thehelix112 Posted September 11, 2006 Share Posted September 11, 2006 Guys, Have some questions regarding the aerodynamics of front wheel wells. I have been reading both Joseph Katz's book `Race Car Aerodynamics' and Simon McBeath `Competition Car Aerodynamics'. I understand that you put vents in the side of the front guard behind the wheel, to vent the high pressure air in the well that builds up because of the `scoop' shape of the guard behind the wheel. That makes sense. You also put a vent on top of the guard right above the wheel to vent the same built up air. This has the added benefit of increasing the static pressure on top of the guard and reducing lift over the front wheels. That makes sense too. You then put a small diffuser on the front splitter/underpan in front of the wheels so it doesn't have to do a sharp 90 degree turn to head up the front of the wheel and out the top vent. That makes sense too. My question is, do you think this diffuser should have a sharp 90 degree inside edge, ala rear diffusers? or be smoothed into the flat underpan that runs under the engine? The idea of sharp edges is to generate vorticies INTO the diffuser to energise that flow and delay flow separation. The only one I have seen (in S. McBeath) had smoothed edges. I was wondering if the sharp edge might not generate enough of a vortex to enable much steeper front diffuser, so maybe its not worth it? I was also interested in the flow-down effect of any underbody vorticies. Would this allow me to run a longer/deeper/sharper rear diffuser? I think thats all I can think of for now. Dave Quote Link to comment Share on other sites More sharing options...
bjhines Posted September 11, 2006 Share Posted September 11, 2006 This is a great question... I would love to see more info on this subject... I have been mulling around with this idea myself... also... some pictures of peoples gill vents and top vents would be nice..... Quote Link to comment Share on other sites More sharing options...
thehelix112 Posted September 11, 2006 Author Share Posted September 11, 2006 Terry's car has a lot of these features: Top guard vents: http://www.fototime.com/ftweb/bin/ft.dll/standard?pictid={DDD2ED6F-8F41-44AA-9D6E-6A283C1D8801} Side guard vents: http://www.fototime.com/ftweb/bin/ft.dll/standard?pictid={A5694E3B-579E-49EF-9EE3-125F751F6A06} Dave Quote Link to comment Share on other sites More sharing options...
Challenger Posted September 11, 2006 Share Posted September 11, 2006 Do you mean fender when you say guard? If youd be able to create high pressure behind the top vent inside the fender the air would be forced out the top. But youd have to set up some kind of ballast so the high pressure wouldnt escape out the rear vent. You could use the rear vent only for air that gets sucked in behind the wheel to escape. Im no expert but It seems like it would work to me. Helix are you going to be using these vents on your race car or just wondering about it? Quote Link to comment Share on other sites More sharing options...
thehelix112 Posted September 11, 2006 Author Share Posted September 11, 2006 Yes I do mean fender when I say guard. Fender/guard/front quarter panel, same same. . I am not entirely sure what you mean by ballast in this context. I think the rotation of the wheels and the necessary shape of the rear fender well will mean there is a high pressure on top of the wheel. That and the air is already headed in that direction, and additionally compounded by the lower-than-ambient accellerated air on top of the fender. This pressure differential will mean lots of air gets drawn out, which will reduce drag and lift. Hence every closed-wheel race car you see has this, from the ALMS to the grand american daytona prototypes to terry's race car. Yes I will be using these vents on the race car. Dave Quote Link to comment Share on other sites More sharing options...
boodlefoof Posted September 11, 2006 Share Posted September 11, 2006 I'm having trouble envisioning the front diffuser you're describing. Could you draw a sketch of what you're talking about? Quote Link to comment Share on other sites More sharing options...
Challenger Posted September 11, 2006 Share Posted September 11, 2006 Im not sure on the air flow inside a fender im guessing some goes under the front airdam to the inside of the wheel (through suspension)and out the rear vent or under the car, some goes out the top and some escapes out the gap between the wheel and wheel well? If the wheels spinning wouldnt it push the air down not up, and there'd be a high pressure zone right in front of the wheel? Im not sure either about your diffuser Im guessing its just a horizontal wing to send air up the inside of the fender and out the top, instead of under the car? Quote Link to comment Share on other sites More sharing options...
thehelix112 Posted September 11, 2006 Author Share Posted September 11, 2006 John, http://www.mulsannescorner.com/diffuser.htm Just in front of the front wheel is a small upward arc. Thats the front diffuser. Mopar, I agree with your hypothesis on airflow inside the front fender. However, the air going around the wheel spinning cannot go down as the wheel meets the ground and blocks it. Hence there is a big stagnation point under the front of the wheel, which greats high pressure and hence pushes the air upwards and over the wheel. Dave Quote Link to comment Share on other sites More sharing options...
280Zone Posted September 12, 2006 Share Posted September 12, 2006 my fender vents, I would like to know they are actually functional : ) http://www.desertzassociation.com/membersrides/Robert/75z/pic/pic9.jpg Quote Link to comment Share on other sites More sharing options...
boodlefoof Posted September 12, 2006 Share Posted September 12, 2006 I think I understand the question now. You want to know whether your front splitter should smoothly transition into the belly pan or whether it should simply be a strip hanging down below the belly pan, right? If so, from what I've read on the subject (who knows how good that info is!) I don't think that transitioning the splitter should really matter directly in front of the wheels. The primary splitter action will be to minimize air going into the wheel area (divert it around the side of the car). Either way though, some air is going to hit the wheel. At the angle you would need to channel the air to pull it up and over the wheel (rather than hitting it head-on) the air would not follow the splitter anyway. From what I've read, at any angle over 10-15 degrees the air will detach rather than following the curve. However, inboard of the wheel, making a gradual transition might help to pull under-car airflow closer to the belly pan and thus might be beneficial if you have a 2/3 or 3/4 length diffuser. It might increase the venturii effect. Again, I'm not an expert in this area though. Just thinking out loud. Quote Link to comment Share on other sites More sharing options...
thehelix112 Posted September 12, 2006 Author Share Posted September 12, 2006 280Zone, If they have a hole in the appropriate bit through the fender, then yes, they are functional. I don't know if they're big enough to vent a whole lot, but some is better than none! John, Its hurting my head to think about that, so I'll be quiet for now. Dave Quote Link to comment Share on other sites More sharing options...
bjhines Posted September 13, 2006 Share Posted September 13, 2006 280Zone... I think your vents look great... but they do not allow much air through... Lets say we put 2 large vertical slots(openings) in the sides for air to escape.... air from all around the inside area can get to the openings... the air flows out in turbulently due to the fact that air comes toward the openings from all angles and exits the holes in the flat openings... A slightly better version would include louvers to smooth the airflow exiting the openings... the louvers would be oriented to act almost as scoops to catch air comming from the front and scoop it toward the openings... the number of different angles air can approach the openings is reduced.. this allows smoother flow and possibly better evacuation of the inner fender area.... Now here is a problem.... air cannot exit the openings from anywhere behind the louvers... they are blocking access to the openings from aft of the openings... this would imply that you need to almost entirely block flow past the openings or you will create a dead end that can only force the air that does not exit the openings to go down... under the car... thus adding to lift at speed... Another problem... mechanical design.. especially with fiberglass would like to have the tops and bottoms of the angled louvers capped in order to stabilize and support the louvers themselves... well again we reduce the number of directions that air can approach the openings... this would increase smoothness of flow... but it would also block air escaping from above or below the openings... thus creating another area that air can get into and only escape by going under the car... again we need something inside the fender to block flow to these areas... we would need another structure to funnel air towards the front edge of the louvers and block any airflow above, below, or past these openings... assuming we can funnel air to where it needs to go... we may find that a cosmetic air opening would not be nearly big enough to do the job... thereby forcing air under the car via a different path.... The top vents could still be used as long as the under-fender sheets to funnel air towards the gill openings still performed thie jobs... ei... not allowing air past the gill openings... ----------------------------------------------------------------------- The diffuser/ venturi aft of the nose splitter, under the chassis at the front wheel section.... is a way of reducing pressure and adding downforce on the front end... it is designed to make use of the small amount of air that cannot be redirected over or around the car... Air that goes under the front splitter flows smoothy along the bottom of the car and reaches a seciton right in front of the front wheels that begins to diverge upwards... as long as airflow is smooth it will flow laminarly up and accelerate in the taller cavity.... this will literally suck the nose of the car downwards... the underbody pan then reconverges with the ground in a smooth manner which will cuase a slight amount of increased pressure as it converges again... the idea is that the downforce is significantly forward of the area of lift... which will increase overall downforce on the front end...the area of lift is minimized by recoverging the surfaces in a gentle manner... Quote Link to comment Share on other sites More sharing options...
bjhines Posted September 13, 2006 Share Posted September 13, 2006 The problems I can see are in the overall complexity of the inner fender bodywork needed to direct air towards the gill openings... The top vents would be relatively easy... A proper set of gill vents would be easier to design with a widened front fender that simply left a tall vertical opening at the entire back edge of the fender... ie... getting rid of the side gill vents altogether and simply allowing a widened fender with a normally spaced door to leave a large vetical opening that would allow any and all air inside the front fender to escape... Quote Link to comment Share on other sites More sharing options...
bjhines Posted September 13, 2006 Share Posted September 13, 2006 These fender vents look really cool from the outside... they are very appealing.... but they have a VERY SMALL opening inside for air to escape... they do not appear to be very functional... perhaps the front inside edge couls be removed to allow more air to escape without visibly changing the appearance... The Ferrari 550 has some great looking vents that are both retro and large enough to be functional... I would love to see some pics of the inner fender to see what they have doen to direct air towards the openings... I think they are louvered inside as well... but I cannot find any good pics... ... Quote Link to comment Share on other sites More sharing options...
thehelix112 Posted September 13, 2006 Author Share Posted September 13, 2006 BJ, I agree with you on the side vents. There are some difficulties, but nothing that can't be overcome. Though I am quite aware its not a matter of simply cutting some holes. I have to disagree on the explanation of the front diffuser. According to Venturi's principle air flowing into an expanding region will slow down. Slower air has more static pressure, so in the case of a diffuser there is INCREASED pressure. This is not that case with a rear diffuser/underbody channel because of more complicated effects coming into play. Namely vorticies spilling inwards from the side of the car, and the reduced restriction to flow due to a more streamlined trailing edge of the car. I posted something on another forum about maybe the front diffuser's job is to minimise the air going into the wheel well. It could do this by having its forward facing section narrower than the rear one (as per the pic in S. McBeath). In this case both the high pressure in front of the wheel, and the higher pressure in the channel itself would, maybe, create a wedge for the faster moving air in between the points of the wedge to be diverted under the car where we want it. Getting complicated. I will have to sift through the mountain of information on Mullanes Corner at some point. Dave Quote Link to comment Share on other sites More sharing options...
bjhines Posted September 13, 2006 Share Posted September 13, 2006 ahh... yes... you are absolutely right... it is hard to keep perspective... it is a cavity not a wing... I was thinking in entirely the wrong perspective... One problem with top vents on the S-30 cars is that the tops of the fenders are not really "open"... the upper support of the chassis sticks out almost ot the outside edge of the fender... another candidate for the extended fender idea.... Quote Link to comment Share on other sites More sharing options...
thehelix112 Posted September 13, 2006 Author Share Posted September 13, 2006 Definitely. My starting point will be the YZ kit. Dave Quote Link to comment Share on other sites More sharing options...
boodlefoof Posted September 14, 2006 Share Posted September 14, 2006 Dave, While considering front diffusers more, an idea occurred to me that I hadn't thought about before. Maybe this isn't such an epiphany (obviously somebody thought of it before as you'll see below), but I hadn't considered it before. It wouldn't work directly in front of the wheel well, but between the them. So here it is... Rather than simply having the front diffuser smoothly transition up to the rear underpan (this limits the angle you can realistically create without raising the floor)... why not have the diffuser actually pass through the underpan and vent upwards? I envisioned the air flowing through the diffuser then connecting to a hood nostril vent, but it could also split to the sides and actually provide the brake ducting with cool air. The latter idea (splitting and exiting out the sides) is what I found while I was poking around at Mullane's myself. The Toyota GT-One used such a system. See http://www.mulsannescorner.com/gtone-1.html I'll have to take some measurements on my frame, but if I can fit it I think I might give this a try on my Z project. Quote Link to comment Share on other sites More sharing options...
thehelix112 Posted September 14, 2006 Author Share Posted September 14, 2006 That is the design of a front diffuser yes. I have finished reading Katz's book and think I've got a much better grasp on them now. Also after poking around Mullannes corner today, it seems that it doesn't matter so much where the air after the end of the front diffuser goes, aslong as it is out of the car and it is free to get there easily. In this case what you are trying to achieve is a front wing. The upsweeping underside coupled with a little high pressure on top, voila. I have just traced the 240z chassis schematics from the brakes & chassis forum from my laptop screen onto paper so I will have a sketch over the next little while and see what I can come up with. Will post results if anyone is interested. Oh, seeing as I plan on using the 280YZ, if anyone who has the kit could give me some measurements (any really) that'd be awesome. Dave Quote Link to comment Share on other sites More sharing options...
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