A unique aero-related observation...
 

In past posts, I've mentioned the addition of a 1"x1.5" rear lip spoiler to the rear hatch of my Volvo 240 wagon. Over the past several months, I've been noticing something different happening behind the car @ speeds over 45mph. While driving on rain soaked roads, there appears to be a "wall" of upward flowing water mist that seems to extend from under the rear bumper(@ ~45deg.) and disapates somewhere above the roofline. Now that Fall is upon us, I've also noticed a similar pattern with fallen leaves on the road. Prior to the installation of the spoiler, there were no noticeable flow patterns behind the car. I realize that this observation is non-scientific, but there clearly is something different happening back there.

Any thoughts??

Sounds like the spoiler is creating a low presure aera at the rear and "sucking" the stuff up from the bottom.

Kinda what I was thinking as well. But is it feasible that a small spoiler could have such a dramatic effect?

Tilt it 90 degrees (straight up) and watch what it does to your aero :p

Honestly, I don't know - but these interactions are rather complex (especially turbulant interactions, so much so that we "know" very little about turbulant interactions).

You could try taking it off and seeing if the same updraft is there ;) There is a possibility that the spoiler is making you look at something you didn't pay attention to before :thumbup:

Actually, its non-adjustable. It has a gradual incline angle to 45deg.


But aren't there general rules regarding high/low pressure zones? Eg: High pressure being drawn/attracted to low pressure areas, attemping to 'fill the void'...so to speak?

Like I mentioned before(...and brucepick might concur;) ), there were no visible flow patterns without the spoiler. I've been driving Volvo wagons for over 20yrs and I've never seen anything like this. I suppose I'm most curious to know if the altered flow induced by the spoiler is beneficial or not.

Can you, or have you posted a pic of your spoiler?

Pics are here....https://gallery.newbug.org/volvo

Why did you choose an upwards angled spoiler? I would say it likely increases the size of the wake behind the vehicle.

Believe it or not, the OE lip spoiler on some of the new Chrysler PT Cruisers was my inspiration. The Chevy Malibu Maxx SS is another "two box" with a positive downforce spoiler.
I don't doubt that my spoiler has increased the size of the wake. But it does seem to be providing 'order'(...for lack of a better word) to the airflow where outright chaos most likely existed before.
I LOVE the new C30 and I appreciate the potential benefits of the kammback design. But as brucepick had mentioned in a previous topic, it would be difficult to install a functional kammback on 240 wagon and still maintain the practicallity of the rear hatch.

OK - so you were intentionally going for downforce, rather than reduced drag. Sorry - I misunderstood!

Spoilers can reduce drag AND give downforce
 

I don't understand making thinks look cool, unless it does cool, sorry.

A spoiler should function. This one does:

www.max-mpg.com

Reduces Cd from 0.38 to 0.30, and gives downforce

Ernie Rogers

Does it actually generate downforce, or just reduce lift? (Not just a question of semantics)

Does it actually generate downforce, or just reduce lift? (Not just a question of semantics)

F = ma
 

That's a sharp question, Metro,

The answer is to be found in Newton's laws. The total force acting on the car must be balanced by an equal reaction, seen as air accelerating opposite to the force.

My wing actually slopes downward slightly toward the rear, and is arranged to suppress flow separation near the top. After the car goes by, there is no air moving upward. So, the total force on the car must be backward and slightly upward (lift) Strictly speaking, the wing reduces lift over the rear of the Beetle.

Ernie Rogers

How was that determined?

I suspect coast-down testing. Ernie's a proponent and has posted a few how-to's in the past.

Not soley downforce...if it was only downforce that I wanted, there are much more aggresive designs I could have chosen. Like the hatch-mounted wings that were used on 'Group A' racing wagons in the 1980's. Besides, how much downforce do you think I'd get from a 1" high lip?

Well, I'm not gonna lie to ya'....yes, there is a part of me that wanted to improve the asthetics of the car which is why I made the extra investment in having it painted to match the body color. But, more importantly, I wanted to explore the possibilities of actual function of the rear lip on a two box after seeing them used in many other OE applications.
After all, what do you think the motivation is for auto mfgrs installing rear lip spoilers exclusively on thier high efficiency models? (eg: Honda Civic Hybrid, Volkswagen Jetta TDi)

Just out of curiosity, Ernie...why are the 'side pedestals' of your spoiler so large? Are there lateral stability benefits there?

might the spoiler work a bit like a turbulator? most of the time the airflow does not keep fully attached to a surface and a turbulent layer tends to build up between the surface and the laminar airflow. a turbulator is usually a zigzag ridge of a few mm high that trips the laminar flow back to the surface at a point where it would otherwise totally shed from the surface thus causing more dragg than the turbulator does

the spoiler might act somewhat similar to a vortex generator also, that extends above this turbulent area and draws in laminer flow.

i imagine that the spoiler might serve a bit like a dam that stops the slow turbulent layer from going over the edge and somehow messing things up there and serve as a sharp solid edge for the "clean" airflow the shed of the back into a more favorable path?

anyway it's just a theort that might not have much to do with actual science... i wish i knew more about aerodynamics.

lunar,

It could - but the spoiler is mounted at the trailing edge of the body... It's going to trip the flow and then there's nothing for the flow to stick to...

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I'm really curious of this whole "clean edge" separation thing... sometimes called "clean separation" - something tells me this isn't really possible. Logically, the flow above a flat spoiler will be higher (lower pressure) than the flow just behind the car/under the spoiler (higher pressure).

I'm starting to get into CFD modeling... maybe I should just make a simple scenario and see what happens (that won't be for a couple weeks though)

I think the majority of those 'spoilers' could be better described as 'extensions'. Most are meant to retain attached flow longer over already tapering body lines, or to move the flow separation point away from bodywork that would otherwise draw the flow inwards or downwards in such a way that it causes more lift/drag.

By the looks of the angles at the rear of your wagon, it already had clean separation on both the top & sides. Unfortunately, since there's little taper to the car's existing shape, there would be little to be gained from simply extending bodylines, such as with the roofline 'extensions' on the C30 Efficiency, Polo Bluemotion, or GMC Yukon Hybrid, for examples.

EDIT: The best bet on your car would be to mimic the partial boat tail setups that have been tested on tractor trailers. Or jwxr7's Metro.

I know it's lots of work, and probably not the look you're after.

Metro was right, I did a coast-down test.

The winglets on the sides suppress corner vortices, which helps lower drag.

Also, there is a small gap between the wing and the body. The positive pressure on the top of the wing pushes separation bubbles through the gap which helps maintain flow attachment.

There is positive pressure on top of the wing because the flow curvature is concave upward there-- this lowers drag and gives downforce.

Ernie Rogers

Thanks for the clarification on the winglets, Ernie.

Metro - Upon closer examination of the Volvo C30 photos you posted, I noticed a tighter wheel gap on the "Efficiency" model. Couldn't the lower cD be more attributed to the car having a lower stance rather than the change in the rear spoiler?

Good eye. The Efficiency engineers achieved the cD reduction through a combination of improvements, and the roof extension was just one of them:

I'm not sure if they're listed in order of largest contribution to the improvement, but I wouldn't be surprised.

I agree. Could be highly effective, and is a lot of work. On a Volvo 240/740/940, it's even more work than on the Metro. I had one on my 240 this past summer and finally took it off. It was a duct tape + coroplast project and I was never able to keep the forward edge of the extension smoothly attached to the roof plane.

One tough part is that the side "cuts" where the tailgate separates from the body RE on the car's rear face. And on the top surface, the cut is in the roofline. This creates all kinds of issues in achieving a smooth edge where the extension meets the side panels, and also the roof. I never suceeded so I removed it. I think it possibly could be done using semi-rigid thin panels and sheet metal screws - if you dare go there.

Something else worth mentioning as a sidenote:
The car dealership that I work for is located next to a major 4 lane highway. It was raining yesterday and I took some time during my lunch break to watch traffic pass by on the wet roads. I found it interesting how well the water mist illustrated the wake of each passing vehicle. It was easy to see the low pressure kick-back swirl behind every semi & box truck. But what I found most interesting was the height of the wake behind even the smallest cars. I watched a Chevy Aveo 5dr(no OE spoiler) pass by and the water mist peaked at least 20ft in the air, about 50ft behind the car. Once again, not really a scientific observation...but interesting nontheless.

It would have been neat to see basjoos go by just then, wouldn't it?

Hahaha....I was thinking exactly the same thing! :thumbup:

But I wonder if he has windshield wipers on that large false windshield.:rolleyes:

You can get a relative idea of wake height (comparing one car to another).... But keep in mind that water has more mass than air and aero drag plays a role on each water droplet (as opposed to a uniform fluid - air).

It's cool though -- I like watching cars drive by in wet conditions for that very reason :p It's interesting to watch the interactions at an intersection where wakes in opposite directions collide :p

Another interesting note:
It was raining again this morning as I traveled on a major highway on my daily commute. Traffic, of coarse, was very heavy. I was behind a full-size Chevy van, following @ a safe distance while travelling ~55mph...when I glanced in my rearview mirror, my "wall" of upward flowing water mist was gone! Naturally, I was a lil' miffed.:confused:
When I finally hit some open highway with no one in front of me, I looked back and the "wall" was there again. That's when it hit me....You commonly hear during a NASCAR race how a driver can "loose the air on his/her spoiler" while racing in traffic. I think this what happened to me here.

Anyone agree?

When I drive down a dirt road, I leave behind a light dust trail that's about 2 to 3 feet high (the height of the tip of my boattail). When I drive down a leaf covered road, the leaves just move a little bit on the road and an occasional leaf pops up to the height of the tip of my boattail. In the pre-boattail days I could see many leaves dancing behind the rear window and it would be covered with dust after driving on a dirt road. Of course I can't see it when I'm driving, but I don't think I am leaving much water mist in the air behind me.