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Location:
MELBOURNE
Registered:
December 2004
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Aerodynamics question.
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Thu, 12 May 2005 10:20
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Hey
Was just wondering if anyone has any pictures/description of the points on the car where air hits and then shoots up/forced down etc. etc. Sorry I'm new to this topic and didn't find much with a search.
Thanks
Troy
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Membership Secretary
Location:
Sydney
Registered:
June 2002
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Re: Aerodynamics question.
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Thu, 12 May 2005 11:30
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what exactly are you asking??
Maybe hit
How Stuff works dot com
Air is deflected all over a car. A car has to punch a hole in the air to get thru.
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Location:
MELBOURNE
Registered:
December 2004
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Re: Aerodynamics question.
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Thu, 12 May 2005 11:54
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Hey
Sorry I'll try to be more specific.
When the air is split at the front of the car I was wondering where the most air is on the bonnet before its deflected up etc. Right now I"m looking at the placement of Scoops so I was just wondering because if you ran a cold air intake out the top of the bonnet with a fibreglass moulding roud it to make it legal would greatly decrease the length the air would have to travel to get to the turbo which I will eventually have so I was just wondering where the best placement would be so I'm just after some animations/drawings/advice/descriptions etc.
Thanks
Troy
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Location:
South Australia
Registered:
July 2002
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Re: Aerodynamics question.
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Thu, 12 May 2005 12:02
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Most air intakes dont really require ridiculously crazy placement - what you are suggesting is a form of 'ram air' induction which probably won't have any significant effect until about 250km/h, so it's probably not worth the bother.
The reason most cars have bonnet scoops is for intercooling purposes, or for heat extraction (using low pressure zones on top of the bonnet to suck out high pressure hot air from under the bonnet). Not for air intake (except in the case of some gay looking hot-rod or muscle cars)
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Location:
Brisbane
Registered:
June 2002
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Re: Aerodynamics question.
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Thu, 12 May 2005 12:56
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Typically speaking you would be looking at the leading edge of the bonnet and base of the windscreen as high pressure points.
Do a search on http://www.autospeed.com They have quite a few really good articles on exactly this sort of thing.
Cheers,
Julian.
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Location:
MELBOURNE
Registered:
December 2004
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Re: Aerodynamics question.
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Fri, 13 May 2005 01:42
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Thanks you guys. I'll have a search on Autospeed and we'll see what happens. Are you serious about the sucking hot air out of the engine bay? Then whats the point of a top mount intercooler. It wouldn't get any cold air flowing into it?
Thanks
Troy
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Location:
South Australia
Registered:
July 2002
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Re: Aerodynamics question.
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Fri, 13 May 2005 03:25
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A top mount intercooler does get plenty of air flow through the scoop but mainly because the flow of air in those engine bays is fairly purposefully designed. You will notice that WRXs, and GT4s (maybe even Mazda Familia's?) have extra vents for heat extraction ASIDE from the main bonnet scoop, this is to ensure air *flow* so that the intercooler can operate effectively. If you only had the bonnet scoop, and the hot air from the engine bay had nowhere else to escape (as in the case of the Pulsar GTI-R I think) then the top-mount intercooler will get very little flow directed to it and only at very high speeds when the pressure is very high near the bonnet scoop will it actually work with some level of efficiency - effectively only when the air is really FORCED into the engine bay.
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Location:
South Australia
Registered:
July 2002
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Location:
MELBOURNE
Registered:
December 2004
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Re: Aerodynamics question.
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Fri, 13 May 2005 11:53
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RWDBoy
Thanks for the info and explanation man! I"ll have a look at a rex next time I see one and see whether I can spot it. Do you know whether at the back of the bonnet( near the windscreen, just in case you don't understand loL! Im not great at explaining things) Would it suck hot air out if you curved the edge up and gave say a 3-4cm opening? That would make things good for me loL!
Thanks
Troy
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Location:
Brisbane
Registered:
July 2004
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Re: Aerodynamics question.
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Fri, 13 May 2005 14:45
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as jkvsnn said ,the base of the windscreen is a high pressure point,this means it is a good position to suck cool air in.this is why ALL cars have their cool air vents for the interior at the base of the windscreen.ALSO why top mount intercooler cars position their intake vent at the rear of the bonnet.
NOW to do the opposite,to remove hot air,you need a low pressure area.look at drift bonnet design.where are the vents typically placed,directly behind the front lip of the bonnet.
It really is worth reading that article in autospeed.Its an excellent resource and cheap to access all the back issues.i appreciate southo putting me onto it when i was sorting engine bay airflow.You will also read how the skirts under the engine contribute heavily to air flow through the engine bay.
Good luck and happy reading.
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Location:
South Australia
Registered:
July 2002
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Re: Aerodynamics question.
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Fri, 13 May 2005 16:42
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Actually most top mount air-to-air intercoolers have their scoop placed around the halfway point of the bonnet, but generally not at the 'rear'. The rear of the bonnet is not really under much pressure and it's a region of very low air flow - this is partly why it's used to cool the in-cabin because it's not as likely to get extremely noisy at very high speed nor will it get turbulent.
Air flow design is pretty interesting, I'm not a genius because I didn't study gas dynamics at university (I only did third year fluid mechanics which only covered newtonian and semi-newtonian 'liquid' flows and only briefly brushed on non-laminar flows) but the principles are similar.
Generally windscreens are 'pitched' more vertically than the bonnet, thus the whole region under the line between the top of the windscreen and the very front of the bonnet has air flow which tends to move slower and slower as you get closer to the surface of both (and farther away from the high pressure points). The highest pressure points are right at the front of the bonnet, and right at the top edge of the windscreen. The reason bonnet scoops are raised is partly because they will catch more air if the frontal area is greater, but also because they need to be higher up (and further away from the bonnet surface) so they get good air flow and speed! The reason that most air vents are placed just behind the front lip of the bonnet is because usually the front bar is being used to scoop up air!
If I am wrong, then I blame the lecture induced narcolepsy - because I slept through ALOT of lectures.... :S
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Location:
sunny coast, qld
Registered:
October 2002
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Re: Aerodynamics question.
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Sat, 14 May 2005 00:23
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| RWDboy wrote on Thu, 12 May 2005 22:02 |
The reason most cars have bonnet scoops is for intercooling purposes, or for heat extraction (using low pressure zones on top of the bonnet to suck out high pressure hot air from under the bonnet). Not for air intake (except in the case of some gay looking hot-rod or muscle cars)
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Oi, I have a bonnet scoop for air intake purposes! It is positioned just infront of my air filter and the air filter will have a heat shield around it with provision for a CAI tube from the front bar also. 
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Location:
South Australia
Registered:
July 2002
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Re: Aerodynamics question.
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Sat, 14 May 2005 00:58
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Cheeky bugger 
I knew someone would be have that so that's why I wrote most cars rather than every car 
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Location:
South Australia
Registered:
July 2002
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Re: Aerodynamics question.
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Sat, 14 May 2005 01:00
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Oh and you can feel free to disregard the gay looking hot rod comment 
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Location:
sunny coast, qld
Registered:
October 2002
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Re: Aerodynamics question.
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Sat, 14 May 2005 01:39
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Thanks mate, no offence taken
Although, I think it does look pretty gay
Anyway, back on topic.
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Location:
Brisbane
Registered:
July 2004
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Re: Aerodynamics question.
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Sat, 14 May 2005 02:14
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excuse accepted.[narcolepsy]
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Location:
MELBOURNE
Registered:
December 2004
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Re: Aerodynamics question.
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Sat, 14 May 2005 23:12
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lol. Thanks RWDboy once again, appreciate the info. Does that mean you could have a setup where a scoop took air in and then a larger opening sucked air out placed somewhere on the bonnet?
Thanks
Troy
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Location:
South Australia
Registered:
July 2002
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Re: Aerodynamics question.
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Sun, 15 May 2005 14:53
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| manmx83 wrote on Sat, 14 May 2005 11:44 |
excuse accepted.[narcolepsy]
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Does that mean I'm wrong 
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Location:
Kita-Ku, Sapporo, Japan
Registered:
January 2003
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Re: Aerodynamics question.
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Sun, 15 May 2005 14:58
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there are some good pics around of both flow simulations and tuft tests, but are a little tricky to find...
i cbb  get out your google and off ya go
sufficient to say base of windscreen = localised higher pressure, 1/3rd of way back from front of bonnet is low pressure etc etc
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Location:
Kita-Ku, Sapporo, Japan
Registered:
January 2003
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Location:
South Australia
Registered:
July 2002
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Re: Aerodynamics question.
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Mon, 16 May 2005 05:09
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If you look at the velocity of the streamlines near the base of the windscreen you can see that the flow rate drops around there. It looks mostly like what I would suspect, although I did mis-judge how high the pressure would be at the tip of the windscreen and how high the pressure would be at the bottom of the windscreen I stand corrected in that regard!
That flow only shows 55mph, at higher speeds the pressure zones would change and turbulence would play a bigger role.
Also it doesn't seem to model the flow going through the engine area and the front grill etc, as if the model is solid? I looked around for more details but couldn't find any.
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Location:
MELBOURNE
Registered:
December 2004
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Re: Aerodynamics question.
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Mon, 16 May 2005 09:08
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ALright, total stupid question but I have to ask it. High pressure zone - where wind is hitting harder? Or more?
Low pressure zone - wind isn't hitting area all that much? Gots to get my deffinitions right! Thanks for the pics OldCorolla!
Thanks
Troy
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Location:
Kita-Ku, Sapporo, Japan
Registered:
January 2003
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Location:
Kita-Ku, Sapporo, Japan
Registered:
January 2003
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Re: Aerodynamics question.
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Mon, 16 May 2005 09:55
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| RWDboy wrote on Mon, 16 May 2005 15:09 |
If you look at the velocity of the streamlines near the base of the windscreen you can see that the flow rate drops around there. It looks mostly like what I would suspect, although I did mis-judge how high the pressure would be at the tip of the windscreen and how high the pressure would be at the bottom of the windscreen I stand corrected in that regard!
That flow only shows 55mph, at higher speeds the pressure zones would change and turbulence would play a bigger role.
Also it doesn't seem to model the flow going through the engine area and the front grill etc, as if the model is solid? I looked around for more details but couldn't find any.
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yup, it's a bitch to find good images, partially cos it's a bitch to model, and ppl who pay for the modelling are 'slightly' protective of their IP.... but you get that
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Location:
MELBOURNE
Registered:
December 2004
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Re: Aerodynamics question.
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Mon, 16 May 2005 10:12
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Hey
I already searched and got those pictures thankyou man! So yes, I'm learning more without being spoon fed
Thanks
Troy
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Location:
South Australia
Registered:
July 2002
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Re: Aerodynamics question.
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Mon, 16 May 2005 14:12
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High pressure just means that the air has been 'squeezed' by some force acting on it. Not that it's hitting anything hard...in fact high pressure air might not be going anywhere at all.
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