MKII CFD Aerodynamics Analysis - MR2 Owners Club Message Board
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post #1 of 345 (permalink) Old December 29th, 2010, 15:55 Thread Starter
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MKII CFD Aerodynamics Analysis

Finally got started on a project I have been wanting to tackle for quite some time now. A pictures worth a thousand words... so:



The model has a few things I need to add before it will really be ready to start testing, but its pretty close. My knowledge of flow works and aerodynamics however, need a lot of work. If anyone wants to see specific types of data or specific configurations tested just let me know and I will try to oblige. My goal is to throw some data out there and let the community analyses it, since in this field I think I would have as good of a chance at getting it wrong as right

Before anyone asks, no I didn't model it from scratch, it started as a Google Sketch Up model that I downloaded from their library and modified. I did spend about a day getting it into a solidworks friendly format, and another half day or so adding internal details.

Last edited by Alex W; July 10th, 2017 at 12:55.
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post #2 of 345 (permalink) Old December 29th, 2010, 16:03
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Would be interesting to see air flow from the side scoops and air flow from a tmic setup. Also flow from a border style hood or a more aggressive cut front hood.

Nice work!
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post #3 of 345 (permalink) Old December 30th, 2010, 06:21
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very cool! I built a rear diffuser and vertical TMIC...love to see what actually goes on under there. My ideas are all theory and very little proof.
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post #4 of 345 (permalink) Old December 30th, 2010, 17:07
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I'd be very interested as to whats happening at the rear of the car!
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post #5 of 345 (permalink) Old December 31st, 2010, 10:05
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very interesting! This could open many many doors for us, to really tweak on our Aerodynamics and get the best bang! like the guys said above me, tmic set up, air flow trought side scoops and air flow with boarder and other side scoops add on, to see if they actually help.

Keep us updated
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post #6 of 345 (permalink) Old January 2nd, 2011, 09:52
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Quote:
Originally Posted by Blurvision
Would be interesting to see air flow from the side scoops and air flow from a tmic setup. Also flow from a border style hood or a more aggressive cut front hood.

Nice work!
this wouldnt tell you much, aggressive cut hoods are generally set up with the radiator slanted so that the air flows across the surface of it primarily, but not through it, where border hoods are set up with the idea that you are relocating the radiator so that air flows through it and out the border vents, terminal velocity through a radiator is about 60mph (which explains the smaller vents used). In the more aggressive style there is less drag (no radiator to flow through), meaning higher velocity and a need for a larger vent.

Modeling and showing an accurate representation of the border hood would be a head ache and would likely be really difficult to get numbers that are useful. For the border hood I would simply take it at face value, the border racing team designed it, tested it and raced it, chances are its probably legit.

The more aggressive style would be alot easier to model, less variables involved there. I would be really interested to see what kind of data it reveals.
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post #7 of 345 (permalink) Old January 2nd, 2011, 11:02 Thread Starter
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Quote:
Originally Posted by Eversor
this wouldnt tell you much, aggressive cut hoods are generally set up with the radiator slanted so that the air flows across the surface of it primarily, but not through it, where border hoods are set up with the idea that you are relocating the radiator so that air flows through it and out the border vents, terminal velocity through a radiator is about 60mph (which explains the smaller vents used). In the more aggressive style there is less drag (no radiator to flow through), meaning higher velocity and a need for a larger vent.

Modeling and showing an accurate representation of the border hood would be a head ache and would likely be really difficult to get numbers that are useful. For the border hood I would simply take it at face value, the border racing team designed it, tested it and raced it, chances are its probably legit.

The more aggressive style would be alot easier to model, less variables involved there. I would be really interested to see what kind of data it reveals.
I don't think it will be too difficult to model a "close enough" border hood. I also don't think that the more aggressive cut hoods are usually setup to bypass the radiator as you are saying. The air still has to flow through the radiator in any of the setups I have seen pictures of.

I do have the radiator modeled as a "porous medium" in my analysis, but I have been unable to find any good data on the actual flow characteristics of a radiator (pressure drop vs flow rate, that sort of thing), so its basically a guess right now.
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post #8 of 345 (permalink) Old January 3rd, 2011, 16:11
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alot of the agressive hoods i have seen on here have the radiator angled like a ramp, where on the border car its an inverted ramp. Assuming the car is facing to the left it would look like this in a side view: aggressive -> "/" where the border car has it more like "\"

in the agressive styles the air going through the radiator goes under the car in the same manner as stock, where with the border style you could seal the bottom of the car from the bumper back. If you can show the radiator even somewhat accurately I would be interested to see which way is more efficient.
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post #9 of 345 (permalink) Old January 3rd, 2011, 16:18
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The problem with the stock rad is the bulkhead right behind it, its like pushing a flat plate through the air. With the bulkhead removed and the air let out an opening in the hood you will get better down force up front and less wind resistance with the bulkhead removed.

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post #10 of 345 (permalink) Old January 3rd, 2011, 16:21
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post #11 of 345 (permalink) Old January 3rd, 2011, 18:33
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awesome... I need that with the passenger and drivers side window down. that exact car please. no spoilers or fancy riceboy crap. One wiper in the middle, with the wiper straight up and down, and one with the wiper in the down position in front of the driver.

thanks

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post #12 of 345 (permalink) Old January 3rd, 2011, 21:33 Thread Starter
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Quote:
Originally Posted by Bill Strong
awesome... I need that with the passenger and drivers side window down. that exact car please. no spoilers or fancy riceboy crap. One wiper in the middle, with the wiper straight up and down, and one with the wiper in the down position in front of the driver.

thanks
I don't know if I will ever get as detailed as to model different wiper versions (although it probably wouldn't be all that hard), but I was definitely planning on trying to figure out how much difference having the windows down and / or T-tops off makes.
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post #13 of 345 (permalink) Old January 3rd, 2011, 21:36
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Would love to see a 91 vs 94 vs 98 wing comparison and what effect (if any) each of them has.

Would also like to see the result of what sealing the rear underbody similar to an elise would do.

Similarly upfront I would like to see the difference of sealing the front underbody and venting air through the rad out the frunk would do but you seem to already have that covered.

Last edited by Levi; January 3rd, 2011 at 21:38.
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post #14 of 345 (permalink) Old January 3rd, 2011, 22:03
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I want to see how bad out 1991 race car is.


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post #15 of 345 (permalink) Old January 4th, 2011, 12:03
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wow, safe to say that car has been put through its paces
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post #16 of 345 (permalink) Old January 7th, 2011, 08:17
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yep... just needs to be waxed, a new front engine mount and a new exhaust made and it will fly again!
http://modsquadracing.blogspot.com/p...g-records.html

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post #17 of 345 (permalink) Old January 7th, 2011, 12:36
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impressive, I was just thinking though about the rack of lights and aero... Not sure if the windshield wiper will make that much difference in comparison.
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post #18 of 345 (permalink) Old January 8th, 2011, 08:27
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we only use those on 24 hour races. With all the body damage the lights really don't make a difference

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post #19 of 345 (permalink) Old January 9th, 2011, 15:11 Thread Starter
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OK, some results!

First of all, a few notes:
1. While I have attempted to include as much detail as I feel is reasonable, a lot of the details I have added are very quick and dirty, guesses at dimensions, and very simplified. However, I think for our purposes the models should be OK. I am more looking to validate concepts and get a general idea than to get perfect results.

2. For reasons unknown to me you will notice velocity vectors coming up off of the road surface. I don't know why, but they do disappear under the car, so I don't think they are significantly effecting anything.

3. When I run my model without the car (just the road), I get a drag force of 18lb and a lift force -2.3lb. I have the road setup as a moving wall at the same velocity as the air stream (80mph), so I would think that it would not have an effect, but it still does.

4. Related to #3, I don't think I can calculate forces on just the car, they can only be done as "global" goals, IE they will include the road. I have subtracted the above "road" values from the totals for each test.

5. Related to #4, unfortunately I don't think I can calculate separate front and rear lift / downforce numbers, just a total.

If anyone knows more about FlowWorks than I and can provide input on the above issues 2-5, please let me know.

6. Obviously I cannot possibly provide all possible angles, flow trajectories, or pressure plots. I have provided some basic plots, but can easily go back and get more details on any particular area if someone is interested.


Here is my basic model with a very simplified engine, exhaust, rear cross member and suspension:




And front end / radiator configuration with a 93 lip:


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Last edited by Alex W; July 10th, 2017 at 12:56.
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post #20 of 345 (permalink) Old January 9th, 2011, 15:14 Thread Starter
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In the above 93 lip configuration I get a lift force of 47 lb and a drag force of 98 lb. The drag numbers seem awfully low to me, but when I calculate a drag coefficient from them I get .302 in this configuration. The published number I have found for the MKII is about .31, so I think I am pretty close.

Velocity, scaled 0-80mph

Pressure, scaled 14.6-14.8psi





Last edited by Alex W; November 11th, 2018 at 22:06.
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