Citroen Xantia "notchback" aerodynamics using Flow Illustrator

[ecoxantia]

Yesterday I discovered "Flow Illustrator"- fantastic fun!

So I made my own bitmap, using digital camera, MS Paint and Irfanview.

Looks impressive to me, not a lot of turbulence and little detachment of the flow.

I have added the mods I am working on right now: undertrays front and rear.


When I made the bitmap I had to go back to the car with a straight edge since I was surprised how curved the profile of the car really is!

This one gives more room to see the "wake":


Settings are default, ie Re 1000.

[lunarhighway]

nice car! one of my favorite cars from the period, and it still looks good today.
i found some interesting info on your car here
Xantia story

it says

Drag coefficient of Xantia is around 0.30 depending width of the tyres.

although "around" seens to mean 0.33 to 0.34 according to other sources

[ecoxantia]

Thanks. I inherited the car, here in the UK they have always depreciated fast so now you can get an HDi for little over $2000, that gets 42 US mpg as standard. I think it has excellent potential for high mpg.

My car is a late model with 205 tyres, maybe I'll look at narrower wheels and tyres at some stage, I didn't know they could have such an effect on drag.

Simon

[lunarhighway]

small changes can have a big effect on drag, that's both good and bad news.

the opel/vauxhall calibra for example hads a drag coeficcient of only 0.26, but that's only true for the 8 valve versions as 16v's had no undertray, more grill opening, and wider tires.

their cd is 0.29

that sugests that adding and undertray, narrow tires and a grillblock, might decerease your cd by 0.03

i think the bodywork of your car is pretty smooth, but as in most cars, the underside will be a desaster, so that's where the simples improvements can be made, and they're mostely out of sight so it won't affect the looks.

unless you're tires need replaceing i wouldn't loose any sleep over it though. you might want to look into wheel dams, they seem pretty universal on current cars, and while i still haven't figured out exactly how big their impact is, their addoption by nearly every carmaker suggests they're a quick aero fix that impoves FE measurably.

that's nice about older cars with fairly good aerodynamics... they don't even have these simple tweaks applied, so little additions might make them even better.

[ecoxantia]

You're doing well on your fuel log. I guess aerodynamics of the Vectra must be very similar to the Xantia. My modifications are going to be similar to yours.

I did think that some extra bodywork to enhance the hatchback area would be needed, but since the flow stays attached to the hatch I don't think this will help. (according to FI anyhow!)

The Citroen C4 coupe and many other cars have a very flat hatch and a lip at the end of it, this will also be interesting to model and easy to implement.

I am physically working on undertrays in between the summer thunderstorms!, next will be rear wheel arch covers. Of course the car that the Xantia replaced, the BX, had partial wheel arch covers...

What do your wheel dams look like?

[lunarhighway]

the vectra is supposed to have a 0.29 CD (it formed the basis for the caliba and considdering it replaced the boxy ascona/cavalier mk2.. so at the time it really was a leap forward) but mine came with slightly wider tires and alloy wheels wich don't look that bad, but might still hurt the airflow.
this could add + 1 or 2 to the CD

than again i have the smallest engine available in this car and my top and bottom grill are partly blocked from the factory. i also removed the rear wiper as i rarely use it and the shape if the hatch is such the air could indeed follow it so the wiper seemed an aero killer.

i've read some honda crx's came with a rear wipers and some didn't, the ones without had a CD that was 0.01 lower so if you have a rear wiper it's only one screw to get rid of it, and you can keep it in the car along with the right spanner in case you want it back.... i can live with the attachement point for now...

so i guess the actuall CD will still be "around" 0.29 i hope the lower end.

the wheel dams i'm curently running are like the second picture

http://ecomodder.com/forum/showthrea...html#post51337

i'd like them to be a bit longer though.

i basically just used short wood screws to attach a bit of plastic to the plastic inner wheel well liner.

personally i think they're to small to reall help a lot, but as they don't hurt fe i'll leave them in place for now.
i'm also considdering rear ones but everythings metal there and it's a classic rust spot on these cars so i'm not going to drill.


ideally the undertray would have covered the sides but the sheet aluminum just wasn't big enough and i didn't want to get another one as i wans't sure it would work. still it seems to deliver an extra 1 a 2 mpg's on mostely off highway but with light traffic so while nothing huge with the curent fuel prices that's quite a nice improvement that paid for the sheet in no time

right now i'm toying around witht the idea if an active top grillblock, as the grills are already quite well designed i don't think it's wise to assume i can get away with a full permanent block all the time and blocking the top grill on a hot day with moderate driveing saw the temp climb quicky to the redline.

so i'd like something that opens when the fan comes on...