printed rear diffuser

Topdownman · July 18, 2018, 15:02

Rear seats would go some way to quashing all those criticisms that the 2 isnt practical.

Otherwise, rear spats would look damn good...

Mr X · July 18, 2018, 16:40

haha rear seats would be good but he reason we have an mr2 is because the engine is in the correct place!

I thought about putting in some spats (or something similar) but that's really counter productive. Function over fashion

I've still not looked at this software yet... I would also need a CAD model of the mk3 (or draw it up myself). Looking at simulations from other cars, it seems being half closed half open is probably the middle of the pack. As an open cylinder (like an F1 car) creates turbulence behind the tyre.
Putting on a spat is like having on a parachute.

Looks like ideally the bodywork should be flush with the outer face of the wheel with a in and out diffuser channel directly behind the wheel from the inside to out. This would normally vent the rear breaks too I would imagine.

I've never even been on the track in my car so it's not as if my ideal goal is to be able to go around hammerhead at 90

dan944 · July 18, 2018, 16:47

Quote from: Mr X on July 18, 2018, 16:40
haha rear seats would be good but he reason we have an mr2 is because the engine is in the correct place!

I thought about putting in some spats (or something similar) but that's really counter productive. Function over fashion
I've still not looked at this software yet... I would also need a CAD model of the mk3 (or draw it up myself). Looking at simulations from other cars, it seems being half closed half open is probably the middle of the pack. As an open cylinder (like an F1 car) creates turbulence behind the tyre.
Putting on a spat is like having on a parachute.

Looks like ideally the bodywork should be flush with the outer face of the wheel with a in and out diffuser channel directly behind the wheel from the inside to out. This would normally vent the rear breaks too I would imagine.

I've never even been on the track in my car so it's not as if my ideal goal is to be able to go around hammerhead at 90

Haha. I get what ya mean though. Again looking at Kyle.engineers work on the tube it can be seen that when he studies it wheel cut outs provide good aero.

Mr X · July 19, 2018, 08:56

I forgot I had a pointcloud..

Talking about spats reminded me I took a scan of the rear corner back when I had a scanner. Just in case I ever wanted to do something with it.
I'm sure something will be on the cards once I'm back from holiday

Mr X · July 20, 2018, 18:44

I just thought. I could just not jack the car up with the edge parts of the diffuser on?? $:-\$

twiglett · July 25, 2018, 09:12

I'll see if I can find some pictures of my racecar aero set-up.
That had a full flat floor with a splitter and diffuser incorporated.

dan944 · July 25, 2018, 15:21

Quote from: twiglett on July 25, 2018, 09:12
I'll see if I can find some pictures of my racecar aero set-up.
That had a full flat floor with a splitter and diffuser incorporated.

This is what I want!!

From what I gather it's the flat floor that will give biggest gains on a road car.

jvanzyl · July 25, 2018, 18:13

This is a good read... You guys do know that Laurence does the aerodynamics modelling for Williams? Lgriffiths I think- you should ask him for his input as I reckon he's rather qualified...

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lgriffiths · July 25, 2018, 23:03

Nice to see some DIY aerodynamics, kudos for finding the motivation to make some physical parts! All I can advise is find a way of viewing the probablem so you can see if you've fixed it. What simulation software you using?

You've managed to fit in a lot of fences. If you simulate it experiment as well with taking a couple out. it's worth examining the area evolution through the diffuser, flow won't take kindly to rapid area expansions, if there are any. At a guess I would imagine the flow in that central section around the cat will want some support though, likely to separate without it.

Try some DIY aero tests... if you can find the time. Could mount a camera looking at the diffuser, stick some wool Tufts all over it and do a video. Or mix up some flowviz paint, just don't do it near your house, that stuff is messy.

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Mr X · July 26, 2018, 22:15

Thanks for the input. I'm actually on holiday at the moment. I took the car out for a good drive on Monday (I left on Tues) and it felt a lot more planted, especially on roundabouts. What I need to look into now is the angle of the exit around the CAT, as you say. This however does become a problem because from what I understand (I didn't take a course on this stuff or goto collage etc lol) the angle that flow separation occurs depends on the speed of the airflow as well as the height of the car, air temp etc. So really it has forever moving goalposts on a road car.

lgriffiths · July 26, 2018, 22:40

It's easy to overthink it. Best practice will sort most of it.

With any aero you need to decide if your going for low drag or high downforce.

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Mr X · August 12, 2018, 17:29

ok, my re-design is done to comply with the 7-11deg suits most cases angle.

The CAT can be seen in this one and red part is the bottom of the original bumper.

Unfortunately it means I need to re-make the whole thing as none of the first version is re-usable.
got quite a length out of it without getting any closer to the CAT. I extended the diffuser out in the middle by 40mm to give it a bit of extra height. I left the dropped section open to draw heat out of the engine compartment.... If i block this section off I presume it would create extra drag.

lgriffiths · August 12, 2018, 22:32

Nice solution to getting around the cat issue, and yes I think leave open too, good for cooling. Would be really interesting to measure massflow through the enginebay with this open and blanked off, I'd expect blanked off to be much worse.
Another option you could try to step around the cat is a double diffuser design. That way you won't expand too aggressively.

Miles better than the nappy!

Would be fun to flow Vis this against the stock nappy.

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Mr X · August 13, 2018, 09:44

Thanks.
Yeah, I'm guessing the effect of the airflow leaving the diffuser would cause a suction effect on the air leaving the engine bay. If i was to blank it off that suction would then cause more low pressure. My exhaust exits to the left right side of the number plate and I read that that will reduce drag as area as I'm effectively pumping air into the low pressure zone.
I've already thought about that too as now I have the exits above. I could channel it from left and right of the engine as there's quite a lot of space there around the suspension components. I'd have to slot the channels as when I jack the car up or go over bumps, the suspension & drive shafts need to be able to move up and down.

I agree, I would love to see what difference its made (I'm presuming quite a bit) but I've been spending time designing and re-printing other stuff for the car rather than looking into openfoam.

lgriffiths · August 13, 2018, 10:07

You can measure drag using coast down tests , just need a stopwatch and some paper, and a bit of maths. It's important that there's no wind, and take a large sample going in both directions.

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Mr X · August 21, 2018, 21:18

Ok, so the newer version is on and I'm quite a happy bunny!

self tapping screw just a little off to the left here causing the bulge where the undercover meets it on the right!

The extra 100mm I put on the inside means the centre sections meets up with the undercover very nicely! Although I didn't quite put one of the screws in the right place and it's warped the flushness it a little (i'll have to fix that).
Now I just need to add a longer extension to the outer sections of the diffuser (so it meets with the outside of the undercover), think about how to finish off the outer inside so air cannot get in from the edges and make sure the exhaust doesn't melt it

.

Mr X · September 3, 2018, 18:05

ok so, another quick update.
The diffuser has had no problems with the heat from the exhaust, the foil has worked very well and I can put my hand on it after a good run.

I made the new version of the side extensions with a sweep up to 11deg and a colour swap:

As you can see, I extended what will be the flat section so it runs along side where the engine cover is. It reaches all the way to to covers beside the fuel tank and sits under them quite well.

The piece you see on the floor is the part that's going to replace the engine under tray. It reaches all the way past the fuel tank and to (what's left) of matt's brace.
This is too far for this section I think. I'm going to make it end at the fuel tank. That gives me some nice points to mount the start of this tray on. Then I can, if i wish, flatten off the fuel tank and matt's brace in 2 separate sections.