43 replies to this topic

[dw1]

So if I'm reading this correctly it also means I wouldn't have to spend so much on cleaning products because the back of the car wouldn't get as much muck thrown up it?  Result! - I wonder if I can convince my other half it would make it cost effective..

Faultless man maths - go buy yourself one

[subseamac]

yes but which one - the TATs look good but I doubt the short version does anything. Maybe the TaT long but he's got it bolted to the undertray at the original angle which is too steep. I wonder if the Jimsan works partially because it reduces the angle of the rear undertray and stick out very slightly. Ideal may be a Jimsan that sticks out further and with the vertical vanes deeper, in line with the mid and front undertrays.

[MAXR]

I've had to fit 10mm spacers to the two main locating bolts on the Jimsan diffuser & 4 x 35mm aluminium spacers to the 4 rear locating points. This lowers the trailing edge & thus the angle of air exiting & also allows a better air flow through the standard circular exhaust vents on the underside.

yes but which one - the TATs look good but I doubt the short version does anything. Maybe the TaT long but he's got it bolted to the undertray at the original angle which is too steep. I wonder if the Jimsan works partially because it reduces the angle of the rear undertray and stick out very slightly. Ideal may be a Jimsan that sticks out further and with the vertical vanes deeper, in line with the mid and front undertrays.

[subseamac]

excellent idea. I may copy that if you don't mind.

I've had to fit 10mm spacers to the two main locating bolts on the Jimsan diffuser & 4 x 35mm aluminium spacers to the 4 rear locating points. This lowers the trailing edge & thus the angle of air exiting & also allows a better air flow through the standard circular exhaust vents on the underside.

yes but which one - the TATs look good but I doubt the short version does anything. Maybe the TaT long but he's got it bolted to the undertray at the original angle which is too steep. I wonder if the Jimsan works partially because it reduces the angle of the rear undertray and stick out very slightly. Ideal may be a Jimsan that sticks out further and with the vertical vanes deeper, in line with the mid and front undertrays.

[subseamac]

Right. Who wants to buy a jimsan with me. He's got a discount if two are bought and delivered to the same address.

[NickB787]

Right.

Who wants to buy a jimsan with me. He's got a discount if two are bought and delivered to the same address.

How much?

[subseamac]

Right.

Who wants to buy a jimsan with me. He's got a discount if two are bought and delivered to the same address.

How much?

2x£180 for two inc black anodising and post to UK.

as seen here

http://www.lotuscars...ype116/shop.php

Edited by subseamac, 17 May 2008 - 10:37 AM.

[Adam_B]

i want one, but no point in me doing the discount thing with you as im in the NW

[Winstar]

Finally got round to doing the plots that show the effects the best

Flow Velocity Vectors without Diffuser


Flow Velocity Vectors with Diffuser


You can see that a diffuser that exits away from the car stop the flow going up the back of the car and creating lift.

It also shows how high you have to mount a rear wing in order for it to get clean air flow.

[subseamac]

very revealing. and what angle do you have your diffuser running in the model? looks horizontal Theoretically the rear diffuser should work best at a 7 degree or so angle from the flat under-engine section, and create negative lift at the boundary between the two. Presumably some of the TAT designs that leave the exhaust hump uncovered wouldn't work too well. Also by how much do you have the diffuser sticking out beyond the body? would 5cm do the trick? Can your model do small vortex generators on top of the roll bar cover (see evo VIII/IX)? in theory this may help get some more flow above the engine cover. Sorry for the million Qs but this is very interesting

Edited by subseamac, 19 May 2008 - 06:50 PM.

[Mr_M]

Quality as always Winstar Very interesting, as you say, to note how high up a wing would have to be to be in truly clean air Subseamac - I've though similar re vortex generators, but had come to the conclusion (and I'm not sure I see anything in Rob's plots to convince me otherwise) that without a smoother rear profile (i.e. exige typre rear cover) they wouldn't really do anything. Very open to being told I'm wrong though..... I did wonder if some sort of (chavtastic) roof spolier contraption on the back of the roll bar cover might help? Not sure I could ever bring myself to do it though

[aron916]

sorry slight hijack...........but; silver car, standard wheels - black or silver Jimsan diffuser?

[Adam_B]

Black. The silver looks pants imo

[pincher]

Black

[subseamac]

Quality as always Winstar

Very interesting, as you say, to note how high up a wing would have to be to be in truly clean air

Subseamac - I've though similar re vortex generators, but had come to the conclusion (and I'm not sure I see anything in Rob's plots to convince me otherwise) that without a smoother rear profile (i.e. exige typre rear cover) they wouldn't really do anything. Very open to being told I'm wrong though.....

I did wonder if some sort of (chavtastic) roof spolier contraption on the back of the roll bar cover might help? Not sure I could ever bring myself to do it though

You can see on winstar's model that with the rear diffuser the flow of air is substantially nearer the boot lid and edge of the boot than without. One thing that's particularly interesting is the change in airflow above the engine lid showing that even though it gets better with the diffuser, it is still very weak and not sufficient to properly cool the engine. I wonder if a combination of VGs on the trailing edge of the roll bar cover associated with an effective diffuser may not - just - get us some proper air circulation above the engine. I've also been thinking about a more discreet very small/thin wing type deflector that could be positioned on the trailing edge of the roll bar on the turbo side to divert some flow down over the engine bay. With a diffuser, the low pressure created above the boot will then help circulate that air.


p.s given the scale on winstar's model I would say that diffuser needs to stick out a good 20cm!!! Winstar could you confirm as anything more than 5cm isn't exactly practical and would end up being used by pedestrians and small animals as an alternative means of transport (..)

Edited by subseamac, 20 May 2008 - 07:41 AM.

[EdButler]

You can see on winstar's model that with the rear diffuser the flow of air is substantially nearer the boot lid and edge of the boot than without. One thing that's particularly interesting is the change in airflow above the engine lid showing that even though it gets better with the diffuser, it is still very weak and not sufficient to properly cool the engine. I wonder if a combination of VGs on the trailing edge of the roll bar cover associated with an effective diffuser may not - just - get us some proper air circulation above the engine. I've also been thinking about a more discreet very small/thin wing type deflector that could be positioned on the trailing edge of the roll bar on the turbo side to divert some flow down over the engine bay. With a diffuser, the low pressure created above the boot will then help circulate that air.


p.s given the scale on winstar's model I would say that diffuser needs to stick out a good 20cm!!! Winstar could you confirm as anything more than 5cm isn't exactly practical and would end up being used by pedestrians and small animals as an alternative means of transport (..)

The engine bay is cooled from the NACA Ducts underneath and the Side Pods... The vents out of the top are to let the rising heat escape (plus the small rear clam holes which are obscured mostly by the Jimsan Diffuser)... The slight amount of uplift will draw the heat into the air-stream which is fine.

[Winstar]

For the diffuser size I think I used an angle of 12 deg, while 7 is whats quoted as the optimium to an extent it also depends on the speed range that you want to diffuse the air at the slower the speed than the larger the angle can be without the the flow seperating, the models is at about 100 mph IIRC. I think it extands behind the car by 50mm, I prob looks more as I squared everything off. However I did it ages ago and ment to redo it with a betfer car model and mesh, so I can't remember exactly what I did. As Ed said the there will be flow in the region above the engine bay from the engine cooling, it's not modelled as it would be far to complex for the code I've got access to, like i said above most of the cars just a rough shape to get the inlet flow to the diffuser about right. The other thing is that the roof of the car is just flat in reality it curves down towards the rear that would angle the flow down abit more. As for Vortex Gen I.m no sure if they'd work they are used to stop the boundary layer seperating away from curved/angled surfaces where as with the step behind the roof then the flow will always seperate, you could possibly use them along with either an exige style back or even just a small extension to where the roof slopes back to turn the flow downwards and try to get the flow to reattach before the spoiler. But all this would require a hard top as the soft top will fcuk the flow right up.

[siztenboots]

Which design of diffuser extends furthest ?

[subseamac]

For the diffuser size I think I used an angle of 12 deg, while 7 is whats quoted as the optimium to an extent it also depends on the speed range that you want to diffuse the air at the slower the speed than the larger the angle can be without the the flow seperating, the models is at about 100 mph IIRC. I think it extands behind the car by 50mm, I prob looks more as I squared everything off. However I did it ages ago and ment to redo it with a betfer car model and mesh, so I can't remember exactly what I did.

As Ed said the there will be flow in the region above the engine bay from the engine cooling, it's not modelled as it would be far to complex for the code I've got access to, like i said above most of the cars just a rough shape to get the inlet flow to the diffuser about right. The other thing is that the roof of the car is just flat in reality it curves down towards the rear that would angle the flow down abit more.

As for Vortex Gen I.m no sure if they'd work they are used to stop the boundary layer seperating away from curved/angled surfaces where as with the step behind the roof then the flow will always seperate, you could possibly use them along with either an exige style back or even just a small extension to where the roof slopes back to turn the flow downwards and try to get the flow to reattach before the spoiler. But all this would require a hard top as the soft top will fcuk the flow right up.

so a 7-12 degree diffuser clearing the rear by 5cm will work just fine. If anything, having it sat 2cm below the clam at the rear (like Max) to clear the exhaust "hump" will improve cooling and reduce the angle.

I like your idea of a small extension to the roof slope maybe with a couple of VG's attached to it. Should be easy to get a 10cm wide piece of transparent perspex to extend the roof and fix on either side, stick on the VG's, attach a few small ribbons to the engine cover etc and check improvements in flow at different speeds (right now water droplest don't even move off the engine cover/ rear spoiler)

[subseamac]

Which design of diffuser extends furthest ?

there isn't a perfect one. The TAT long version extends out a bit but He's cut it in the centre to clear the exhaust hump so not so useful, the Jimsan seems to be a decent bet but it doesnt' extend very far out, also the vertical vanes on the Jimsan look a bit shallow. If I had to chose right now I would go for the Jimsan and mount it like Max, but I'd rather think about this some more & maybe come up with a better design. this thread has proved very useful