Over and Under or around the sides

[greenpower dude reloaded]

again, real work etc. has got in the way of me replying faster.

The channels is a great idea, I've really looked into it but unfortunately our chassis is packed together so tight that we couldn't fit anything in without really giving the air a hard time. I've always liked the idea of opening up some holes in the body and running air directly through to the other side (with smoothly moulded ducting of course,) but again, I think there is more hassle involved than we would actually benefit.

I'm pretty much there now with openFOAM, I just need to work out how to position the model correctly (soo difficult without some sort of GUI)my first run of Super Swoosh saw the car cut in half and gave us a Cd of 4.8 or something silly, thats deffinately not right! I've been given 0.28 before for the current model and with a nasty windscreen it knocks that down to 0.25.

SO, once this minor detail has been dealt with (answers on postcards btw, I want to solve this problem post haste! blimey that's a bad pun, I assure you it wasn't intended) I'm going to want all of your suggestions that you want me to try. Obviously a CFD run take a bloody eternity to run but I intend to try pretty much anything to get this shape perfect. Shall we aim high? (or low I suppose) and set a target Cd of sub 0.15 PAC CAR II a shell Eco-Marathon Car although fully enclosed managed a Cd 0.75 I feel sub 0.15 is possible.

[greenpower dude reloaded]

OK, just running my first full simulation of a slightly developed shape, still really square etc. but it's coming out with a lift coefficient of around 0.45 which although not really knowing much about lift it suggests to me a very high number.

How does every body feel about that? is lift really a bad thing? I suspect it's going to be costing energy and that it's likely to have adverse effects. As I think Breezy may have mentioned,

I'll try and post pics of it in Paraview tomorrow but you'll notice I've lowered the angle of the tail as we were getting a bit of unwanted separation there. I may try and widen the tail a touch so as to be able to shake off a touch of separation around there too.

I've got a few ideas of what I'd like to do with this shape but only time will tell if it'll actually benefit, and as I said before your ideas are all welcome!

[flynfrog]

Ideal you would have .00 cl and I would think that sub .15cd is easily possible the question then becomes what other compromises are you making to get it. Handling fitting the driver ect all start to add up. You should figure out what a point of CD is worth to you in power terms then decide what you can gain in other areas.

[greenpower dude reloaded]

Very good point Flyn and one I know a lot of teams have started to lose sight of.

Which, is another reason to use this years chassis which has already proved itself to be a safe yet competitive chassis in it's own right. (and now reliable, but we'll see if testing brings anything up)

We are likely to be raising the CofG a touch with the Ground clearance potentially being raised to 75mm, which isn't something I'm 100% happy with so I have left that decision down to the driver, because at the end of the day she's the one who's going to be tearing around a track.

There are one or two cars that have gone as extreme as 200mm ground clearance (ours is currently 60mm)and for the minimum track width of 500mm (ours is 530mm.) But we'll see how it goes. It's worth noting that the team that have raised up to 200mm achieves about the same speed as their 800mm tracked 40mm (minimum GC) car. The higher one is probably a bit quicker but the drivers don't have the confidence.

So really what I'm looking at is a development of this current shape. I'll be trying to package it all a lot smaller and making a few other changes, I'm open to any suggestions at all. I really want to try and get a nice organic(ish) looking shape.

So why is the lift so detrimental? BTW the Cl was much lower by the time the simulation had run its full duration, but the Cd was higher, so I have no idea where I stand really. I think I'll have to ask my OpenFOAM expert.

[flynfrog]

Very good point Flyn and one I know a lot of teams have started to lose sight of.

Which, is another reason to use this years chassis which has already proved itself to be a safe yet competitive chassis in it's own right. (and now reliable, but we'll see if testing brings anything up)

We are likely to be raising the CofG a touch with the Ground clearance potentially being raised to 75mm, which isn't something I'm 100% happy with so I have left that decision down to the driver, because at the end of the day she's the one who's going to be tearing around a track.

There are one or two cars that have gone as extreme as 200mm ground clearance (ours is currently 60mm)and for the minimum track width of 500mm (ours is 530mm.) But we'll see how it goes. It's worth noting that the team that have raised up to 200mm achieves about the same speed as their 800mm tracked 40mm (minimum GC) car. The higher one is probably a bit quicker but the drivers don't have the confidence.

So really what I'm looking at is a development of this current shape. I'll be trying to package it all a lot smaller and making a few other changes, I'm open to any suggestions at all. I really want to try and get a nice organic(ish) looking shape.

So why is the lift so detrimental? BTW the Cl was much lower by the time the simulation had run its full duration, but the Cd was higher, so I have no idea where I stand really. I think I'll have to ask my OpenFOAM expert.

If you are creating lift you are wasting energy by moving air that you don’t have to. Ideally you wan the air to be slipping off the car as if the car was never there. If you are creating lift you and throwing air up or down.

[wesley123]

maybe build a longtail? also if the track width minimujm is 500 why not make itthe minimum?

[greenpower dude reloaded]

Thought about making it minimum track, but the work involved would be massive. The steering arms would need to be redone, the rear track width made smaller leaving no room for the brakes, and the whole of the drivetrain would need to be moved, plus we'd lose a little bit of stability.

I have thought about it but I'm just not 100% sure it's worth it. Problem everything in any form of competition vehicle depends on everything else so change one thing and the rest of the bloody vehicle needs modding.

so we'll save that job for when I inevitably make the bodywork 30mm too small haha.

I can't really make the tail much longer at the moment, we may be able to take a bit of meat off the front and add it on the back but i'm not sure (we were 20mm oversized last time but shhh!!)

Thanks for the explanation Flyn, when my housemate asked why you didn't want lift (after the usual questions of "what the hell is all of that jibberish scrolling on your screen" and "why is your start bar up at the top of the screen" after explaining I was meshing and that I was using Linux (and then explaining what Linux was) I gave him a similar answer but thought it probably wasn't the case and that I was getting a bit ahead of myself.

[flynfrog]

The best way to think of it is to put your hand out the car window. (Airplane sounds optional.)

When your hand is horizontal there is little drag and no force pushing your hand up or down. Now tilt the leading edge of your hand up. the force pushing back on your hand(drag) increases and your hand now wants to go up (lift). Depending on your angle of attack the force pushing back increase with the force going up untill you stall then there is no gain in force pushing up but the force pushing back is much greater.

[greenpower dude reloaded]

right! as promised!

Ahh I always assumed that the force felt on my hand when I altered the angle of attack with my hand out of a car window was down to turbulence and separation on the back of my hand...

I spose that's why you shouldn't assume you always end up making an ASS out of U and ME

[flynfrog]

are you modeling the ground in these simulations?

also a strait on side shot would be nice to see.

[flynfrog]

also that shape looks like it will be easy to build using flat sheets of foam and glassing it up.

[greenpower dude reloaded]

et voila!!

I think I still need to do a litte bit more work on the tunnel..It's coming across at a slightly skewed angle but gives a genrally pretty good idea.

Yeah your right flyn, I could, we'll see how the final design ends up though..

It appears theres a lot of low pressure behind the wheels so next test will include small fairings

[flynfrog]

et voila!!

I think I still need to do a litte bit more work on the tunnel..It's coming across at a slightly skewed angle but gives a genrally pretty good idea.

Yeah your right flyn, I could, we'll see how the final design ends up though..

It appears theres a lot of low pressure behind the wheels so next test will include small fairings

to do it properly you need to simulate the wheels turning. that being said getting 90% there with the basic shape will put you way farther ahead than trying to fine tune fairings. Get your basic shape right then worry about fairings.

to answer your lift question if you look at the rear of your traces and see how they have a downward velocity that is your lift. If you could curve your bottom a little more you would reduce some of your lift and drag. this is assuming you are modeling ground effect in your simulation.

BTW great work so far.

[BreezyRacer]

I think there are a couple of small design changes that would help to minimize the lift you're getting ..

First, and I think you referred to this .. take some taper out of the sides of the body. However if you follow my lead below about a flat floor on the sides you might not have to do this.

Second, it looks like your diffuser area is creating a low pressure area, as it should, but that low pressure is helping to "pull" air from the top and sides down into the diffuser area. To fix that a couple of thoughts. First add sides to the diffuser by extending the sides down roughly to a level plane. This will contain the low pressure area to being under the car. Secondly, extend a flat floor out along the sides, starting where the body is tapering back. That will isolate your side flow from the underside flow, resulting in decreased lift because the side airflow will not be encouraged to be sucked under the tail of the car. It should also help your diffuser in yaw as well.

Finally one more point about the lift .. your rear axle location in relation to the rest of the body is perhaps leveraging the nose of the car and perhaps producing lift on the front wheels. Rather than just having a lift reading, it would be helpful if we could figure what the aero loads are, front axle vs rear axle. Having said that you are doing great work. One more thing .. are you emulating the road surface in your CFD? If not, you really need to do that. It will change a lot.

[greenpower dude reloaded]

Yes, sorry, I forgot to answer your question before. The floor is indeed modelled in this and the wheels should be turning but I'm not sure they will be. I tried to keep the model itself as simple as poss to cut down on meshing etc. for my first full run.

Very good point about the lift. See! it's stuff like that I post on here for. It all seems glaringly obvious to me now but you just get so close to it all you just can't see the wood for the trees.

Fair point about the fairings, I keep getting drawn in by images of PAC CAR II.

At the risk of sounding quite demanding is there any chance you could try and draw any suggestions on a picture of the car in say, paint or w/e? Just to make sure I get it right... would hate to spend all night modelling/meshing/running something different from what you meant.

At the moment I'd be tempted to just raise the height of the tail but, I worry about cross winds a little bit.

I had wondered about rounding off the whole of the undertray say 50mm radius to help air get out from underneath, if you see what I mean but I suppose that really it could just as easily help suck air under....