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Old 11-19-2007, 07:09 AM   #1
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Getting our priorities straight...

I was just looking over my notes from a course I did at uni about solar car design. I came across these nine rules for minimising drag. Its no so much the rules that are interesting, its more the priority given to each rule. They are given in order so changes affecting the rules lower down the list should only be made if they don't compromise the top ones. Trying to get these priorities right might help in making decisions about aero-mods. NB: this list is specifically for very aerodynamic and streamlined solar cars, the priorities might be different for normal cars.

1. Ensure attached flow over every surface of the vehicle.

2. Minimise wetted Area (i.e. surface Area exposed to air flow).

3. Maximise laminar (non-turbulent) flow over as much of the vehicle as possible.

4. Surface finish should be as high a quality as possible (especially ahead of the transition point to turbulent flow).

5. Minimise lift/down force.

6. Minimise wing tip drag (i.e. where two flowing streams of air come together).

7. Decrease frontal area (this is pretty far down the list because it generally compromises the other 6 rules).

8. Minimise interference drag (drag caused by the combination of boundary layers when two shapes meet).

9. Minimise ventilation drag.

It might be good for some of the more well informed aero-dudes to edit the order of this list for conventional cars. I'm sure things like wing tip drag are less of an issue and laminar flow is virtually impossible to maintain but as I said above, getting our priorities straight would help a lot in making aero-mods.
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Old 11-19-2007, 09:07 AM   #2
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Quote:
8. Minimise interference drag (drag caused by the combination of boundary layers when two shapes meet).
I guess compared to everything else, it should be so far down... But it's interesting to point out that a huge (if not the most amount for any single component) amount of money is/was invested in modeling/designing the surface that joins a large aircraft's wing to the fuselage.... Of course, they're top speeds are much higher

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I'm sure things like wing tip drag are less of an issue and laminar flow is virtually impossible to maintain
Yeah - bluff body versus streamlined body As far as wing tip drag... I'll bet the flow that spills over the C pillar and collides with flow that comes over the roof is a car equivalent

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7. Decrease frontal area (this is pretty far down the list because it generally compromises the other 6 rules).
Even so... holy crap - the stereotypical photo of a solar car shows an incredibly small cDA
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Old 11-20-2007, 08:13 PM   #3
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Yeah solar cars are apparently the most efficient powered vehicles ever made. An average car takes about 10.5kW to travel at 100km/h (60mph) a solar car used 1.5kW. That's right with 1.5kW you can either blow-dry your hair or commute on the highway at 100km/h. I'm so frustrated at the crazy amount of excess space we have in our cars just in case 4 other people spontaneously ask us for a lift home from work (never happened to me, but if it ever happens I'll be ready! =P). And it means everyone has to keep making big cars because otherwise a little efficient car gets squashed by the hummer.

I was interested by how far up the list surface quality was. That was the first major question I had about Basjoos car. It does 1 and 2 pretty well but i'm sure some smoothing of all those shapes and caulking would make a great deal of difference in drag (as well as looking very nice). I remember the lecturer even describing how they had to be careful that the sponsor's stickers weren't tripping the air into turbulence so they covered them in several clear coats to smooth the transition! Is the difference between bluff and streamlined body mainly the lack of boat-tailing (i.e. big wake) or is it the smoothness of the car as a whole?

I think the priority list may give the wrong impression, the things on the bottom of the list are still a huge priority for solar cars. They just are in that order because otherwise they compromise more important stuff. Even for the last priority (ventilation drag) they still only extracted air from already turbulent flow, ducted it to the drivers neck for most efficient cooling of the body and ensured all remaining air was vented out the back of the car. They just did all that without breaking the other 8 rules!

Can anyone recommend any books that talk about aerodynamics as is applicable to normal road cars? There is plenty on racing (all about downforce, tiny bit on drag) and heaps about aeroplanes etc but i can't find anything that just talks about how to make a good aerodynamic car. I am trying to learn as much as possible but its hard to piece together information from different forum posts.
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Old 11-21-2007, 12:31 PM   #4
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I don't see much on the interface between the vehicle and the ground except maybe frontal area.

And....8. Minimize interference drag where the body and the road are 2 surfaces?

For most people it will be how to incorporate aero mods into/onto an already completed design?
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Old 11-21-2007, 10:33 PM   #5
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Originally Posted by ZugyNA View Post
I don't see much on the interface between the vehicle and the ground except maybe frontal area.

And....8. Minimize interference drag where the body and the road are 2 surfaces?

For most people it will be how to incorporate aero mods into/onto an already completed design?
The main problem of ground interference is generally the lift that is produced. There are a few low altitude planes that use this to maintain an efficient glide. The two main ways of dealing with this are to have the car on rather high wheels as to avoid ground interference or to alter the shape of the solar car so that the added lift is countered by more downforce so the net force is zero. The same principle applies to road cars. We can either keep our cars high and put a full belly pan etc or can lower the car and the air dam so that ground force counters the additional lift. I see advantages to both approaches.

Number 8 is to do with the joining of sections of the car. On solar cars the example given was between the wheel fairings and the body but on any car with multiple curved sections that don't meet smoothly (i'm imagining the beetle as an example), some effort to smooth out the join would reduce interference drag.

These priorities are just as viable for altering the car as for designing one. How affective they are is determined by how the car maker prioritised them in the design phase. If a high priority on this list was low for your designer then you have a big opportunity for improvement!
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Old 11-21-2007, 11:02 PM   #6
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The main problem of ground interference is generally the lift that is produced.


Interactions like this I've been trying to figure out how to do moving ground modeling

When you think about it - the balanced forces must have reactions that terminate somewhere. Air shears under any force, so it eventually gets translated into the ground.

For a solar car, I wouldn't think down force is particularly desirable (although, at some of those speeds - perhaps a little?). Increasing rolling resistance seems unnecessary unless increasing stability is necessary.
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Old 11-22-2007, 10:21 PM   #7
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1. Ensure attached flow over every surface of the vehicle.
This makes sense, especially with most of the cars we are starting with. Hint: as long as there is that big area of suction at the back because of detached flow, this will be holding you back (literally), and the larger the area, the more it holds you back.

If you look at most of the common aero mods on this site... this would apply to boattails, skirts, undertrays, mirror removal, smooth rims on wheels. Deflectors are a compromise. I suppose even grille blocking could be considered here, because it's not like you are going to have attached air through the labyrinth that is your engine bay.

And what's left? uhhhhh pretty much nothing. Which is an indication that we are on the right track.

But really, I think that this is kind of poor phrasing. It's not detached flow that kills you, it's the area of suction created by the detached flow. Consider something like the prius versus something with equal cross sectional area, but going straight back like a typical van. Both have same amount of attached flow, the prius has better drag coefficient.

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2. Minimise wetted Area (i.e. surface Area exposed to air flow).
Given that we are stuck with the height of our vehicles (from chassis bottom to roof height) unless modifications cost $$$$$, requirement 1 precludes any focus on requirement 2. i.e. don't worry about the extra wetted area from the partial boattail, we are reducing detached flow.
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3. Maximise laminar (non-turbulent) flow over as much of the vehicle as possible.
This is only really possible on the front part of your vehicle, because after the cross sectional area starts decreasing we are heading into turbulent territory (i.e. laminar flow wings put the thickest area of the wing further back, comparatively, to maximize laminar flow). Perhaps modifying your wipers to point up (albeit goofy looking) at the front would reduce this. Caulking as well.

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4. Surface finish should be as high a quality as possible (especially ahead of the transition point to turbulent flow).
This is identical to point 3, not sure why a separate point was made.

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5. Minimise lift/down force.
I think this should rate higher, up near number 1. Lift/down force has the potential to absolutely DESTROY your drag. There is a reason why formula 1 cars have absolutely terrible drag coefficients, but stick to the road like glue. If your car is causing air to leave your car upwards or downwards in a major way after it passes through, the energy to do that came from somewhere. Hint: it's probably your engine.
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6. Minimise wing tip drag (i.e. where two flowing streams of air come together).
I'm not really sure how to do this, except perhaps instead of an angular boattail (like mine), it should be smoothed out to a gentle curve (more like a prius).
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7. Decrease frontal area (this is pretty far down the list because it generally compromises the other 6 rules).
Not really an option in most of our cars, other than removal of antennae/mirrors.
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8. Minimise interference drag (drag caused by the combination of boundary layers when two shapes meet).
Not sure what this is and how it differs from point 6.
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9. Minimise ventilation drag.
Appropriate place for it.
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Old 11-22-2007, 10:41 PM   #8
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If I had to make a list:
1. Minimize the cross sectional area of the plane normal to the path of the car created by detached flow. This is cumulative. If two separate cross sectional areas overlap, both are counted. If minimizing this minimizes frontal area (i.e. window removal), so much the better. See my previous post. There is so much stuff to be done here.

2. (More along the lines of equal to 1). Minimize lift/down force. (If you are forcing air to go up or down a distance behind the car in order to propel you the other way, you have supplied it energy and that energy came from your engine.)

3. Forcing air to flow in close proximity to the ground requires energy, from your engine. The bottom of your car should be fairly flat for this reason - angles will cause movement to or from the underbody of the car. If you can't do this, adding extra frontal area via a spoiler is a compromise, lowering your car would be better. (Thanks ZugyNA)

See all solar cars for evidence of this, and consider why the ground effect works - as you get closer to the ground, the air between car/plane and ground does not like to move. Getting it to move costs a lot more energy than air nearer the atmosphere.

4. Minimize wing tip drag. I suspect that this would go here somewhere, along with laminar flow. This probably applies to boattails.
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