How do you rid a car of a fart?

Chris Smith put this question to Jess Wade from Imperial College London.

Chris - I thought that Jess, never in your whole career did you think that you would be coming to reach this moment? To apply the rules of physics and fluid dynamics to ventilate a car?

Jess - Especially to be asked from California - I thinks that’s brilliant. There’s obviously a few different issues here about how you get a fart out of a car. And one of them is about which windows you choose to open, the other one is what kind of temperature the fart is at, as disgusting as that sounds, because that affects how quickly it expands when it comes out into the atmosphere? Then the other one is which kind of gases are in the air already?

When the fart occurs: it comes out, it expands, it moves forward to the front of the car because that’s the region of lower pressure if you’re moving along because the higher pressure region is at the back.

Chris - Why is that?

Jess - Because the air is accelerating forwards, so it’s pushing backwards increasing the pressure at the back.

Chris - You’re assuming the car is getting faster?

Jess - I’m assuming here that the car is moving forwards…

Chris - So the air is being left behind a bit inside the car?

Jess - Exactly.

Chris - So the farts going to make it’s way towards the front so the driver is going to experience it more than the back seat.

Jess - Exactly.

Andrew - Well that’s made me realise the worst thing you can do when someone farts in a car is try and accelerate away from it.

Chris - Yeah, so you should put the brakes on - is that what you’re saying?

Jess - It is the worst thing you can do. And if you’re in the passenger seat and you’re the one who’s done the fart it’s going to be very obvious, very quickly because it’s going to go forward to the driver. If you try and open the windows, which is what you’d probably try and do, there’s interesting things that happen with kind of fluid flow. You’d think if you were this little air particle in the car you’d want to rush out, right? As soon as you get outside you realise very quickly that the air there is not actually at the lower pressure you assumed it was at and it’s much lower pressure inside the car, so the air there isn’t moving with you, it’s outside. So you rush back into the car and so the smell doesn’t leave if you just open one of the windows, which I think is interesting.

Chris - So you need to open more than one window then?

Jess - To get this fart out - Karen in California - you have to slightly open the front window and open the back window so that you create a flow through the car. You can get really interesting graphics of it, a kind of turbulent flow going through these two open windows.

Chris - I’m forming a few mental images right now.

Jess - I’m not going to make any judgement here. You could also get smart materials to make your car seats out of that absorb odours, so smart materials and materials that have a function when you do something to them. Materials like ones that you can jump up and down on and then they produce electricity pseudo-electric materials.

You can also get clever ones that can trap the smells. Smells are usually carbon based molecules and you can get ones that get trapped in the fabric so you don’t smell those at all. So that is one option but the other option is, obviously, also doing this with the windows.

Chris - OK. So a front window open a bit. A back window open a bit. And that should achieve the right pressure differential to vent the fart?

Jess - Entirely right.