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Affect of front air-dam opening on splitter effectiveness


thehelix112

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Hey guys,

 

I was pondering over coffee the possible effect of the front air-dam on the splitter's ability to generate downforce.

 

From the diagram, if you increase Y (downwards) and decrease X, it would seem to me (assuming that the air flowing into this opening can do so at an adequate rate) that you will reduce the frontal downforce generated by the splitter.

 

The way a splitter works is by placing a high static pressure (stagnation point) region on the upper surface, and allowing air to flow quickly underwards, creating a low static pressure region. High pressure on top, low pressure underneath means downforce.

 

By reducing X (whether by increasing Y or not) we are reducing the amount of air that will slow down (and increase static pressure) on top, and thereby reducing the pressure differential and reducing downforce.

 

Does that make sense?

 

Dave

splitter_thumb.jpg

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Yeh I've thought about the subject too Dave and think your reasoning is correct.

 

Of course there are other factors at work too that make it all very complicated, such as the vertical effect (lift and downforce) of air passing through the front openings on the surfaces of such openings.

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The way a splitter works is by placing a high static pressure (stagnation point) region on the upper surface, and allowing air to flow quickly underwards, creating a low static pressure region. High pressure on top, low pressure underneath means downforce.

 

That's what I though too, but its not quite right. A properly designed splitter works by channeling and accelerating air underneath the car into a front diffuser. A properly designed splitter actually points up at about a 2 or 3 degree angle to accerate and compress the air under the front of the car before the air enters the front diffuser.

 

Its counter-intuitive and took me a while to understand, but a splitter by itself is just a fancy spoiler/air dam. Adding a diffuser at the front wheel centerline makes a splitter really work. Even without a front diffuser, a splitter in conjunction with a flat undercar works by extending the length of the low pressure area under the car.

 

http://www.mulsannescorner.com/audir8nose.htm

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John,

 

I would dispute that that is not how a splitter works. A splitter does not have to be feeding a diffuser to act as a splitter and generate downforce.

 

The latest trend for splitters is as an inverted aerofoil shape, as you say, for the purpose of accelerating the air up into the diffuser. Note: this will not compress the air, but will see a drop in static pressure along the underside surface.

 

I don't find it that counter intuitive at all, a front diffuser will only ever work if its exit path is extremely low-resistance. Like out the side of the car where a very low pressure vortex exists (as generated by a dive plane).

 

 

I definitely agree though that a splitter in complete isolation is far inferior to a splitter with a flat underbody, which is also far inferior to a splitter with a front diffuser.

 

 

Dave

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Have you all seen the NASCAR Car of Tomorrow (COT)? Butt ugly thing, but they recessed the splitter and in doing so left a lip right above it. I think the idea there is that it will be largely protected by the "bumper" above it, and the splitter itself will still produce downforce. Might take a look and see if you can determine what shapes they used on the bottom lip for the bumper, because that seems like it would be important. I would tend to think you would want to radius the BOTTOM of the Y area, and the bottom of the grill opening as well, and would want a sharp area at the hood and at the top of the lower opening. My thought is that this would be more inclined to keep the flow attached to the lower surface that you want pressure on, and less inclined to have attached flow on the upper surface. That's a WAG though...

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Hmmm... just to make sure I understand, in the picture, the diffuser is the piece in front of x. Is that correct? I have always increased x to increase downforce (which gets ya in trouble with curbs etc..). Does anone know the relationship between the length of the diffuser (out from x), and the length of x to downforce? I would like to be able to reduce x somewhat and am not sure what would be the optimum lengths. If you reduce x and add a diffuser what will happen to the downforce?

 

Thanks!

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I work for Nascar busch and truck shop the noses on the trucks look simular in shape to your drawing . for less drag on the trucks all the front edgdes that face the wind are radiused , the traling edges are sharpened to 90 degrees ( ie back off wheel wells back side of A , B posts . On our down force stuff every thing is sharpened break the wind. I cannot comment on the relationship between the area between the splitter and the bumper cause we have templates to follow :shifty:

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demidion,

 

The splitter is indeed the bit out from x, but really as johnc has pointed out, the whole front undertray is closely related.

 

I'll have a hunt through the aero books I have to see if they mention the relationship between x and splitter length. From what I remember though, they only discuss the splitter length in isolation, but we'll see.

 

BattleKat,

 

Radiusing the front edges makes sense. Though I don't see why you wouldn't radius some trailing edges as well, depending on whether or not there is anything else downstream. In the case of the trailing edge of the front air-dam (that leads out into the side of the wheel), I will be radiusing the inner edge, and adding a small groove (will try to find the pic on mullannes corner) to redirect the air outwards. This is all in an attempt to give the high pressure zone in front of the wheel somewhere to bleed off (out the side).

 

Thanks for your input, its always great to hear from people actually using this stuff day-to-day. :)

 

Dave

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Helix , the way this was explained to me was simple using a flat bench take an air blower and blow air parallel of the edge using your free hand find the path of air . It should be 6 -8 inches away from the edge of the table. Now take a round object ( atleast an inch in diameter for this experiment ) and place it on the edge of the table and blow the air across it you should find the air 6-8 inches below the table , the air will follow the radius and a sharp edge will negate the flow

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Helix.. It is my understanding that the radiator/intercooler presents a very odd flow restriction...

 

In a nutshell... It will restrict the air flowing through it to a little less than 20 MPH no matter WHAT speed the car is moving... I have seen data on this that showed speeds all the way to 200MPH and virtually no change in airspeed through the radiator...

 

So.. that faster you go the more stagnant this area becomes... and you will find that even though the radiator may be several square feet in frontal area.. You only need a 2"x4" slit to allow air to enter the nose at highway speeds and above... even if you don't mask off the front of the car.. It will only allow this much airflow at speed.. so It might as well have a mask on it...

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BJ,

 

Tis a good point. Even if it is a ducted intake as I was assuming, the whole point of ducting the intake is to slow the air down as it passes through the car and to stop unnecessary reversion between the front of the car, and the radiator. I think...

 

In the case it is a nicely ducted intake, the air will still be slowing down as it flows through it, so I do agree that the airspeed in there is probably (on average) quite slow. And as the airspeed on the underside of the front splitter/diffuser is (should be) quite fast, I do agree that this would generate downforce.

 

My apologies to battlekat and his speedway engineer. My mistake.

 

Dave

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Great thread... I'm learning a lot! (At least I think so)....

 

Ok, so what it sounds like is we could eliminate either the opening y or the opening above y without changing the amount of air flowing through the radiator... I would think that at lower speeds the fan (if ya have a good one), would still pull enough air through the radiator, correct? So if we were to eliminate the opening y, x would go all the way up to the next opening ( which would then become y I suppose), and it should at that point, while not nessesarily contibuting to more downforce, it will cause more air to flow over the car instead of bunching up in front of the radiator, therby lowering the cars coefficient. Or am I way off here?

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Great thread... I'm learning a lot! (At least I think so)....

Good to hear someone other than me is learning things. I like learning.

 

 

Ok, so what it sounds like is we could eliminate either the opening y or the opening above y without changing the amount of air flowing through the radiator...

Thats kinda a simplistic view. What BJ is saying is that you can't force anymore air through the radiator than its capable of flowing. Ie, it is the bottleneck, so its no point having an opening that supports more than this flow. As soon as the opening flows less than the radiator, then the opening is the restriction, and you are lossing cooling capacity.

 

 

I would think that at lower speeds the fan (if ya have a good one), would still pull enough air through the radiator, correct?

Not if the opening won't flow it. Pulling by a fan, pushing by the wind, same same: the idea is to create a pressure differentail between one side of the radiator and the other. The higher the differential, the more flow. I think.

 

So if we were to eliminate the opening y, x would go all the way up to the next opening ( which would then become y I suppose), and it should at that point, while not nessesarily contibuting to more downforce, it will cause more air to flow over the car instead of bunching up in front of the radiator, therby lowering the cars coefficient. Or am I way off here?

 

That sounds simple, but its a very interesting question. Don't have time to think about it now. Will get back to it.

 

Dave

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