# Why does div(v)=0 for a fluid means that the fluid is incompressible?

1. Feb 7, 2006

### asdf1

why does div(v)=0 for a fluid means that the fluid is incompressible?

2. Feb 7, 2006

### Galileo

The fluid doesn't diverge or compress, so that the density in a certain fluid element always stays the same.

Use the divergence theorem. Take an arbitrary volume region V of the fluid with surface S, then according to the divergence theorem:

$$\int_{S}\vec v\cdot d\vec a=\int_{V}\vec \nabla \cdot \vec v dV$$

Do you know how to interpret this for your fluid?

3. Feb 8, 2006

### asdf1

do you mean that the surface integral is the same as the volume integral?
@@

4. Feb 8, 2006

### Galileo

No, the left integral is the fluid FLUX through the surface. The right one is a volume integral of the DIVERGENCE of the fluid.

Have you heard of the divergence theorem before?

5. Feb 8, 2006

### asdf1

i looked the theroem up!
thank you!!!
so in other words, (the amount of fluid leaving)- (the amount of fluid entering)=0=incompressible...

6. Feb 8, 2006

### Galileo

Right, that's basically it. Since div(v)=0 the flux through any closed surface is zero. The flux is a measure of how much fluid flows through the surface. Since the surface is closed there is no net accumulation of fluid in the volume.

7. Feb 9, 2006

### asdf1

^_^
thank you very much!!!