# Aerodynamic drag of a vehicle with an aperture inside

1. Dec 14, 2012

### Filmcavity

If a moving vehicle (van) had a 10cm diameter aperture in the bodywork at the front, which forms the mouth of a sealed container within the vehicle, would the drag experienced by the vehicle increase in proportion to the size of the sealed container? In other words, would it matter if the sealed container had an internal volume of, say, 10 litres, or considerably larger?

Likewise, would the shape of the container have any bearing on the drag: i.e. would a conical container with its base at the opposite end to the mouth perform differently than an inverted cone shape with the mouth in the widest part of the container?

I will be grateful to anyone who can provide me with the relevant maths.

2. Dec 14, 2012

### ThereIam

My intuition is that the drag for a given speed (after driving at that speed for a sufficiently long time) would be determined by the size of the aperture, not the volume within. I think this because the force opposing the motion of the van would come from the pressure inside the cavity in the direction opposite the motion. The force is going to be the area of the aperture times the pressure. This is assuming that as the van drives forward, air that gets trapped in the cavity contributes to a pressure there. The maximum pressure is probably a function of the speed of the van and the ambient temperature. I wonder if this makes sense?

I think the size of the volume would probably determine the rate of the pressure increase...

And I think that this all makes very intuitive sense if one just thinks of a sail! They basically have an aperture size not much smaller than the sail itself, and they don't care about making the volume of the pocket of the sail bigger in an ideal world (I don't think).

As far as the shape, I dunno! My intuition says it doesn't matter because I don't see how it could obviously affect the pressure at the aperture... I hope other people respond to this thread.

3. Dec 15, 2012

### Filmcavity

Therealam,

I share your intutitive view, but I would also like to know that it is actually correct. When I think about this problem, I reach the conclusion that when the pressure inside the sealed container is equal to the ram pressure at the front of the vehicle, as it will be when the vehicle is travelling at constant velocity, then the effect of there being an aperture at the front of the van is of no consequence, irrespective of the size of the aperture and the volume of the sealed container.

4. Dec 15, 2012

As long as the aperture does not cause the overall frontal area to change or the profile of the front of the vehicle to change, having an aperture there will have little to no bearing on drag at all.

5. Dec 15, 2012

### Khashishi

Doesn't the aperture, by definition, change the profile? The effect on the drag can be dramatic.

6. Dec 15, 2012

### Filmcavity

If the aperture is a "ram scoop" that sticks up from the hood, then this would increase drag. In my original posting, I am suggesting that the margins of the aperture to the sealed container lie flush with the surrounding body panels so this wouldn't materially alter the front profile of the van.

7. Dec 15, 2012

### AlephZero

The pressure over the whole area of the aperture might not be constant, and the "flexibiility" (compressibility) of the air in the cavity will not be the same as original structure of the vehicle. The viscosity of the air will mean that the flow past the aperture will always cause some motion of the air inside - there can't be a sudden change in velocity across the aperture between "inside" and "outside".

For a small opening and cavity volume these are probably small effects, but think about for example opening one of the vehicle windows completely and considering the whole interior of the van as the "cavity".

8. Dec 15, 2012

### Filmcavity

So if the aperture of 10cm diameter is at the front of the van, and the whole interior is the sealed cavity, how much would this affect the drag? Likewise, would a larger aperture opening into the whole interior of the van create more drag, and if so, by how much and is this proportional, in any way, to the increase in size of the aperture into the van?

9. Dec 15, 2012

### Khashishi

I heard that above 60mph or so, it's more efficient to use the A/C than to open your windows. Also, I remember watching a short video on the record for fastest human powered bicycle. They close the body of the velomobile around the bicyclist and put tape over all of the seams to reduce the drag. There are small gaps in the bottom, barely big enough for the tires to fit through. They explained that the drag from the small holes around the tires was equal to the drag from the rest of the vehicle combined.