Teardrop trailer aerodynamic advice

[Frank Lee]

I doubt the table wind tunnel gives results that are any better than sitting there imagining flow. If it was that easy there wouldn't be much need for multi-million dollar testing facilities.

[Tugger]

In order to gain experience in visualising air flow, "Feeling" turbulence when trailing vehicles on a motorbike, is a HUGE help, as is designing a simple wind tunnel., to visually seek out the turbulent flows, from the less than ideal designs, that produce a turbulent wake and flow separations.

And to declare that a simple kitchen top as worthless, as both a scientific instrument and research tool, simply because the "real ones" cost millions of dollars.

You see my well intentioned friend - the only difference between what you call a worthless junk kitchen table wind tunnel and a "real wind tunnel" - costing millions of dollars, are two things, size and air velocity.

And a simple wind tunnel, is a great aid to the imagination.....

I map thermal gradients and air flows, from the stratified air layers in my own home with an incense stick.

The cooler night air coming in, filling up the rooms from the bottom, as the warm air runs out under the tops of the doorways, and vis versa in the morning when the sun rises and the air outside becomes warmer than the air in the house.

All of the air flows, and turbulence from the delaminating boundary layers, projections etc - everything, can be tracked using an incense stick, in air velocities as low as 1/4 of walking speed.

[elhigh]

Back in the day my brother, dad and I built a small wind tunnel for my brother's science fair project. It used a box fan, a convergent section, and an observation section where the test piece went on a small, homemade balance scale. The weights for the scale were on the downwind side, but in retrospect it would have been much better to have them outside the chamber entirely.

It was a step up from a kitchen table top, but the investment in it was some cardboard and green paint. Looking back I think the paint was very, very close to Matthias Wandel Green.

The wind tunnel's test section was about 8" x 8" x 12", and its design was for measuring differences of lift in wing sections based on airfoil profile and angle of attack. We drew up plans for how to measure drag using a small spring scale, but time got short and that aspect was shelved. Even without that, we were able to quantify our results and compare them to known attributes of the various profiles we tested, and our numbers more-or-less lined up with the stated facts. As experiments go it wasn't anything groundbreaking, but if nothing else it pointed up that we could get good results with minimal equipment. For quantifying the lift of the wings, we used pennies as counterweights.

My point is that Tugger, though a bit condescending, isn't wrong. You don't have to have the big expensive facility to get scientifically valid results. An incense stick and a contrasting background WOULD be excellent for viewing where the flow breaks.

The only trick here would be making really good models so what you test at the tabletop scale translates usefully into real world applications.

[Tugger]

The idea is not for absolute precision - but more to get a stream of air about 5 - 25Kmh (walking speed to bicycle speed), and to see how the air moves around the object; or as the stationary object, moves through the moving air.


Consider a plain cardboard box - a good container shape, compares to a torpedo or boat tailed bullet shape - a good at moving through liquid shape.

The flip side is the box moves through air like a parachute, and the really good air penetrating shape of the torpedo or spire pointed, boat tailed bullet, is an incredibly **** shape to get things in and out of, and transport things in.

So by seeing where all the power chewing features of the box are, with a wind tunnel, where the air flow separates from the shape and creates all the turbulence from, that is where you can add the aerodynamic (or hydrodynamic) properties of the torpedo's shape, to the box shape of the trailer.

And it's not much of an effort to turn a totally **** aerodynamic shape, into one that is a remarkably good improvement.

At the end of the day, you can't make a shoe box operate like a javelin, and you can't make a javelin as useful as a box - but you can make a very aerodynamic shoe box.

Basically what your after is the general shape of an aircraft wing, with rounded over ends - for the sides.

Have a look at the curves and blended tapers on the bus and the nascar.

The bus is technically a house brick, but at it's slow speed, it's aerodynamic profile basically eliminates all significant sources of drag - making that component, a significant part in reducing it's fuel consumption.

And the NASCAR, although these run at 320Kmh, and not 100Kmh, like a typical trailer would, all the principles of shape, blended curves, tapers that you need in your trailer design.

https://en.wikipedia.org/wiki/File:Leading_edge.jpg

https://en.wikipedia.org/wiki/File:Tempo_SR.jpg

https://en.wikipedia.org/wiki/File:K...ge_Charger.jpg

Get something like a shoe box, or smaller, and make it from clay or blocks of watery plaster (easy to shape) and start working on that - an incense stick for your smoke generator and even an open window with a bathroom fan sucking the air into the room, through the window.

You now have all the info you need, study the subject, do your tests, and then make it work.

[freebeard]

Originally Posted by Tugger

You see my well intentioned friend - the only difference between what you call a worthless junk kitchen table wind tunnel and a "real wind tunnel" - costing millions of dollars, are two things, size and air velocity.

The third thing is Reynolds number. Stick around and you might learn about it.

Originally Posted by Tugger

I map thermal gradients and air flows, from the stratified air layers in my own home with an incense stick.

All of the air flows, and turbulence from the delaminating boundary layers, projections etc - everything, can be tracked using an incense stick, in air velocities as low as 1/4 of walking speed.

As have I. The incense smoke dissipates at half-walking speed.

...and even an open window with a bathroom fan sucking the air into the room, through the window.

Oh, come on. That's no longer a tunnel. It does solve the problem of tunnel blockage though.

I don't know if you lurked before emitting 8 posts in 24 hours, but in case you've not found them:

http://ecomodder.com/forum/showthrea...ead-26678.html

http://ecomodder.com/forum/showthrea...els-23040.html

http://ecomodder.com/forum/showthrea...el-3432-2.html

At the scale you're working in a water tunnel is appropriate:

http://ecomodder.com/forum/showthrea...ies-31047.html

[Tugger]

And yeah, been on and off this forum for years...

And no - didn't read up on anything else, from this site.

Was researching something - and came across a link to this topic and shared my expertise on the subject.

And it's an odd thing that when coming home with a big load on the motorbike, into a cross / head wind, where I am forced by circumstance, to ride through the wakes of the passing vehicles.

And the wakes of aerodynamically messy vehicles, ARE remarkably different, to very low COD vehicles....

I am not a car fan per sae, but aerodynamically speaking, I really like the shape of this carriage.

https://upload.wikimedia.org/wikiped...-12-28)_02.jpg



When getting passed by an aerodynamically messy vehicle, you and the bike are getting pushed from the "bow wave", pushed sideways from the compressed air envelope, and sucked back in behind the vehicle as it pulls ahead....

The bow wave, is the "slug of air" being pushed off the front of the vehicle, the "suck in ' suck along" effect along the side of the vehicle, is the vortices and turbulent air flow, behind the bow wave, and the suck in after the vehicle, is the huge zone of negative pressure behind the vehicle....

You can see it in theses ship driver training videos - but when your on a motorbike - you can feel it - in the air.

https://www.youtube.com/watch?v=9jwb0-r0haw

https://www.youtube.com/watch?v=dq8EQVl7GG0

Where as the low COD vehicles slide cleanly through the air, and hardly creates any disturbance.

[Tugger]

But you can make it a very aerodynamically efficient house brick.

Tapers, blended curves - all the functional features of the modern car.

[Tugger]

Aerodynamics - aerodyschmamics...

The Bugattie Veyron, Meh - a high priced set of wheels, and racing cars, have been using the same old stream lining and wind tunnel tested designs that were used on stream liner trains, cars, aircraft when power and range and fuel consumption - and loads = running costs; were beginning to take real priority over cheapness of construction.

And as Zepplins, stressed metal skin monoplanes, racing cars and motorcycles, and similar all got going, especially after WW1, all the basic principles of a LOW coefficient of drag, were established, and well many things have improved, but air had the principles of friction, and drag have remained constant.

So the Bugattie, or more so the photo of it, shows off the blended curves, tapers and it is a more or less very low COD shaped box trailer.

Take away the passenger compartment details like a windscreen, and cooling ducts etc., and you have a nuclear submarine hull shape, made into a shoe box profile.

When you look at the Douglas DC-3 - a pre WW2 aircraft design, well all the parameters still stand, and yet while the formula 1 racing cars, did, for a while, look like supersonic jet fighters, yet they were no faster than the DC-3.

i.e. - there were a lot of stupid designers around.

So the nice rounded nose, the middle bit, and the tapered rear...

It all still stands.

The fastest cars in history: 1946 to now

[freebeard]

And yeah, been on and off this forum for years...

And no - didn't read up on anything else, from this site.

http://ecomodder.com/forum/showthrea...ists-7118.html

http://ecomodder.com/forum/showthrea...ion-21952.html

[aerohead]

I began with my grandfather's old 3-speed fan and a cigarette.Later I made a cardboard tunnel for 24th-scale models, which had a thread leading over a pulley, down below the floor of the tunnel to a pan which you could balance the drag force by adding B-Bs as weight to the pan until the model stopped being blown back in the test section.
Being a 24th-scale model,they never generated a proper drag coefficient,so none of the drag measurements had any real meaning,but you could do what Alex Tremulis called 'pick-and shovel' aerodynamics with it.
I say,go ahead and play with it,but understand that, as others have mentioned already (and they are correct) whatever you come up with would NOT be recognized as real science.