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 Post subject: Downforce
PostPosted: July 6, 2016, 2:01 pm 
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I am starting to work on Scrap Metals body and one of the first things I am going to do is make the diffuser. It is probably the most important pice for adding downforce so I want to build that the way I want it and work everything else in the rear around it.

It just so happens our pool has a waterfall giving the perfect opportunity to visually see the effects of downforce. I was surprised how strong the pull was. I'm going with a curved shape which will be a little harder to make but we all love a challenge right.
Attachment:
IMG_20160706_132634.jpg

Attachment:
IMG_20160706_132659.jpg

The two photos show fluid flow at different angles the top one about 60 degree and the 2nd around 45. It seemed like a gradual curve ending up around 45 degrees made the ideal flow. I also did some testing in the sink with similar results.

Any experts out there have a better suggestion for angles. I know that hunting for downforce in an Autocross car is somewhat of a losing battle but every little helps. I expect it will have more use than the current rolls of lead ballast.


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 Post subject: Re: Downforce
PostPosted: July 6, 2016, 3:18 pm 
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I'm not an expert, but I believe that all the water pulling away from the under side of your test piece constitutes the undesirable effect of flow separation. To the best of my understanding, what you'd be looking for is essentially the the steepest gradient that keeps the flow fully attached to the under side of the test piece. Typically, I've seen the rule of thumb that somewhere in the 7-10 degree range is about as much as the average person will get away with before flow separation starts to occur.

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 Post subject: Re: Downforce
PostPosted: July 6, 2016, 3:52 pm 
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Yes I have read about the 10-14 degrees angle but I believe that is in relation to a flat diffuser.

The water going around the side is because the waterfall is about 12" wide compared to the 4" wide model car. The water actually connected pretty well to the curve but it is not easy to test because the water pulls to model hard against the bricks, then no water can get below so I was attempting to maintain a relatively small gap from the wall.

When it was in the sink with the narrow flow there was virtually no separation. No ideal how the speed would scale up to a real car though. The faster the water speed the better the downforce. No idea if is that will represent 20mph, 50mph or 200mph.

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 Post subject: Re: Downforce
PostPosted: July 6, 2016, 7:00 pm 
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If you want to replicate aerodynamic effects with water, you should try to match the reynolds number of the two fluids at various speeds. A calculator can be found here:
http://airfoiltools.com/calculator/reynoldsnumber
Since the kinematic viscoscity of water is about an order of magnitude lower than air at the same temperature, you need to use either a model about 1/10th scale or scale your velocity in water by about 10, or a combination of the two. Keeping the same reynolds number should result in the same flow characteristics in the two fluids. So, if your model diffuser length is, say, 1/4 the length of your final version, you want your water speed to be 1/2.5 the design speed of the airflow.


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 Post subject: Re: Downforce
PostPosted: July 6, 2016, 9:36 pm 
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+1 to Posthumane ..... Reynolds number matching should help model diffuser performance. Absent that, your swimming pool test is but a notional concept demonstration of why a diffuser should work, but not whether any particular one will.

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 Post subject: Re: Downforce
PostPosted: July 7, 2016, 10:17 am 
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Well looking at the calculator I instantly realize I am in over my head with the calculations. I don't have much time to complete the body so I am going to go with what looks good and fits well in the space. I may do some ribbon/string tests on the car to see what the air is actually doing. Ultimately I do not think that aero is really going to do anything to my times on an autocross course. I think my best hope it to try to reduce some of the lift the car naturally generates.

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 Post subject: Re: Downforce
PostPosted: July 7, 2016, 11:18 pm 
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Well here is what I ended up with. Not the best quality but it should be sturdy enough once it is connected to the floor, fenders and rear (Next on the to do list). Not sure how well it will work but it is at the bottom rear of the car so a good place for ballast if it is useless.
Attachment:
IMG_20160707_212544.jpg
Attachment:
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Attachment:
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 Post subject: Re: Downforce
PostPosted: July 8, 2016, 2:51 am 
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That's starting to look good but it needs to be much bigerer and turn it 90 degrees to face the wind! :rofl:

Maybe the end plates can be hinged plexi or lexan and be windows on the side of the car.


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 Post subject: Re: Downforce
PostPosted: July 8, 2016, 8:14 am 
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haha. I would love to make a roof wing but it is not class legal. I actually have a fairly nice 2 element wing, if and when I decide to track the car I will probably make a removable roof mount for it.

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 Post subject: Re: Downforce
PostPosted: July 8, 2016, 2:15 pm 
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wrightcomputing wrote:
haha. I would love to make a roof wing but it is not class legal.
As long as the upper and lower surfaces are not converging, it's not a 'wing'... :wink:

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 Post subject: Re: Downforce
PostPosted: July 8, 2016, 4:44 pm 
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Full side plates above and below help to reduce wind "spillage" off the sides of the wing and loss of lift.
It stops the trailing vortices. I always loved those words.


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 Post subject: Re: Downforce
PostPosted: July 9, 2016, 9:17 pm 
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Here she is mounted to the car. The front is tucked under the floor about 2" and held in place with a few sheet metal screws. In the center there are 2 grade 8 bolts going directly to the chassis. I think I will also add a plate under those bolts to make sure the large washers don't rip through the think aluminum. I added the triangle plates on the rear which made the sides plates solid. There is also a 1" aluminum bar at the top which will also connect the the fenders and rear of the car eventually. I think I did a pretty good job.
Attachment:
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 Post subject: Re: Downforce
PostPosted: July 9, 2016, 9:56 pm 
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wrightcomputing wrote:
... I think I did a pretty good job...

Other than those sharp corners ready to make you bleed. Please take a file to those to protect yourself and other!

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 Post subject: Re: Downforce
PostPosted: July 9, 2016, 11:20 pm 
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KB58 wrote:
wrightcomputing wrote:
... I think I did a pretty good job...

Other than those sharp corners ready to make you bleed. Please take a file to those to protect yourself and other!

lol, How in the world would someone get cut by my diffuser.

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 Post subject: Re: Downforce
PostPosted: July 10, 2016, 12:21 am 
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Virtually every diffuser I've ever seen also has several strakes between the 2 end plates to further divide the air and keep it from separating.

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