# Air velocity from a tyre

1. Jul 7, 2009

### Paul245

Can any one tell me what the velocity of air coming out a valve of a tyre pumped to 200 kPa would be?

What equations to you use?

2. Jul 7, 2009

### Thaakisfox

Bernoulli equation...

3. Jul 7, 2009

### Paul245

incompressable flow

4. Jul 7, 2009

### Thaakisfox

What do you mean? The bernoulli equation can always be applied.. (i'm not talking about the "original" bernoulli equation proposed by bernoulli but the general one where we dont integrate the pressure function at once..)

5. Jul 7, 2009

### Staff: Mentor

I think that was Bernoulli with the incompressible flow assumption...

6. Jul 7, 2009

### FredGarvin

The flow will certainly be choked at the valve. All you need to do is plot the density as a function of time and find the flow area of the valve and you can calculate the mass flow.

7. Jul 7, 2009

### Staff: Mentor

200 kpa is 29 psi, isn't it? So it isn't anywhere near choked flow, which, iirc, is somwhere around 65 psi.

8. Jul 13, 2009

### raknath

Can i twist this a bit and ask this question

At what velocity what the tyre be propelled forward, what would be the limiting conditions

Till what time would this actually keep moving.

My guess: (i am still working on the math)

At some bore radius where the force is large enough to push the tyre along.

So as the tyre keeps moving its mass decreases(air is escaping) and at some point the mass going out(change in mass) would equal the pressure(now decreasing)

At that point the tyre would stop, am i right here

9. Jul 13, 2009

### FredGarvin

I suppose I should have paid attention to the pressures. Whoops.