Aerodynamic chat
Discussion
Yes that's the usual consensus, however Doogz says he's done some CFD work which might suggest that airflow under the bonnet may affect this high pressure region.
I'm not sure personally, but as was stated on the STNPHS thread, fluid flow isn't something you can 'imagine' as it's highly non-linear...
I'm not sure personally, but as was stated on the STNPHS thread, fluid flow isn't something you can 'imagine' as it's highly non-linear...
GadgeS3C said:
Nice to see an informed discussion on this on PH - hoping the thread gets as much debate as the other one!
+1!While I'm sure the airflow in the engine bay does affect the pressure at the bottom of the windscreen, you're still going to struggle to stop it being a very high pressure area. Indeed it's beneficial to have it as high pressure for a high-performance application, because it generates downforce.
A typical pressure distribution for a hatchback:
On a list of places I'd want to put a cooling outlet, the top of the bonnet is pretty far down!
GadgeS3C said:
Nice to see an informed discussion on this on PH - hoping the thread gets as much debate as the other one!
I was enjoying it on the other thread too 
I don't have anything useful to add unfortunately, although I now understand why my blower on setting one blows much more air into the cabin on the m'way 
The answer is it depends...
Typically you will find the cooling inlet is located at the front stagnation point where static pressure coefficient is 1. At the base of the windscreen, whilst it is a high pressure region (pressure coefficient is greater than 0), it generally doesn't reach 1 as flow here, on most vehicles, isn't completely brought to a stop. Although it is slowed significantly.
Therefore flow will move through the cooling system from the inlet to the base of the windscreen as static pressure is higher at the inlet than at the base of the windscreen.
All that said its easier to exhaust the underbonnet flow under the car as pressure is much lower here. Therefore locating an outlet under the car will drive a greater mass flow through the system than it would if the outlet was at the base of the windscreen.
Typically you will find the cooling inlet is located at the front stagnation point where static pressure coefficient is 1. At the base of the windscreen, whilst it is a high pressure region (pressure coefficient is greater than 0), it generally doesn't reach 1 as flow here, on most vehicles, isn't completely brought to a stop. Although it is slowed significantly.
Therefore flow will move through the cooling system from the inlet to the base of the windscreen as static pressure is higher at the inlet than at the base of the windscreen.
All that said its easier to exhaust the underbonnet flow under the car as pressure is much lower here. Therefore locating an outlet under the car will drive a greater mass flow through the system than it would if the outlet was at the base of the windscreen.
Quite - and while the pressure coefficient at the bottom of the windscreen is almost always much less than at the inlet, it's not a simple flow right through the engine bay with nothing in the way! I think at very high speed it could start to work, and would become a little useful if you wanted to reduce the air pumping underneath the car generating lift. But at low speeds I don't think very much air would exit through it.
CoolHands said:
has anyone noticed how manufactures don't advertise the coefficient anymore? I remember when they used to go on and on about it and all the adverts featured it. Like the latest ford sierra had a drag coefficient of 0.31 or whatever. All a load of baloney if you ask me, but anyway...
Without knowing the frontal area of the vehicle it's rather a meaningless stat anyway.syncro. said:
Also if you supliment the flow under the car with that from the cooling system you actually reduce lift. Increased mass flow under the car therefore greater velocity to satisfy continuity thus lower static pressure, reducing lift and in more extreme cases generating downforce.
If you had a sealed full effect undertray, yeah, you could use your engine bay outlet to help pump it. But I would imagine that, on a road car with large ground clearance and open sides and pipework and crap everywhere, all you'll do is create more turbulence (exacerbated by the temperature difference) rather than effectively increasing the velocity?This flow is highly turbulent anyway, the simple fact is you're adding mass flow, velocity goes up, pressure down. Granted some does leak out due to the higher ride height but as your between the wheel wakes they do a pretty good job of keeping it under the floor between the sills. Speak to youre head of dept about it tomorrow 
Kozy said:
Carried over from the 'Stupid things non-petrol heads say' thread.
So, do bonnet risers work as a cooling aid?
I say no...
They do, for one critical reason - they make a lot of difference when you are travelling more slowly but in a high stress environment, say towing up a hill or in stop start traffic on a summer day. Certainly on my Dolomite Sprint (with the hinges at the front of the bonnet) it's a very handy auxiliary cooling aid once running the heater on full power in midsummer isn't enough.So, do bonnet risers work as a cooling aid?
I say no...
syncro. said:
This flow is highly turbulent anyway, the simple fact is you're adding mass flow, velocity goes up, pressure down. Granted some does leak out due to the higher ride height but as your between the wheel wakes they do a pretty good job of keeping it under the floor between the sills. Speak to youre head of dept about it tomorrow
Fair enough - I didn't expect much meaningful velocity gain to be possible that way (compared to the drag you'd get). I forgot about the wheel wakes helping bound the flow though. Always interesting to get general ideas of what would happen in particular real-world situations! What sort of percentage of the underbody flow would you expect to be coming from the engine bay?And thanks
Kozy said:
Yes that's the usual consensus, however Doogz says he's done some CFD work which might suggest that airflow under the bonnet may affect this high pressure region.
I'm not sure personally, but as was stated on the STNPHS thread, fluid flow isn't something you can 'imagine' as it's highly non-linear...
Like I said in the other threadthe high pressure region is needed to force air into the cabin for the heating and ventilation, so I don't see how you can suddently 'loose' the high pressure region so that air can escape from the low pressure under bonnet.I'm not sure personally, but as was stated on the STNPHS thread, fluid flow isn't something you can 'imagine' as it's highly non-linear...
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