Brilliant way to visualise/see air/fluid flow

[Vman455]

Originally Posted by freebeard

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?

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.

[aerohead]

Originally Posted by freebeard

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

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 ).

[j-c-c]

Originally Posted by aerohead

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.

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.

[freebeard]

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

You are welcome to do better.

[aerohead]

Originally Posted by j-c-c

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.

* 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.

[Logic]

Originally Posted by aerohead

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.

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!

[freebeard]

There are no 'teardrop' trailers in existence [within the 'aerodynamic' context of 'teardrop] as conceived within aerodynamic parlance.

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.

[aerohead]

Originally Posted by Logic

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!

* 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.

[Piotrsko]

The difference between black arts and engineering is advanced mathematics

[freebeard]

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 0.0

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