Spin the teardrop around

[printer]

Seeing the squaredrop designs and some of the more curved designs I am wondering about which would have less drag. While the teardrop shape looks wing-like the great slope of the back may not be doing much to aid aerodynamics. From what I recall a slope greater than 8-12 degrees results in separation of the airflow from the surface. With the teardrop spun around it would seem to cut the air better, the question is on the wake generated. With a raised back some have incorporated a lip on the back resulting in the air on top of the surface to curl under and fill in some of the negative pressure area reducing the wake.

Or so the theory goes. Anyone have some wisdom they can give to a confused builder?

[saywhatthat]

your theory dose not work you have a tow vehicle in front messing up the airflow

[tony.latham]

I agree with SayWhatThat.



The tow vehicle changes the front-end drag game.

Tony

[printer]

So due to the tow vehicle the theory does not work for a teardrop the normal way or the theory does not work for it turned around? Obviously having the edge of the reversed teardrop below the tow vehicle's bumper will not do much. I am talking about a slope up from about trunk lid height. And what is considered a mid sized car now days. Not a SUV or truck.









This one is more like a reversed teardrop.

[Tom&Shelly]

Seeing the squaredrop designs and some of the more curved designs I am wondering about which would have less drag. While the teardrop shape looks wing-like the great slope of the back may not be doing much to aid aerodynamics. From what I recall a slope greater than 8-12 degrees results in separation of the airflow from the surface. With the teardrop spun around it would seem to cut the air better, the question is on the wake generated. With a raised back some have incorporated a lip on the back resulting in the air on top of the surface to curl under and fill in some of the negative pressure area reducing the wake.

Or so the theory goes. Anyone have some wisdom they can give to a confused builder?

A square-back will have air separation at the back corner. At separation, the air will become turbulent. The energy to create that turbulence comes from your TV's engine which is another way to say it increases drag, and reduces gas mileage. (Also, if you want to use a laser communication system through the turbulent air, you will have problems. You probably don't, but that's how I happen to know what I know about turbulent air.)

So to minimize or eliminate the "boundary layer separation" it's better to have a tapered tail. Very tapered is better, but that makes a lousy galley, so we have to compromise. Ideally from the point of view of eliminating turbulence, I suppose, it would look like the tail of an airplane without the tail planes or rudder.

A square front on a trailer might work, if it works with the TV. What you would really like is one smooth body, like an airplane fuselage, so the air flows smoothly around it, with no sharp edges for the boundary layer to separate. Again, you can't do that for practical reasons, and so you want a body that picks up the air behind the TV and encourages most it to reattach to the sides and top of your trailer.

That's about as far as intuition can take you on this problem. It can be modeled mathematically for specific shapes of TV and trailer, but it literally takes a super-computer to try each specific shape. Or one can build a model and either put it in a wind tunnel, or drive it down the road. Lot's of us have done the latter, and it would be interesting to collect data. But roughly, it seems like the differences in detail make minimal difference in actual performance. (We really need a "shrug" smilie icon.)

Also, of course, a rounded leading edge on a teardrop, like the Benroy design, makes the thing stronger, as a bent piece of plywood (or any other sheet material) cannot bend in a different direction, while two flat pieces of plywood can bend all over the place.

Then there is the matter of aesthetics. Jeeps are square because they always were, and we Jeep owners know we're cool, even though we're driving a box down the road. Designers of Cars, trucks, and TVs that get better gas mileage have gone through the trouble of modeling their creations on supercomputers. Too bad few or none have tried different trailer designs with their models.

Helpful?

Tom

[printer]

Thank you for the detailed answer. I have been involved in making RC airplanes and do get the ideal shape concept. I also get that if the air separates from the wing it messes up the air profile. With a teardrop shape I would expect a large separation to occur and turbulence behind the trailer. But I have noticed some doing the opposite, with the sloped front and the tall back. One saying it works great for boats, it works great for them. So even though I knew from common knowledge that the teardrop should be better, but, well, just because it is common knowledge it may not be right. I have set up experiment in other fields just to confirm what was common knowledge was right and actually got a surprise when it wasn't. But enough of that.

Actually you do not need a lot of computing power to get an answer now. Not to say it did not take me some work to get to that point. I searched for a profile drawing of my car, cut it out and transferred it to card stock, took a picture of it, uploaded it to a simulator and, problems. Finally figured out it did not like a picture width greater than 1400 pixels. Then, I found out it did not like my yellow car (it was yellow card stock) I need to take a picture of a black card on a white background. So I painted the car and trailer outlines, mounted them, took the pictures, played with the minimalist photo editor in Windows, junked it and imported into Paint. The black paint was reflecting the flash, the white paper did not seem whit, so I traced with a black pen, filled in the insides with black, traced around it with a white pen, painted around it with white, was a time consuming effort. Mind you better to do it now than suffer later.

So I uploaded the picture of the TD to the simulator software and while it was running I finished up drawing the square back trailer picture. I downloaded the TD movie (shows a 10 second clip) and the turbulence behind the TD was not that bad. I did put a propane box between the trailers and the car as I knew the larger the gap the worse the performance. Even so there was some turbulence because of the gap. By now the squareback was finished its simulation ready for me to download. Opened it up and, well there is a song called 'She ain't pretty she just looks that way', or close to that.

I mist have missed whiting out an area or two, there seems to be more turbulence over the car than on the TD video. The air over the first half of the square back is well behaved, not bad over the top to the tail. It leaves the top and the bottom with nice little vortexs and when they meet up, EVERYTHING GOES TO HELL!!! It is almost comical how bad things get. The top vortex grows until it is the height of the trailer then sheds itself and the vortex on the bottom of the trailer grows to the height of the trailer then shes, then the top repeats itself and... you get the picture. So I will have no concerns with my choice in making the teardrop now.

[tony.latham]

But I have noticed some doing the opposite, with the sloped front and the tall back. One saying it works great for boats, it works great for them.

Tony

[printer]

I thought of putting wings on the back but I might forego that on this one.

[Tom&Shelly]

Actually you do not need a lot of computing power to get an answer now.

I've seen programs that allegedly compute turbulence, but when you look closely they really do it in two dimensions. "For illustrative purposes only". To do it for real (three dimensions) takes quite a bit more computing power. To do it in a reasonable time, it might take a thousand or so, $2000 engineering workstations.

Tom

[western traveler]

You guys are awe inspiring…
(I can do celestial navigation but I hate fractions and electricity).

[tony.latham]

I thought of putting wings on the back ...

For what function?



Tony

[DrewsBrews]

Power boats use the flat back because they intentionally want the flow separation. That is how they get on-plane at higher speeds. However boats that need efficiency at slower speeds, such as sail boats, will have a more smooth transition to the transom where it eventually cuts off. I think the idea is to try and keep the slope within the range to keep flow connected as long as possible until you just run out of trailer (or boat). Ideally it would keep going to a point, but that would lead to an unpractically long length. From what I hear, on teardrop trailers the flow typically separates near the galley hinge once the slope becomes too great.

[printer]

I thought of putting wings on the back ...

For what function?



Tony

The cool factor.

[printer]

Power boats use the flat back because they intentionally want the flow separation. That is how they get on-plane at higher speeds. However boats that need efficiency at slower speeds, such as sail boats, will have a more smooth transition to the transom where it eventually cuts off. I think the idea is to try and keep the slope within the range to keep flow connected as long as possible until you just run out of trailer (or boat). Ideally it would keep going to a point, but that would lead to an unpractically long length. From what I hear, on teardrop trailers the flow typically separates near the galley hinge once the slope becomes too great.

Power boats use the flat back because they intentionally want the flow separation. That is how they get on-plane at higher speeds. However boats that need efficiency at slower speeds, such as sail boats, will have a more smooth transition to the transom where it eventually cuts off. I think the idea is to try and keep the slope within the range to keep flow connected as long as possible until you just run out of trailer (or boat). Ideally it would keep going to a point, but that would lead to an unpractically long length. From what I hear, on teardrop trailers the flow typically separates near the galley hinge once the slope becomes too great.

I thought that it would separate around the hinge line myself and therefor my question. I was unsure what happens behind the trailer, I had no experience in that regard. I used to do sole RC sailplane flying but the airflow behind the wing is much better behaved than a trailer. I read the easily accessible tractor trailer theories on aerodynamics but they are dealing with long bodies, although the square backs have relevance here. I just did not find anything regarding a car pulling a trailer. I was going to do the 1/5th scale model approach with foam models but then had the thought to see if there was any crude modeling software out there to get a rough idea. Not like I was trying to tweak a shape for best performance, more to just get an idea. Now will the difference in trailers make much difference to most people, probably not. But I have a sail boat inspired car with inadequate drive train and brakes from accounts.

While the air does get turbulent over the rear hatch what the 2D modeling software shows is the vortex being sucked down to ground level rather than flapping all over in the wind behind the trailer. There still is some turbulence but not the major buffeting seen with the square back model. Mind you, there is still the air from the sides of the trailer to factor in but I doubt the square back (or should I say rectangular) would fair better. I also wanted to know if a small lip off the back helped resulting in air curling down to fill the negative pressure region. It did not seem to have a significant effect. I did do another shape with a curved back top corner which was interesting. Much better behaved but the video clip of it stopped at ten seconds just when it looked like thing might get interesting. The top and the bottom flows seem to be equal to each other and counteract each other rather than one or the other using the total energy at the back of the trailer to produce a large vortex. Another 5 seconds might have shown them acting as in the square back, I think I may play a little more with it. I was also surprised that most of the air above the car's surface curled down after the trunk and under the trailer.

Aw heck, some pictures.









I would have like to see what was going to happen next if it was going to throw up the disturbance above trailer height. I also added a couple of 'wing flaps' at the cutout areas, they seemed to help.

The semi-teardrop.









And the square back. I did have a different picture for this one and it seems to have more 'noise' in the air. You can see it on the car also. I was just learning a little how to use the software here. But it still shows the difference.







I want to redo my vehicles, use one picture and modify it, that way have a consistent parameters.

Much better when viewing the video. I hope to redo them and maybe post them yet.

[tony.latham]

Camp-Inn says that their Raindrop gets better milage than their Classic Teardrop. Something to ponder. But I think one would be more labor-intensive (and technical) to build.



Tony