Does putting an opening on the front of a vehicle nose increase drag? 1

[robinfryer]

Since usually a stagnation point would occur on the front of the vehicle nose, if we make a small hole in the nose (for a radiator for example), the stagnation points would move to the area around the hole. How does this affect the total drag.

I know there would be an increase in drag due to internal aerodynamic drag, but are there any other effects?

 

[btrueblood]

Wellll....

For a Mig-13, or F-86, the hole in the nose actually greatly diminishes the vehicle drag, since that hole directly feeds airflow to the engine, where it contributes to thrust...

Even a radiator, if designed well (e.g. Mustang P-51), can generate net thrust relative to the frontal area of the inlet.

I think you are asking if a stagnation port in the nose can cause excess drag - and I would think not, unless the edge of the hole is poorly faired, or sharp-edged. A quick look at Hoerner or some NASA reports on pitot tube designs might help you.

 

[robinfryer]

But the fact it produces thrust doesnt affect drag? Drag is purely the aerodynamic force, opposite to the direction of motion, the fact that the air contributes to increasing motive force doesnt reduce drag, it just increases the net force in the forward direction. I am 100% certain that a ford mustangs drag would decrease if you created a smooth rounded surface at the front of the vehicle, although the car would break down eventually.

The only example where i can think of it not incerasing drag, is if that airflow was ducted, and then pumped by a fan internally and the exausted/ejected out into a turbulent wake trailing the object (assuming it had one).

 

[btrueblood]

"and then pumped by a fan internally"

...or had energy added to it by other means, such as heating, thus the reference to turbine engines, ducted radiators, ramjets, etc.

If you had a solid object, say a bomb, and drilled a hole straight thru the body, the frontal area (cross-sectional area) decreases, and I would be pretty surprised if the overall drag did not also decrease. The drag coefficient C_D (which includes the ratio of drag force to frontal area) might very well increase when the hole is added, if that is what you mean.

 

[robinfryer]

Actually i think the opposite would happen, if you drilled lets say like you said a hold through a bomb shape.

The flow inside the hole (which would be like flow inside a pipe) would create a massive skin friction drag and a turbulent flow inside the pipe. Flow inside pipes typically have a high drag.

This flow would pass out the back of the bomb into a region which (with no hole) would have been undisturbed and so this would maybe create a tubulent region behind the bomb, increasing the pressure drag.

In short, the frontal area is NOT everything. Even though the frontal area decreases the Cd is no longer relevant, that is a property of the geometry and by changing the geometry you change the Cd. You also increase the wetted surface area, thereby increasing the skin friction drag, which in streamlined bodies is usually more signifficant than the pressure drag.

 

[KENAT]

Well, are we talking incompressible or compressible flow here?

Many aircraft do have a 'hole' in the nose as this is where the pitot is.

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[robinfryer]

For my case i would say incompressible, but what is the effect of compressible flow, i know there are alot of investifations into the shape of air intakes with respect to supersonic flow...

 

[btrueblood]

"The flow inside the hole (which would be like flow inside a pipe) would create a massive skin friction drag and a turbulent flow inside the pipe. Flow inside pipes typically have a high drag.

This flow would pass out the back of the bomb into a region which (with no hole) would have been undisturbed and so this would maybe create a tubulent region behind the bomb, increasing the pressure drag."

I think you overestimate the effect of skin friction vs. pressure drag, but I will grant you I may be thinking at a Re much larger than you are which may affect things. I.e., your arguement might hold at low Reynold's numbers. At anything over Re = 10k or so:

Take the viewpoint of an air molecule, approaching along the centerline of the bomb. The closed bomb forced that air molecule to diverge around the maximum diameter of the bomb, and all the adjacent air molecules move too. At the back of the bomb, there is a turbulent wake, and a seperated, low-pressure region.

Take the second case, same setup, the air molecule blows thru the center of the casing, retarded somewhat by adjacent molecules via skin friction to the "pipe". But the pressure drag, by far the larger effect, is greatly reduced by the air flow into the base region from the hole. Any skin friction in the "pipe" can only be generated by a pressure differential from front-to-back on the body, and so cannot be high enough, or remove more energy from the flow, than the body is capable of generating in the first place.

Take the thought experiment to the extreme - a 4" diameter bar, vs. a 4" pipe with 1/8" wall. Which has the lower drag?

Any flow thru the hole will reduce the overall body drag, since less air is required to be disturbed by the body's passage than if the hole was not there.

 

[GregLocock]

Depends whether skin drag outweighs pressure recovery drag.



Cheers

Greg Locock


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[robinfryer]

"Take the thought experiment to the extreme - a 4" diameter bar, vs. a 4" pipe with 1/8" wall. Which has the lower drag?"

That is a COMPLETELY different scenario, your comparing a blunt body to a streamlined body wihich is ridiculous. You also dont have any figures to substantiate that claim, or examples of technical papers investigating it.

For a streamlined body, cutting a whole through the middle would almost definetly increase drag.

I am certainly not overestimating skin frictional drag. High streamlined bodies, such as a bomb or teardrop, the drag component from skin friction may even be 1 higher order of magnitude compared to pressure drag. I recently did an investigation of a HPV, Cd was 0.045 and that had a skin friction drag 10 times higher.

If you compared a doughnut to a sphere, i dont really know which one would have a higher drag, that would require investigation to be sure. My initial question was basically, what are the effects of moving a single stagnation point to multiple stagnation points around the hole created. Maybe it is a useless question.

Basically im designing a car body using CFD, i may need to cut a hole in the nose for radiators, but im basically wondering if i could cut a hole in the underside of the vehicle and whether this would be better.

 

[MikeHalloran]

Them NASCAR boys seem convinced that a hole in the front causes drag, and tolerate some pretty high coolant temperatures to keep the hole as small as possible.

I think you'll have to model the flow through the radiator and under the car to catch up to them.



Mike Halloran
Pembroke Pines, FL, USA

 

[GregLocock]

Typically 6% of the total drag on a car is due to internal flow, and that is very carefully modelled on production cars primarily in order to get cooling air where it is needed. When panels were added under the engine in the 80s for noise reg reasons we expected cooling to be a problem. It turns out that this didn't really happen, the less chaotic airflow made it easier to sort the cooling out.

In general I would expect holes in streamlined bodies to increase drag.

Cheers

Greg Locock


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[mattyduk]

Take a look at current Formula 1 nose designs. Most if not all have a hole in the nose

 

[robinfryer]

Yes but the hole in front of the formula one car is for cooling the drivers feet basically (well thats what im told), i would very much doubt its for reducing drag, if anything its for some other aerodynamic effect.

I think as greglock says internal flow will increase drag, except in some extreme cases.

Ive come to realise that cutting a hole in the front will have some effect on the area the stagnation pressure acts on. By cutting a hole, stagnation does not disapear, it would move to around the hole. If it acts on a bigger area this will mean bigger drag. I expect this effect is minimal compared to internal flow.

 

[mattyduk]

Thats actually a common misconception. It helps equalize the pressure in the cockpit area, effectively reducing drag.

 

[robinfryer]

Ahh i see, yes that would make sense, silly me for listening to F1 pundits. Thats a good idea actually, thanks mattyduk!

I have to say though that it wouldnt work with a closed cockpit car obviously and so the internal drag for a typical car would still increase.

 

[nobog]

Some good background info:

http://www.pumaracing.co.uk/TOPSPEED.htm