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Old 06-10-2020, 02:26 PM   #11 (permalink)
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vorticity

Vorticity is a form of transverse boundary layer contamination, reported very early by NACA (precursor to NASA),during airship hull developments in the 1920s.
All vortices are a result of shear. The shear is a symptom of any adjoining flow fields of varying velocity/pressure (Bernoulli Theorem) which can physically come into contact with one another.
As high pressure always moves towards low pressure, in an effort to reach equilibrium pressure, then we're talking about 'slower' air flowing towards 'faster' air ( again, Bernoulli).
On an automobile, typically, the fastest air (and lowest pressure air) resides on top of the car, as the greenhouse eruption forces it to travel the longest path, while meeting the slowest air at the same time at the rear. ( this is where Julian is dead on,with respect to local pressures).
The only way this can happen is to travel faster over the top.
Since all streamlines are isoenergetic, due to 'conservation of energy' (Thermodynamics), the only way the faster air can gain kinetic energy of motion, is to lose potential energy of local pressure.( It's remarkably like the mass/energy trade-off in Einstein's Theory of Relativity ).
Slower, high pressure air alongside the vehicle is attracted to the low-pressure,fast-moving air above the vehicle, and their velocities where they come together determine whether or not shearing forces will be great enough to spool the intersecting flow up into a vortex ( like the Jet-Stream rolling a rising column of warm,moist,super-cell,Gulf air onto its side to create a tornado).
We know from experiment that, even on simple prismatic bodies, that all vorticity can be 'tuned' out, by careful treatment of the roof angle, side bot-tailing angle, and diffuser angle.
On more rounded shapes the same holds true.
Some longitudinal vortices can be killed simply by properly softening (radiusing) the side/roof/C-Pillar intersection.
What we're looking for is a 'spike-free' pressure environment. Streamline bodies have this architecture. It's why Hucho recommends them. And why I do too.

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Last edited by aerohead; 06-10-2020 at 02:28 PM.. Reason: spelling
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Old 06-10-2020, 06:40 PM   #12 (permalink)
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Quote:
Originally Posted by aerohead View Post
Vorticity is a form of transverse boundary layer contamination, reported very early by NACA (precursor to NASA),during airship hull developments in the 1920s.
All vortices are a result of shear. The shear is a symptom of any adjoining flow fields of varying velocity/pressure (Bernoulli Theorem) which can physically come into contact with one another.
As high pressure always moves towards low pressure, in an effort to reach equilibrium pressure, then we're talking about 'slower' air flowing towards 'faster' air ( again, Bernoulli).
On an automobile, typically, the fastest air (and lowest pressure air) resides on top of the car, as the greenhouse eruption forces it to travel the longest path, while meeting the slowest air at the same time at the rear. ( this is where Julian is dead on,with respect to local pressures).
The only way this can happen is to travel faster over the top.
Since all streamlines are isoenergetic, due to 'conservation of energy' (Thermodynamics), the only way the faster air can gain kinetic energy of motion, is to lose potential energy of local pressure.( It's remarkably like the mass/energy trade-off in Einstein's Theory of Relativity ).
Slower, high pressure air alongside the vehicle is attracted to the low-pressure,fast-moving air above the vehicle, and their velocities where they come together determine whether or not shearing forces will be great enough to spool the intersecting flow up into a vortex ( like the Jet-Stream rolling a rising column of warm,moist,super-cell,Gulf air onto its side to create a tornado).
We know from experiment that, even on simple prismatic bodies, that all vorticity can be 'tuned' out, by careful treatment of the roof angle, side bot-tailing angle, and diffuser angle.
On more rounded shapes the same holds true.
Some longitudinal vortices can be killed simply by properly softening (radiusing) the side/roof/C-Pillar intersection.
What we're looking for is a 'spike-free' pressure environment. Streamline bodies have this architecture. It's why Hucho recommends them. And why I do too.
So in summary, you subscribe to Dick Barnard's approach, which is the one I have largely covered in the book. Unfortunately, some very well qualified practising car aerodynamicists don't agree with this.
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Old 06-11-2020, 02:51 PM   #13 (permalink)
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I posted this in the 911 thread as well. I think it should be Stickied somewhere every Ecomodderer will see it.

The dead fish is creepy, but it validates Viktor Schauberger.
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Old 06-11-2020, 04:08 PM   #14 (permalink)
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... I posted this in the 911 thread as well. I think it should be Stickied somewhere every Ecomodderer will see it.

The dead fish is creepy, but it validates Viktor Schauberger.
That was really very, very good. I watched with my daughter, who is headed into an aerospace engineering program this fall. The description of Reynolds Number, the explanation of friction drag and growth of turbulancein the boundary layer, that votices dissipate into heat, the dead fish, the vortex generators... good vid!
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Old 06-12-2020, 12:20 PM   #15 (permalink)
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Quote:
Originally Posted by JulianEdgar View Post
So in summary, you subscribe to Dick Barnard's approach, which is the one I have largely covered in the book. Unfortunately, some very well qualified practising car aerodynamicists don't agree with this.
I subscribe to fluid mechanics.
Vorticity is an artifact of shear. ' ... shear stress Tau is proportional to the velocity gradient du/dy.' ( Hucho page-48)
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1) There are practicing cardiologists in the USA who prescribe the Standard American Diet to their patients, while they,themselves are practicing vegans, who know that the SAD causes the , morbid obesity, type-2 diabetes,high blood pressure, artereriosclerosis, enlargening of the heart, stroke, and myocardial infarction.
2) there may be a corollary with respect to aerodynamicists. ' It's hard to get a man to understand something,when a condition of his salary is to not understand it.'
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Old 06-12-2020, 03:49 PM   #16 (permalink)
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I'd feel better if either of you would comment on the video instead of lobbing grenades at each other.

Comment on this: https://en.wikipedia.org/wiki/Chaos_theory#History
Quote:

Turbulence in the tip vortex from an airplane wing. Studies of the critical point beyond which a system creates turbulence were important for chaos theory, analyzed for example by the Soviet physicist Lev Landau, who developed the Landau-Hopf theory of turbulence. David Ruelle and Floris Takens later predicted, against Landau, that fluid turbulence could develop through a strange attractor, a main concept of chaos theory.

"Jerk_systems" LOL
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Old 06-12-2020, 03:57 PM   #17 (permalink)
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video

maybe in my next life. i'm kinda busy.
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Old 06-12-2020, 04:12 PM   #18 (permalink)
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I'd feel better if either of you would comment on the video instead of lobbing grenades at each other.
The year was 2020, it was total war.
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Old 06-12-2020, 04:24 PM   #19 (permalink)
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The year was 2020, it was total war.
I agree with freebeard and jakobnev. I'll add that you both--Phil and Julian--make important contributions here that many often find valuable. I'd love it if you would call a truce.
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Old 06-12-2020, 09:05 PM   #20 (permalink)
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Two more with no comment on the video. Nobody saw the situational humor in 'jerk systems'?

I'd watch it again but in another thread Snax has me listening to a 2.5 hour audiobook.

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