (x,y,z)

4. Jun 17, 2010

So, write u down in terms of its components and calculate the divergence.

5. Jun 17, 2010

### coverband

Ok, so u = (fx,fy,fz)

Therefore div u = 3f !!! Now.. !?

6. Jun 18, 2010

### coverband

Or div u = 3f + xf_x + yf_y + zf_z ? Little help!

7. Jun 19, 2010

### billiards

Think I can help here. Not sure where your getting stuck as you haven't properly written out your thoughts.

First thing you need to do is get the general expression for divergence in terms of your scalar function. Here are the key things for the x-component (and they have the same form for y and z).

div u = d(ux)/dx + ......

ux = f(r) rx

so write out div u making the substitution.

But remember the product rule. http://en.wikipedia.org/wiki/Product_rule

There are some further steps before you can get your answer, but this is a good start.

Last edited: Jun 19, 2010