Diffuser Tuft Testing - LotusTalk - The Lotus Cars Community
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post #1 of 34 (permalink) Old 01-14-2018, 01:09 PM Thread Starter
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Diffuser Discussion & Testing

I recently installed a 5 Element Railer diffuser from Difflow, taped on some wool tufts to the two outer channels and took some video of how the airflow causes them to move.

I did 3 tests: 1.) standard 5 element railer 2.) 5 element railer + aero plates 3.) 5 element railer + aero plates + extensions

TLDR; airflow appears to be turbulent in the large, outer channels of the 5 element railer diffuser. Aero plates didn't make a notable improvement. Extensions that start ~250mm forward of the standard strakes and extend 50mm closer to the ground resulted in a notable improvement, as witnessed by the wool tufts.

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post #2 of 34 (permalink) Old 01-14-2018, 03:31 PM
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Very nice work. Thanks. Good info.
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post #3 of 34 (permalink) Old 01-14-2018, 04:34 PM
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I have been told by the "Aero Gods" that what you want to do on the outer down rakes is place a one inch 90 degree brake pointing towards the tires. This should help the problem of making the outer rakes so long thus hurting the ground clearance problems.

Later,
Eldon
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post #4 of 34 (permalink) Old 01-15-2018, 11:10 AM
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Nice work!

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post #5 of 34 (permalink) Old 01-15-2018, 11:38 AM
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Originally Posted by eldonz View Post
I have been told by the "Aero Gods" that what you want to do on the outer down rakes is place a one inch 90 degree brake pointing towards the tires. This should help the problem of making the outer rakes so long thus hurting the ground clearance problems.

Later,
Eldon
So basically, 90 degree bend on the outside strake like in this diffuser?


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post #6 of 34 (permalink) Old 01-15-2018, 11:39 AM
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Correct!

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post #7 of 34 (permalink) Old 01-15-2018, 11:47 AM
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Originally Posted by jdawson View Post
I recently installed a 5 Element Railer diffuser from Difflow, taped on some wool tufts to the two outer channels and take some video of how the airflow causes them to move.

I did 3 tests: 1.) standard 5 element railer 2.) 5 element railer + aero plates 3.) 5 element railer + aero plates + extensions

TLDR; airflow appears to be turbulent in the large, outer channels of the 5 element railer diffuser. Aero plates didn't make a notable improvement. Extensions that start ~250mm forward of the standard strakes and extend 50mm closer to the ground resulted in a notable improvement, as witnessed by the wool tufts.
Great work, and it confirms the study in Racecar Engineering that the outside elements of the Lotus diffuser seem to have too great of an angle of attack to work properly without much longer strakes.

Coincidentally, I just installed a new diffuser on my Lotus and tested it at Streets of Willow yesterday. Picture here: https://www.instagram.com/p/Bd4TAC0j...=last111_lotus (Follow me on Instagram! Or don't...) The S3 diffuser isn't quite the same as the S2 diffuser, but Lotus actually extended the larger outside elements all the way right next to the exhaust.



For my diffuser, I actually copied the angle of attack geometry of the center element (the one containing the exhaust) and extended it all the way across the diffuser. Thus, my new diffuser has a lower angle of attack compared to the old one. However, through Turn 1 and the chicane/waterfall at Streets of Willow, running clockwise, the car was a bit more stable. So, at least my butt is telling me that the lower angle of attack is actually beneficial.

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post #8 of 34 (permalink) Old 01-15-2018, 11:53 AM
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Like this!

Keep in mind, too, a splitter works almost entirely against the diffuser since it is sending air up rather than through the underbody. Side skirts are also helpful for keeping the air trapped under the underbody rather than allowing it to escape before the diffuser.
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post #9 of 34 (permalink) Old 01-15-2018, 12:49 PM
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Thanks for this, I know what I'm doing before the diffuser goes back on the car!

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post #10 of 34 (permalink) Old 01-15-2018, 01:27 PM
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Like this!

Keep in mind, too, a splitter works almost entirely against the diffuser since it is sending air up rather than through the underbody. Side skirts are also helpful for keeping the air trapped under the underbody rather than allowing it to escape before the diffuser.
I'm afraid I'll have to disagree with your statement about the splitter. After watching all of the teams test different splitter designs and radiator pan configurations, you can get them to create a considerable amount of down force on the front. Then the air moving under the splitter is used all the way to the back of the car. Creating a vertical lip on the leading edge of the splitter will create more down force on the front of the car.

Later,
Eldon
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post #11 of 34 (permalink) Old 01-15-2018, 01:30 PM
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I'm afraid I'll have to disagree with your statement about the splitter. After watching all of the teams test different splitter designs and radiator pan configurations, you can get them to create a considerable amount of down force on the front. Then the air moving under the splitter is used all the way to the back of the car. Creating a vertical lip on the leading edge of the splitter will create more down force on the front of the car.

Later,
Eldon
Image please?

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post #12 of 34 (permalink) Old 01-15-2018, 01:52 PM
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Kevin,

Doing this type of lip is hard with a car that places the splitter close to the ground is difficult. You will have a tendency seal off the splitter or tear off the lip going over curbing. I have no image to show you, all I can do is describe it. At the leading edge of the splitter, place a vertical lip down towards the ground. The length of the lip will vary depending on how high you splitter is off the ground. A minimum distance for the lip would be at least 3/4" What you are trying to do is create a wicker bill effect like what you would do with the trailer edge of a wing. The difference is that the low pressure is behind the lip thus increasing the speed of the air under the splitter. The splitters have become nothing more than an inverted wing for the front of the cars. You also want the center of the splitter to be higher than the edges near the outside. This also encourages the air to move down the center of the car and not get disturbed by the front wheels.

Later,
Eldon
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post #13 of 34 (permalink) Old 01-15-2018, 04:34 PM
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Kevin,

Doing this type of lip is hard with a car that places the splitter close to the ground is difficult. You will have a tendency seal off the splitter or tear off the lip going over curbing. I have no image to show you, all I can do is describe it. At the leading edge of the splitter, place a vertical lip down towards the ground. The length of the lip will vary depending on how high you splitter is off the ground. A minimum distance for the lip would be at least 3/4" What you are trying to do is create a wicker bill effect like what you would do with the trailer edge of a wing. The difference is that the low pressure is behind the lip thus increasing the speed of the air under the splitter. The splitters have become nothing more than an inverted wing for the front of the cars. You also want the center of the splitter to be higher than the edges near the outside. This also encourages the air to move down the center of the car and not get disturbed by the front wheels.

Later,
Eldon
I understand, thanks. I will have to see if I can find a way to apply this to mine... Good point about being higher in the center. I wonder what I can make happen there.

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post #14 of 34 (permalink) Old 01-15-2018, 04:59 PM
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Originally Posted by eldonz View Post
I have been told by the "Aero Gods" that what you want to do on the outer down rakes is place a one inch 90 degree brake pointing towards the tires. This should help the problem of making the outer rakes so long thus hurting the ground clearance problems.

Later,
Eldon
Basically, trying to keep the “dirty” air off the rear tires from adversely affecting the operation of the diffuser. One also can move some of the underbody airflow smoothly around the rear tires to minimize the problem.

After adding two small vortex generators on each side of my underbody ahead of the rear tires to guide the air, I had to increase the length of my front splitter and bridge the underside gap to the body to regain my aero balance. Previously alluded to this here: https://www.lotustalk.com/forums/f163...3/#post3690641
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post #15 of 34 (permalink) Old 01-15-2018, 05:27 PM
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Basically, trying to keep the “dirty” air off the rear tires from adversely affecting the operation of the diffuser. One also can move some of the underbody airflow smoothly around the rear tires to minimize the problem.

After adding two small vortex generators on each side of my underbody ahead of the rear tires to guide the air, I had to increase the length of my front splitter and bridge the underside gap to the body to regain my aero balance. Previously alluded to this here: https://www.lotustalk.com/forums/f163...3/#post3690641
Yes, you definitely do not want a gap from the back of the splitter to the underbody.

Later,
Eldon
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post #16 of 34 (permalink) Old 01-16-2018, 03:14 AM
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I'm afraid I'll have to disagree with your statement about the splitter. After watching all of the teams test different splitter designs and radiator pan configurations, you can get them to create a considerable amount of down force on the front. Then the air moving under the splitter is used all the way to the back of the car. Creating a vertical lip on the leading edge of the splitter will create more down force on the front of the car.

Later,
Eldon
When you have to have the full power of professional teams, a full-scale wind tunnel, PhD engineers, etc, to provide an exception to my point then I think you missed the general sentiment.

I'll rephrase - a generic splitter design such as is on nearly every road car and lowish-budget race team tends to divert more air upwards than downwards, especially under braking when the nose dives. Just let this digest and think of examples and you'll see that it is true.

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post #17 of 34 (permalink) Old 01-16-2018, 04:18 AM
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When you have to have the full power of professional teams, a full-scale wind tunnel, PhD engineers, etc, to provide an exception to my point then I think you missed the general sentiment.

I'll rephrase - a generic splitter design such as is on nearly every road car and lowish-budget race team tends to divert more air upwards than downwards, especially under braking when the nose dives. Just let this digest and think of examples and you'll see that it is true.
What you said is what has been believed to be true for years. What I'm trying to tell you is that what is being found out is something different. And "Yes", having access to professional teams, a full-scale wind tunnel and PHD Aero engineers is how I'm learning these things. It is not the air going over the car that is important, it is the air going under the splitter that is creating the added front down force.

Later,
Eldon
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post #18 of 34 (permalink) Old 01-16-2018, 04:35 AM
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What you said is what has been believed to be true for years. What I'm trying to tell you is that what is being found out is something different. And "Yes", having access to professional teams, a full-scale wind tunnel and PHD Aero engineers is how I'm learning these things. It is not the air going over the car that is important, it is the air going under the splitter that is creating the added front down force.

Later,
Eldon
Right, with a contoured underside to the front splitter I am sure. To split hairs though, that is a second component to the splitter. That's in the works for mine once I get through higher priority things. Adding a profiled underside is with almost no disadvantages. It even doesn't hurt ground clearance too bad if the lowest point aligns with the front axle.

Going back to the diffuser, more low-hanging fruit are the NACA ducts that bleed air away from the diffuser. I taped mine off and used an oil temp gauge to confirm they did nothing. They might cool the transmission, but I am not aware of that ever being a failure point.

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post #19 of 34 (permalink) Old 01-16-2018, 07:55 AM
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Right, with a contoured underside to the front splitter I am sure. To split hairs though, that is a second component to the splitter. That's in the works for mine once I get through higher priority things. Adding a profiled underside is with almost no disadvantages. It even doesn't hurt ground clearance too bad if the lowest point aligns with the front axle.

Going back to the diffuser, more low-hanging fruit are the NACA ducts that bleed air away from the diffuser. I taped mine off and used an oil temp gauge to confirm they did nothing. They might cool the transmission, but I am not aware of that ever being a failure point.
You would be surprises to know that you do not even have to go that far to make the basic gains. You would be even more surprised by what they are finding on production cars. I have already mentioned one of these things to do.

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post #20 of 34 (permalink) Old 01-16-2018, 08:24 AM
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You would be surprises to know that you do not even have to go that far to make the basic gains
No surprise here. To make it relevant to this thread, just look at the gains to be had on our diffuser despite the fact that we have one of incredibly few cars on the road with a functional one to begin with. Start with a street car that favors looks over performance (just look at the number of rear bumper valances that try to look like diffusers with strakes...), and the pickings should be easy. Even with sports cars, this is still true - remember I worked in factory supported sports car racing for several years.

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