Aerodynamic drag of a vehicle with an aperture inside

  • Context: Graduate 
  • Thread starter Thread starter Filmcavity
  • Start date Start date
  • Tags Tags
    Drag Vehicle
Click For Summary

Discussion Overview

The discussion revolves around the aerodynamic drag experienced by a vehicle with a 10cm diameter aperture at the front, which leads to a sealed container within the vehicle. Participants explore how the size and shape of the container, as well as the aperture, influence drag, considering both theoretical and intuitive perspectives.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions whether the drag would increase in proportion to the size of the sealed container, suggesting that the internal volume may not significantly affect drag.
  • Another participant proposes that drag is primarily determined by the size of the aperture rather than the volume of the container, linking this to pressure dynamics within the cavity.
  • Some participants express uncertainty about the influence of the container's shape on drag, with one suggesting that it may not impact pressure at the aperture.
  • A later reply emphasizes that if the pressure inside the container equals the ram pressure at the front of the vehicle, the aperture's effect on drag may be negligible.
  • One participant argues that the aperture does change the vehicle's profile, potentially affecting drag significantly.
  • Another participant notes that if the aperture is flush with the vehicle's body, it may not materially alter the front profile, thus having little effect on drag.
  • Concerns are raised about the flow of air past the aperture and the effects of viscosity and compressibility on drag, especially when considering larger openings.
  • One participant draws an analogy to vehicle windows and their impact on drag when opened, questioning how this relates to the aperture's size and drag implications.
  • Anecdotal evidence is provided regarding the efficiency of using A/C versus open windows at high speeds and the drag considerations in specialized vehicles like velomobiles.

Areas of Agreement / Disagreement

Participants express a mix of intuitive views and technical reasoning, with no clear consensus on the effects of the sealed container's size or shape on drag. Multiple competing perspectives remain on how the aperture influences overall aerodynamic performance.

Contextual Notes

Participants highlight various assumptions about pressure dynamics, airflow, and the effects of vehicle design on drag, but these remain unresolved and depend on specific conditions and definitions.

Filmcavity
Messages
4
Reaction score
0
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.
 
Physics news on Phys.org
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.
 
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 traveling 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.
 
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.
 
Doesn't the aperture, by definition, change the profile? The effect on the drag can be dramatic.
 
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.
 
Filmcavity said:
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 traveling 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.

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".
 
AlephZero said:
but think about for example opening one of the vehicle windows completely and considering the whole interior of the van as the "cavity".

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?
 
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.
 

Similar threads

Is Evolutionary Theory Undermined by Creationist Claims?
  • · Replies 266 ·
9
Replies
266
Views
32K
Graduate I have lots of questions on physics
  • · Replies 10 ·
Replies
10
Views
4K