Brilliant way to visualise/see air/fluid flow
 

This is a brilliant way to see the flow around a body of any shape.
Just brilliant!! And a fellow South African judging by the accent.

https://www.youtube.com/watch?v=CncGLXxG3ZE

Agreed. No response to my thread from yesterday yet.

ecomodder.com/forum/showthread.php/rheoscopic-fluid

Did not see your post yesterday, but yes this is certainly very visual.
Now can I see a "teardrop" trailer test?:)

All one needs is a test trailer and a custom receiver hitch. Scientific equipment uses a ring that deforms with a sensor to accurately measure when it goes out of round. So the [removable] hitch would have a ball on one end and a Class 1 or 2 tongue on the other. As the [thick steel] ring in the center goes out of round it's width narrows.

Ask Him...

Or get some graphite etc. :)

That is a measurement. The previous test shared here gave visual results, staying with that line of thinking, I want to "see" what the "teardrops" actually deliver with this innovative IMO test.

' 2-Dimensional Tow-Tank flow visualization '
 

1) In order for the 'scale' flow to represent 'real flow' ( verisimilitude ) we need a minimum Reynolds number ( Rn ) of around 1,000,000, which is the 'supercritical Rn' for a fully developed turbulent boundary layer to exist.
2) This is essential if 'real flow' effects are desired in a scale model.
3) You'll need the formula for Rn.
4) You'll need the kinematic viscosity of the 'fluid' your choosing for the water table.
5) Knowing the kinematic viscosity, and supercritical Rn requirement, you reverse-engineer the water table flow velocity requirement necessary to balance the equation.
6) If you fail to satisfy all conditions of verisimilitude, what you observe will be 'bogus.' And there's no way you could trust scaling up your results in order to create a 1:1-scale 'body'.
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7) If testing for a 'teardrop' trailer body, 2-D flow will not reveal the extremely high vortex-drag which exists in 3-D flow. This was tested to high precision by Fachsenfeld, who presents the wind tunnel photographs, plus associated drag tables, in his self-published ,1951, Aerodynamiks Des Kraftfahrzeugs.

I'm having trouble sorting out whether the viscosity of the fluid needs to be greater or less at smaller scales.

Gnats feel air like we do water, so maybe a less viscous fluid like ethanol?

Here's a CFD analysis of fifth-wheel teardrops: www.sciencedirect.com: Shape optimisation of teardrop trailers to minimise aerodynamic drag in articulated lorries

https://ars.els-cdn.com/content/imag...000538-gr6.jpg

' greater or less '
 

1) It all come down to Reynolds number, which must always be 'supercritical.' ( one-million or above ).
2) 'Kinematic viscosity' is part of the equation used to calculate Rn. It must be known, just as with air density ( rho ) in the drag force calculation.
3) 'Scale' is the other consideration, since 'Length' of the body under consideration is literally one of the factors used in the equation used to calculate Rn .
4) 'Velocity' is the limiting factor in scale testing, as there will be a point where flow is so great that air becomes 'compressible' and supersonic shockwave drag can be present around the body, due to it's super-velocity effects.
5) We must limit ourselves to 'low-speed' aerodynamics, of 'subsonic' flow.

Now that I've thought about it: upside down, *in water*, with hydrogen bubbles.

The kinematic viscosity of water is an order of magnitude smaller than that of air--so using water instead of air, you can use an object an order of magnitude smaller in length, or test at a speed an order of magnitude smaller, or some combination of both.

' upside down...'
 

1) 3-D underwater tow tests are performed with the model attached to an overhead 'sting', supported from an overhead gantry-crane, which travels the length of the tank, wheels of the model 'rolling' in contact with the tank floor.
2) The 'sting' may contain six piezo strain gauges from which pitch, roll, yaw, front lift, rear lift, and axial drag forces can be captured.
3) At Texas Tech University's tow tank facility, Ford Motor Company chose a 1/3rd-scale model for their Taurus testing, at a cost of $68,000.
4) A relative of mine, Paul Bowers, of Bower's Plastics, constructed the different noses for the model testing, designed by Jack Telnack, Global V.P. of Design at Ford, groomed by Alex Tremulis, former chief of advanced design at Ford.
5) Tufts can be used in underwater testing.
6) Food coloring emitted from pre-drilled orifices in the surface of the model.
7) When the water becomes clouded by the food coloring, chlorine injection quickly clears the water. Chlorine content is carefully monitored to protect the safety of the SCUBA divers who must descend to the bottom of the tank, to alter the model for each run.
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8) Hydrogen bubbles would be limited soley to aerodynamic testing, as the mass of the hydrogen gas, plus the mass of the 'bubble' just equals the mass of the air. Surface tension of a 'bubble' would be destroyed underwater, with the gas rising to the surface.
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9) Submarines tested underwater at the US NAVY's, David Taylor Model Basin, in Carderock, Maryland, have also been tested, since 1953, at NASA's Langley, Virginia wind tunnel facility, where tufts and smoke can provide flow visualization which cannot not be found in the water ( Project: USS Albacore ).

Rn aside, I don't see how the lorry "teardrop" pictured and very useful IMO above shares ANY real world aero similarity to the RV "teardrop' trailers discussed on this thread.

It was simply the closest match I could find to the subject.

You are welcome to do better.

' real world '
 

* I'm of a similar opinion.
* While the semitrailer, involved in 'commerce', must justify some rational economic justification for it's inclusion, compared to 'conventional' dry vans, the RV 'teardrop' trailer ( the entire RV industry ) remains a 'want to have' luxury, hoping to attract dollars away from competing products, with the nebulous promise of some vague, perceived advantage, over other products in same price range, while offering no actionable contrasting information which might actually 'inform' a potential customer about 'real' attributes.
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While the trucking industry participated in, and contributed to, the 1980's semitrailer aerodynamic technology developed alongside NASA's R&D at Edwards AFB, California; it appears that, until the advent of electric mobility, nobody in the RV industry, with the exception of two new startups, are even aware of what was 'discovered' by 1980.
There are no 'teardrop' trailers in existence, within the 'aerodynamic' context of 'teardrop,' as conceived within aerodynamic parlance.
No one except AeroStealth, in his YouTube videos, has shared anything in the public domain, remotely approaching the actual fluid mechanics of RV trailers of any stripe.

Quite so. Cant argue with all that.

However:
The lay person has no cooking clue whatTF airflow looks like around a wing or cylinder etc-etc.
This is simply a clever way to plant the seed that then might flower into the likes of your understanding of aerodynamics.

I have also heard it said that aerodynamics is so chaotic/random that if you hear an aerodynamicist sounding certain; he doesn't know his job! :)

Makes my head hurt. The only good teardrop is a dead teardrop?

It would be interesting to put a balance beam in an aero/hydrodynamic tunnel trailing scale models of equal frontal area. I'd start with a teardrop and a Cybertruck. Or an old Beetle?

Or a teardrop with a 4% radius on the edges.

' chaotic/random '
 

* it would be incumbent upon the naysayer to provide the caveats / conditions under which they are clouding the waters with respect to 'aerodynamics.'
* within the context of EcoModdder.com, and streamlining a car to make it 'aerodynamic', and respecting the narrow definition of 'aerodynamic,' as laid out by Dr. Wolf-Heinrich Hucho, in his ground rules of fluid mechanics, ' chaos / random ' is what we'll be eliminating, with the exception of the entropy of the turbulent boundary layer, which is totally inescapable, and the knowledge of, not even 'actionable' information.
* I can only promise you, with 100% certainty that, if you follow Hucho's recipe for 'aerodynamics', you can arrive at road vehicles in the range of Cd 0.09,to, Cd 0.07., with 'off-the-shelf' technology.
* 'low-drag' is 100% understood.
* 'low-drag' is not a 'black art.'
* anyone speaking to the contrary is completely out of their depth, with unspeakable consequences.

The difference between black arts and engineering is advanced mathematics

...if you follow the recipe; it's difficult to respect ingress/egress, breakover angles and worst of all, rear view mirror requirements.

' recipe '
 

1) If you're talking about headroom, then, 'solutions' have been available for that for a long time.
2) Approach, breakover, and departure angles are a nothing burger.
3) I had a CDL for a dozen years and never relied on a center rear-view mirror in commercial vehicles. Current 'synthetic' vision is superior to optical systems, personally, I don't see an issue.

How'a about legally?

To my limited knowledge, I have not seen a minimum size required in rear view mirrors in any jurisdiction...... In Cali, dental mirrirs have been used for decades. Couple that with a camera/monitor system, should be golden.

' legally '
 

Everyone's waiting on the DOT to move the dial.

But still conceding both have the potential of achieving positive outcomes?
I like to remember the example of giving an engineer a dimpled plastic-coated sphere a little over 1.5" in diameter and ask for the calculations of what forces, angles, etc are needed to place it within 2ft? of a 4" round hole 180 yards away, or just hand the same ball to Tiger and see in done in approx 90 seconds.:D

In my world, calling 4% a teardrop anything is how the public gets misled by marketing.

Teardrops don't have edges? :confused:

Nor does a golf ball.

Hard to put doors on a golf ball.

https://ecomodder.com/forum/member-f...qevko1-250.jpg

The reduction to practice would a 37 mm adaptoer for the AR-15 Can Cannon (normally 67 mm). 90 seconds vs subsonic.

' teardrop edges '
 

Only a 'half-body', based on a 'teardrop' ( streamline body of revolution ) would have 'edges'; at its lower interface with the underbody.
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The ' 4% ' radii may be from Hucho's reporting on the R&D of the Volkswagen Vanagon, for which they discovered that forebody drag reduction hit 'saturation' with leading edge radii = 4% of body width.
Any additional 'softening' of the corners was rewarded with 'zero' additional drag reduction, hence 'saturated.';)
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On 'trailers', some use the square-root of the frontal area to use in the calculation, instead of width or height ( they're just creating an 'square -rectangular' cylinder of equal height and width, to simplify an averaging dimension ).
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I use it in the dimensional analysis of each vehicle, reducing them to a rectangular cylinder, for which to create the bar-graph from which the comparisons can simply be made.
( the one car associated with the 'shortest' cylinder, compared to all of those from those associated with the same drag coefficient, is, by 'default', the 'optimum' , most 'efficient' body shape for the entire field.
It would be your 'template' for modifications.

Aren't all cylinders rectangular? Excepting [s]non-parallel[/s[ right angle end caps?

My point had to do with putting a balance beam with two models in a wind tunnel. (Or on a platform on a moving vehicle).

At Permalink #29 I showed Wally Byam's teardrop with a 50% radius on the front edges. Airstream have brought this back in their Base Camp model.

IMO to retain some authenticity to the "teardrop" moniker, seems at the least the overall shape needs to be nonsymmetrical on at least one plane.

Else aerohead's half-body in profile with inadequate fineness ratio.

My geodesic approximation of the half body is asymmetrical on two planes.

https://ecomodder.com/forum/member-f...07-7-35-02.png

' cylinders '
 

* I'm not in possession of the necessary data from which to accurately answer that question.
* I chose 'square', as, from a 'forwards', or 'rearwards' observation point, most RV travel trailers have a 'rectangular' footprint.
* If you design edge radii from the 'height', it won't match the 'sides,' and vice versa.
* By using the square-root of the frontal area, you get a 'compromise' radius which agrees all-around ( this was formally presented as an an aerodynamic 'solution' in Hucho's 2nd Edition book ).
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* I don't comprehend what you're describing when you you use the term 'balance beam', and I'm reluctant to speculate.
* I see no advantage to Wally's design, nor the Base Camp's.
* 'Aerodynamically,' there would be no gap between the TV and TT.
* Their frontal areas would be equally matched also ( take a look at El Paso ( Texas ) Sun Metro's, NFI Group's, low-floor, articulated, 100% gap-filled, pusher-trailer transit buses ).https://www.flickr.com/photos/drum118/48193865117

' inadequate fineness ratio '
 

That's a good way to put it!
When the aft-body portion is less than 350% of the body's 'thickness' ( V ),the attendant super-deceleration of the flow creates the adverse pressure gradient that triggers flow separation ( at the heart of what Fachsenfeld / Kamm found at the FKFS, around 1935 ).
These 'bluff' rear ends are responsible for crappy, vortex-induced drag and lift coefficients.
The 'double-frontal-area' pressure signature of the 'double-hump' TV/TT 'system' aggravates the situation.
A 'real' aerodynamicist would insist on an aerodynamic 'singularity.'

Being from Texas, perhaps your familiar with a singletree?

Quote:

https://chimacumtack.com › blog › 2017 › 09 › 02 › singletrees-eveners Singletrees and Eveners | Chimacum Tack According to Webster's New World Dictionary a single tree is a bar attached at the center of the hitch on a wagon, carriage or plow which is hooked at either end to the traces of the horse's harness. The function of the single is to balance the weight being pulled. The singletree is mounted on a post or pin

An aerodynamic compromise to accommodate interior headroom?

' balance beam '
 

here's some considerations:
1) in order to guarantee reliable quanta, you'd be looking at, at least, 40%-scale models.
2) considering the depreciation in the value of the US Dollar between 1990 and now, a single model would run you about $152,000.
3) there are no wind tunnels to my knowledge that will accommodate two models, side-by-side in a test section.
4) you'd be limited to testing each separately, then comparing the data.
5) from the left coast, to the right coast and back is a 6,000-mile trip.
6) at 14-mpg, pulling a two-model trailer, and $3.30/gallon, your fuel would be $1,414.
7) motels would run around $1,680.
8) meals about $840.
9) wind tunnel time, in a rolling-road test section is gonna be $4,000 / hour, times 2, for $8,000.
10) for one person, one 'test', about $ 316, 348.
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11) if you had your two, ($ 304,414) models, and could get them 3-D scanned, and imported into something like AirShaper, you could save a little over $ 3,900 using CFD.
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12) the models would be too large to place side-by-side atop even the first of three flatbed railcars, pushed from behind by a single locomotive, as has been done in the past for 'single' models.
13) The flow interference between the two would 'wreck' the test, and vibration from the rails would probably ruin the load cell null calibration and any subsequent measurements.
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14) if your talking about TVs & TTs tested as a unit together, you've just 'doubled' your expense, and I'm unsure who could even navigate that kind of testing. NASA's Ames Research Center in Palo Alto, California is one of the few tunnels that can handle TV/TT combinations. And you typically need a DARPA grant to even gain access to the facility, the ' world's largest '.