BT Tech rear spoiler??

My working hypothesis was that the spoiler works by reducing lift, thus reducing the susceptibility to sidewinds. Could your flight instructor mind illuminate us further?

 

I'm only surmising, but I suspect Toyota's main goal was to reduce drag. Even if test results from the wind tunnel (either real or virtual) had shown increased lift at higher speeds, they may have assumed that most people wouldn't be driving it that fast anyway, or perhaps did not consider large sidewind vectors.

I recall from earlier discussions on this topic something about a "Kamm back". As I understand it (and I'm an accountant, not an aerodynamicist) the truncated shape of the car creates less drag than if the rear were streamlined to a point, by eliminating the surface friction of the extra material. I would guess the oem spoiler has a great deal to do with this that may be negatively affected by the BT spoiler.

Did I read you correctly that a racing car wing works via Newtonian effects, rather than Bernoullian?

 

Hate to be a know it all... (actually I didn't know the following for a long time) but wings don't fly due to "Bernoulli" effect lift in the way most people understand it. That's a myth, and pretty counter intuitive if you think about it. I was taught that as a kid making model airplanes, and accepted it as authoritativly true, but I never thought it made any sense. Turns out, it doesn't.

The Prius doesn't generate any lift either. The reason is complicated, but basically it's not acting like a wing to curve airflow downwards as a wing does. The misconception probably arrises because the Prius has a longer top shape and looks kind of wingish at a glance, but really it's not the right shape to be a wing or generate lift, it's more a wedge. A wing with the same profile as a Prius would not fly nor would a wedge.

The spoiler is purely for airflow to reduce drag, but not downforce really. A bigger spoiler won't help. Although, a bigger spoiler may look cooler I suppose. When drawing cars, I always add them!

Some links on the subject of lift myths, inclding some from NASA with cool Java simulations:

http://www.eskimo.com/~billb/miscon/miscon4.html#wing

http://www.grc.nasa.gov/WWW/K-12/airplane/wrong1.html

http://www.grc.nasa.gov/WWW/K-12/airplane/wrong2.html

http://www.grc.nasa.gov/WWW/K-12/airplane/wrong3.html

and on from there. Each NASA page has a "next" button at the bottom.

 

OK, air flow on the Prius is becoming a hobby for me.

Observation 1- Air flow past the side windows flows up to the roof starting at the pillar between the driver and passenger and this upward flow happens all the way to the rear. You can see this from dew in the morning after driving a short distance at 70 mph. Look at the roof and the rear window. I think Toyota did this to allow these flows to meet at the rear before they separate from the rear (lower drag).

Observation 2- The curved distance described in Ob 1 is longer then the straight shot under the car so a negative pressure (relative to the underside) exists on the mid to rear top of the car. Therefore lift is created.

Guess number 1- cross winds and high speeds created faster flow across the rear window area, thus increasing lift and decreasing stability.

I'll have some 3D CFD particle flows and pressure distributions done in about 2 weeks (with and without the BT spoiler). Theoretical yes, but information none the less.
p.s.- does anyone have an IGES 3D model of the Prius, it's a pain to model in 3D.

 

I think you may want to read those links from NASA I provided above. Simply having a longer top than bottom does NOT create lift. That is a myth, or more specifically a bad misunderstanding of the physics involved that has become so widespread even many educated people think it's true. Even HS physics professors (and probably even some colleges) often teach that incorrectly.

As the NASA site points out, air traveling over a longer top surface does NOT create lift due to a so called"bernoulli" effect, because the assumption the top velocity remains constant perpendicular to the direction of travel, is false, due to conservation of volume.

Also, in the case of the Prius, the front spoiler slightly impedes airflow under the car, which is the purpose. That creates a very slight negative pressure under the car relative to pressure on top, i.e. negative lift. The Prius is not a wing, and air flowing over the top relative to air underneath is pressing the car down, not creating lift.

Also, cross winds are not creating "lift" either. They're applying lateral pressure which cause any car to tilt, just as if you lean on one side a car will tilt. That will make any car wander to some extent, and that in turn makes steering feel loser which may prompt overcompensating. The Prius may be slightly more vulnerable to that due to flat sides, i.e. the sides are more perpindicular to lateral forces = more resistance.

 

Your statement on
"As the NASA site points out, air traveling over a longer top surface does NOT create lift due to a so called"bernoulli" effect, because the assumption the top velocity remains constant perpendicular to the direction of travel, is false, due to conservation of volume."

makes no sense. The Bernoulli effect contributes to lift but is not the SOLE source of lift FOR AN AIRPLANE WING. The Bernoulli effect does explain lower pressures in higher velocity flow areas which creates a differential pressure(lift).

For the Prius (which is not a wing with variable AOA-angles of attack relative to the oncoming flow) I am not ASSUMING the top of the car velocity remains constant- IT INCREASES because of the further distance it has to travel TO TRY TO MEET the conservation of volume at the rear.
I was speaking specifcally about the MID TO REAR of the Prius. Are you trying to argue that downforce (total) is created in this area ????

Yes I've been to many NASA websites and studied papers and software written by them on CFD. I'm a pilot and a Chemical Engineer so if want to talk technical then go ahead.

Here is a clip from the NASA site discussing the Bernoulli effect on a wing-Note how they point out that the Bernoulli effect does create SOME lift:
{The upper flow is faster and from Bernoulli's equation the pressure is lower. The difference in pressure across the airfoil produces the lift.} As we have seen in Experiment #1, this part of the theory is correct. In fact, this theory is very appealing because many parts of the theory are correct. In our discussions on pressure-area integration to determine the force on a body immersed in a fluid, we mentioned that if we know the velocity, we can obtain the pressure and determine the force. The problem with the "Equal Transit" theory is that it attempts to provide us with the velocity based on a non-physical assumption as discussed above.

 

You may be misunderstanding what you cited, which is a common mistake. Unfortunatly the NASA examples sometimes use slightly vague terminology as it's a difficult subject to describe without actually running fluid simulations.

Yes, Bernoulli's theory is true, generally speaking. So are Newtonian physics generally, not talking Quantum or such. However, there is no "Bernoulli theory of lift" nor a "Newtonian theory of lift" because they didn't create one.

What the so-called Bernoulli theory of lift does is take a true principle (Bernoulli's) and misapply it, misunderstanding lift. Specifically the incorrect theory is the "equal transit" theory which was debunked in fluid dynamics long ago, after it had already been taught in popular physics unfortunately. Your mention of the top being longer than the bottom creating lift, that is the mistaken assumption of the equal transit theory. In general when someone states a longer top = lift, they're mistaking the principle, as that is not a given.

The "thin wing" is one example explaining why the "equal transit" i.e. "long top + Bernoulli = lift" theory is false. Imagine a wing of infinite thinness, or just really thin like a strong sheet of paper or titanium, or whatever. It is shaped to be curved downwards at a gentle slope towards the rear. Importantly: its top surface is the same length as the bottom. It still generates lift. In fact, it generates more lift than a longer top surface wing. Hence the lie of the "equal transit" theory of lift which misapplies Bernoulli’s theory to presume a wing works principally due to a longer top surface creating a vacuum, as air particles rush to catch up to the bottom surface.

Actually, a conventional wing works because it curves airflow down which lifts the wing (hence downwash) and the top being longer than the bottom is simply due to needing thickness for structural reasons, and the conventional wing shape is a decent compromise. Still though, the top surface being longer is NOT the lift, the overall curve of the wing shaping airflow downwards is the lift. An important distinction.

Is the Prius curving airflow like a wing? No, even though it's top is longer. Why? It's becasue the Prius is more of a wedge (another form with a long top) which doesn't create positive lift, it creates negative lift.

You were citing principles of the "equal transit" theory regarding top being longer. Many pilot textbooks incorrectly cite as true the "Bernoulli/equal transit" theory for lift. I remember them from flight training. I think Jeppeson flight manual probably still does, but it's not written by fluid dynamics physicists at NASA.


The front of the Prius compresses air a lot, hence downwards pressure. The air is still compressed above the body as it travels the short distance to the rear of the vehicle and air pressure has not had the chance to dissapte above the body. It is still exerting downwards force on the rear of the vehicle. That pressure then spills off the rear spoiler as a vortex, filling the vacuum behind the body efficiently. The vacuum behind the vehicle also indirectly exerts downforce due to airflow.

You also mentioned airflow going up on the side towards the rear, and presumed that means lift. Actually the Prius displaces air sidewards and upwards creating pressure all around for inwards force. Towards the rear of the vehicle there is realativly more pressure on the sides (due to flat sides) than there is on top, due to the inwards/downwards arching rear roof. Hence airflow from side to top. However, that is a relative difference between them, it is NOT lift, becasue there is still inwards pressure on both the top and side. The roof is shaped for that purpose, bowed slightly in the center, to try and keep pressure flowing straight down the roof rather than spilling off the side.


The air underneath the vehicle is actually a very slight vacuum, due to the front and side spoilers limiting airflow in, while the moving car leaves the air mass behind, creating a slight vacuum. That (and forward down pressure) is what front spoilers are designed for.


The whole body is designed to provide downwards air pressure across the top with minimum drag, create low pressure beneath, and to control airflow into the rear partial vacuum. It's not a wing, it's a smoothed wedge, and not creating lift, by design. Over the entire car forces are back (drag) and slight downwards pressure, more strongly in the front and weakly in the back, but always down.

Yes, adding a larger spoiler will create slightly more downwards force, and more drag. Neither of which is needed. Guys who put big wings on their cars, they're just adding weight and drag at maneuvering speeds, it’s not helping handling characteristics any more than a trunk full of bricks really.

 

Kozmik, Thank you for being polite and patient.

Yes my description of longer path over the top was in reference to "equal transit" mis-theory.
I would like to break this discussion into parts.
1- let's focus on the back half of the Prius, so front spoiler, wedge, etc. can be discussed later
2-We really don't know if the air under the car is positive or negative pressure. Hopefully my CFD work will add to this knowledge.
3-I don't think it's fair to compare flat plat-AOA simulations to the Prius since the car is at a fixed AOA and has thickness (zero degrees , there really is no cord ?)

So I still think your second to last paragraph statement that
"...weakly in the back, but always down."
is still incorrect.
I think there is a low pressure area covering the rear window and this is partially shown by the dew streamlines flowing INTO this area. If the pressure above the rear window was positive the streamlines would be flowing out and down(i.e toward the tailights).
Either way, only CFD or windtunnel like testing will show the effects of the BT (or other) spoiler. I am using a true Navier-Stokes 3d solver for this work.
Also if you look closely (from the side) at my 04 Prius the front actually inclines up from the front wheels to the bumper. This wedge will create higher pressure then a straight or down sloping front overhang.

 

Now THIS is the kind of stuff I like to read!

I've had trouble with the "Prius causes lift at high speeds because of the wing cross-section" theory since I first heard it.

And even if the Prius profile DID create some lift, how would that little spoiler reduce it.... beyond what the factory spoiler already does? As far as I under stand the rear spoiler of any factory car - it is meant to tumble the air at the back, lessening the vacuum that always exists at the rear of a vehicle. The sharper the edge of the spoiler the better (the factory one is pretty rounded!). But you'll soon find that I know just enough about aerodynamics to make me dangerous, so I'll just shut up and follow what you guys end up doing. Thanks for letting me watch.

 

You're partially correct, in that the inwards slope of the back roof does create space for the compressed airflow to expand, somwhat. Relative to the side (and relative being key) it is a low pressure zone, because the side is flat and pressure more constant. Hence the air flows from the side to the top rear. HOWEVER, both side and top rear are still high pressure, compressed air. Therefore, there is no lift.

Another way to look at it is this: you take a high pressure jet and blow on the side of the car, and another blowing on the top. The top has a little more room for expansion so some of the pressure from the side will flow onto the top as well. However, both top and side are still under pressure exerted inwards, which is sideways on the sides, and down on the top. Bottom line, no lift.

The air flow is laminar over the entire top, and departing from the rear spoiler, that's how I know there is no lift, and that there is always positive pressure on the top of the vehicle. Now, I know exactly what you're thinking: "but doesn't laminar flow = lift on a wing? Isn't that backwards?" Yes, but the Prius is NOT a wing, and again that is the mistaken equal transit thoery leading to mistaken assumptions. A wedge can have laminar flow up to the rear edge where it seperates, and not generate lift, becasue it's not a lift generating shape.


As far as the bottom goes, front spoilers are generally designed to keep air under the vehicle as neutral or negative pressure relative to top and sides. Everything about spoilers deflects air up, over, and out for exactly that purpose. That is another reason why streamlining the bottom isn't such a big deal. You don't want air under the car to begin with.

 

It doesn't create lift, it's just one of those urban legends. The car body isn't generating lift to begin with, pretty much no car does. Again that's the equal-transit myth, and that to some people cars look like wings. They're not though. If it was that easy to build a wing accidentally, we'd have been flying a lot sooner.

Those giant wings some people put on Civics and such, really pointless. It's just weight, and drag. At really high speeds it does create downforce, and a LOT of drag, but it's silly because the Civic isn;t generating lift to begin with. At speeds where the Civiv is actually capable of manuvering, i.e. where extra downforce might be nice, then the car is going too slow for the wing to have much effect. Duh.

May as well put bricks in the trunk and sandpaper the paint job for the "performance" mod, cause that's all it is: drag and weight. But, some people like the look, and that's all it's really about anyways, which is fine I guess.

 

Cars DO create lift in some areas.
Here is a clip from the Lotus Elise website
Note how the REAR of the older S1 creates positive lift (the S2 was touted in a recent Popular Science as the FIRST road car to create total negative downforce-and the Prius was discussed also).

http://www.elises.co.uk/components/s2/bodywork/index.html

The Elise S2 has a Cd of 0.407 and a CdA of 0.651 with the roof on. The Elise is more aerodynamic with the roof on. The front and read lift co-efficients are Clf -0.02 and Clr -0.04 assuming a 130/130mm F/R ride height and roof on. These coefficients need to be multiplied by the appropriate surface area before the downforce in kg can be calculated. These compare well to the equivalent figures for the standard S1 car which are Cd 0.408, CdA 0.653, Clf -0.030 and Clr +0.053. This assumes a standard ride height of 140/140 F/R and the roof on.

Lotus reduced the front downforce and the rear lift on the S2 to achieve almost neutral balance which delivers consistant handling balance at all speeds, while the S1's high front downforce and rear lift contributed greatly to the cars tendancy to oversteer at high speed. It could be argued that reducing downforce is a retrograde step and that leaving the front in S1 configuration and then increasing the rear downforce further to achieve a Clr in the region of -0.3 would generate more lateral rip, but testing showed the drag penalties associated with doing so were unacceptable. The roof of the S2 was lowered to also improve the quality of air flow over the rear of the car and hence make any rear spoiler more efficient. The Elise S1 111S and Sport 160 spoiler efficencies were reduced by the higher roof line.

 

???

Every object creates lift, positive or negative, simply by moving through the air. But very few heavy objects create any meaningful lift relative to weight. A brick thrown through the air depending on random circumstance may create minute lift, positive or negative, as it tumbles through the air. But it's still dropping like a brick because the lift is irrelevant to its mass.

Cars that bother to consider aerodynamics at all, usually try and design for neutrality. Certainly that is true of the Prius for the reasons I mentioned earlier. The Prius is about 2900 lbs dry, though that varies by several hundred pounds depending on cargo, passengers, fuel, etc. It takes a lot of aerodynamic lift, positive or negative, to exceed its normal envelope of operation variance.

That site... well they're selling body kits aren't they? So buyer beware. Also, I wonder how accurate their modeling really is. As the saying goes, garbage in, garbage out.

Anyways, have fun with it. That's the main thing. Beware though, if fine tuning is really your goal, the Prius is already highly tuned. The modeling of Toyota engineers is going to be as accurate as it gets including undercarriage an such. So, I think it'll be difficult to exceed that level of modeling precision. And the "tuner" community is actually the "de-tuner" community for the most part.

Having said all that, I did think the aftermarket rear spoiler looked kind of cool. Doubt it would do much, but that's ok. Get some blinking brake light on it too, and it'll be stylin.

 

Actually, I just realized which spoiler people were talking about.

There are two I've seen. One extends the replaces the rear spoiler and is longer, and looks kind of cool, although beauty is in the eye of the beholder. I doubt it does anything though.

The other is a stick on thing that goes on the back winshield. That's just silly. That'll do nothing at all, except for limit rear visability which isn't cool. May as well drive with the rear window wiper on and call it a performance mod.

 

I'm done. Thanks for the discussion.

 

Well, thanks for the input, 200Volts. I was enjoying the discussion, but I'm still not convinced one way or the other about the BT spoiler.

Anybody know of a wind tunnel animation of the Prius?

 

I could be wrong too.