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Old 09-04-2020, 11:32 AM   #11 (permalink)
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inviscid

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Originally Posted by JulianEdgar View Post
You do realise that the diagram you nominate is for inviscid flow?

That is, it's for for an imaginary fluid having no viscosity?

You did read the description on the next page that states:

In the real, viscous flow there exists a drag force, but it cannot be explained by considering an ideal, inviscid fluid.

If you are using this diagram - and others with similar concepts - no wonder your theory is so divorced from the reality of car aerodynamics.

Addition:

In fact, for people interested in seeing how confusion (and weird theories) can develop, it's worth having a good read of pages 51 and 52 of Hucho, second edition. It explains really clearly how a simplified aerodynamic model (ie non-viscous fluid) cannot be used to explain what happens on real cars.
1) Free flow, outside the boundary layer IS considered inviscid flow.
I stated that we're looking at 2-D flow. Which is basically what your looking at when you do your centerline pressure profiles.
2) And of course, we don't live in a d' Alembert's Paradox world of non-viscosity.
3) The value of the schematic, was the example of a body experiencing positive pressure downstream of 'lift', which is germane to streamlined bodies.
4) You like wings, Hucho goes on elsewhere to show the same thing for a RAF 101 symmetrical airfoil, at 4-degrees angle-of-attack, and zero lift. This is a real foil in a real laboratory, and real air, at supercritical Reynolds number.
5) And as I shared with you many months ago, Abbott and Von Doenhoff's book demonstrates zero-lift conditions for every extant airfoil known at the time of their publication. Your aeronautical engineer friend will have that. My aeronautical engineer friend Larry Mauro does.
' The drag and lift of a body depend strongly upon the angle of attack.' Hucho, page 202, Re: Stollery & Burns Ref. 4.83.

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Old 09-04-2020, 11:35 AM   #12 (permalink)
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Blockage Ratios

Is this something we want to explore? I posted to you on this many months ago. You act as if you didn't get the memo.
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Old 09-04-2020, 02:30 PM   #13 (permalink)
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Quote:
Originally Posted by aerohead View Post
1) Free flow, outside the boundary layer IS considered inviscid flow.
Aerohead, can you post some visuals on this?

I did find the below, and it is different than the boundary layer I had imagined you were describing.

I imagine that the first layer of boundary air can cause all sorts of drag, lift and vortex formation so we better not ignore it, right?

https://www.google.com/amp/s/www.res...1_30496832/amp
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Old 09-04-2020, 03:36 PM   #14 (permalink)
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image

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Originally Posted by kach22i View Post
Aerohead, can you post some visuals on this?

I did find the below, and it is different than the boundary layer I had imagined you were describing.

I imagine that the first layer of boundary air can cause all sorts of drag, lift and vortex formation so we better not ignore it, right?

https://www.google.com/amp/s/www.res...1_30496832/amp
1) You're looking at a 'laminar' wing section in 2-D flow.
2) The 'center-most' structure, all in white is the actual wing.
3) The black region around it is both the thin laminar boundary layer ( LBL ), then, where the wing is thickest, the air the fastest, and pressure the lowest, you see the immediate jump to thicker turbulent boundary layer ( TBL), which continues to thicken with distance, caused by the higher skin friction of a TBL.
4) For a wing, the TBL aggravates drag due to the higher surface friction.
5) The outer inviscid flow travels over the boundary layer as laminar flow.
-------------------------------------------------------------------------------------
A) In a road vehicle, Earth's boundary layer turbulence almost completely prevents the existence of a laminar boundary layer. One paper published for a full-scale RAM pickup determined that the truck had 30mm of LBL, right at the leading edges.
B) A TBL can withstand a higher positive pressure gradient in the direction of flow, so it's actually an advantage for a motor vehicle.
C) Since road vehicle drag is dominated by pressure drag, and pressure drag is a function of separation, the whole point of streamlining is to minimize, or totally eliminate separation.
D) Lift is solely a function of pressure.
E) Pressure is solely a function of local velocity.
F) Vorticity is a function of intersecting flows of varying velocity ( pressure). You might witness a fast moving storm-swollen stream enter into a slow-moving river, and where the two meet you'll see swirls, eddies, gyres, and turbulence, which can actually eat away at the riverbank. This is happening in the Antarctic ice sheets right now. And Clear Creek, along our property.
G) Streamlined shapes don't have pressure increases ( especially spikes) of sufficient magnitude to overcome the TBL's ability to adhere to the surface it's flowing over at any point.
H) Kamm and Fachsenfeld took a full-length streamliner and then just started chopping away the tail and recording the changes.
-------------------------------------------------------------------------------------
I'll have to figure out something about images. I lost over $770 worth of pictures when photobucket decided extortion was going to be their new business model.
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Old 09-04-2020, 05:42 PM   #15 (permalink)
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Quote:
Originally Posted by aerohead View Post
1) the published drag coefficient is Cd 0.32.
2) the published frontal area is Af 24.5-sq-ft ( 85.078% of gross area )
3) a scaled comparison to the template suggests perfectly attached flow at the decklid spoiler.
4) at 100-mph, the car generates 30.6-pounds of rear lift, retaining 98.3 % of it's static wheel loading.
5) EPA test weight of the car is 4298-pounds.
6) 48 % of weight is borne by the rear axles ( 2063-pounds )
7) at 172-mph, rear lift is 90.52-pounds, compared to 207-pounds for a 1969 Volkswagen 1600 Squareback.
8) Hucho stated, in his 2nd-Edition, that if the car's aft-body trailing edge was simply raised to the elevation of the production spoiler, both lift and drag characteristics would be improved. One can infer what they wish.
9) the fastest posted speed limit in the United States is 85-mph.
10) rear lift on the Cadillac at this velocity is statistically meaningless.
11) it would be less with the template. By default.
1) OK, so now you can work out the front and rear coefficients of lift for us - based on frontal area and not the deceptive plan view as you did before.

2) and 3) Seriously, you're still applying the template to guess where airflow goes? Even when I have run multiple tuft pics showing the airflow not following the template at all?! That's just very odd.

4), 5) and 6) Yes, what is your point? We're not expecting the car to fly. Maybe read pages 180 and 181 of my book that reference the SAE papers showing how small a lift force needs to occur before it impacts car stability?

7) Yes, lift coefficients have dropped considerably since 1969. What is your point?

8) Page reference please for that specific statement? I am not aware of any such statement in the book.

9) and 10) See above.

11) Lift would in fact be more with the template - repeatedly saying something doesn't make it true.
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Old 09-04-2020, 05:51 PM   #16 (permalink)
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Quote:
Originally Posted by aerohead View Post
1) Free flow, outside the boundary layer IS considered inviscid flow.
I stated that we're looking at 2-D flow. Which is basically what your looking at when you do your centerline pressure profiles.
2) And of course, we don't live in a d' Alembert's Paradox world of non-viscosity.
3) The value of the schematic, was the example of a body experiencing positive pressure downstream of 'lift', which is germane to streamlined bodies.
4) You like wings, Hucho goes on elsewhere to show the same thing for a RAF 101 symmetrical airfoil, at 4-degrees angle-of-attack, and zero lift. This is a real foil in a real laboratory, and real air, at supercritical Reynolds number.
5) And as I shared with you many months ago, Abbott and Von Doenhoff's book demonstrates zero-lift conditions for every extant airfoil known at the time of their publication. Your aeronautical engineer friend will have that. My aeronautical engineer friend Larry Mauro does.
' The drag and lift of a body depend strongly upon the angle of attack.' Hucho, page 202, Re: Stollery & Burns Ref. 4.83.
Normal mix of Aerohead irrelevancies and the justifiable of the unjustifiable.

Fact 1: Aerohead cited the diagram and said of it:

The Cadillac poses only a small perturbation. It's virtual lack of rear lift is a testament to the pressure-producing capability of a streamlined shape.
If you'll revisit Figure 2.4, page 51 of Hucho, you can see how,over the last 14.5% of the body, local pressure rises all the way back to local barometric pressure. Depending on rear overhang, and low pressure under the body, due to a diffuser, rear lift can be zero, like the VSPORT's


Fact 2: The diagram is for an imaginary, viscous-free (inviscid) fluid. The accompanying body text says of the diagram:

On the rear part of the vehicle's upper surface a steep pressure rise occurs, and it is in this region where considerable differences exist between the real flow of a viscous fluid and the inviscid flow shown here.

Fact 3: Aerohead's description of what the diagram shows is completely contradicted by the text, which makes the clear point that the diagram does not apply to real cars, especially in terms of pressures on the rear half (exactly the area Aerohead references).
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Old 09-04-2020, 05:58 PM   #17 (permalink)
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Quote:
Originally Posted by aerohead View Post
Is this something we want to explore? I posted to you on this many months ago. You act as if you didn't get the memo.
1. I've already quoted the best reference that I am aware of on subsonic wind tunnel testing, and what it says about blockage factors.

'Low Speed Wind Tunnel Testing' (Barlow/Rae/Pope):



2. To suggest that car manufacturers, F1 teams (etc) build huge wind tunnels when there is no need to do so is simply not credible.
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Old 09-04-2020, 06:10 PM   #18 (permalink)
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Quote:
Originally Posted by kach22i View Post
Aerohead, can you post some visuals on this?

I did find the below, and it is different than the boundary layer I had imagined you were describing.

I imagine that the first layer of boundary air can cause all sorts of drag, lift and vortex formation so we better not ignore it, right?

https://www.google.com/amp/s/www.res...1_30496832/amp
All cars operate in boundary layer flow ie flow where the viscosity of air is important. So as soon as you see a description of inviscid flow, you know it's purely theoretical (and of course any pressures that are derived from an inviscid flow model are going to be wrong on real cars).

To give you an idea, consider the following diagram of flow around a cylinder:



With non-viscous (ie purely theoretical) flow, the drag is zero! To put this as my father did to me when I was a little kid, to move the cylinder upstream or downstream would take equal force! Clearly, ignoring fluid viscosity isn't a good idea when trying to make aero changes in the real world...
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Last edited by JulianEdgar; 09-04-2020 at 06:19 PM..
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Old 09-09-2020, 12:45 PM   #19 (permalink)
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OK

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Originally Posted by JulianEdgar View Post
1) OK, so now you can work out the front and rear coefficients of lift for us - based on frontal area and not the deceptive plan view as you did before.

2) and 3) Seriously, you're still applying the template to guess where airflow goes? Even when I have run multiple tuft pics showing the airflow not following the template at all?! That's just very odd.

4), 5) and 6) Yes, what is your point? We're not expecting the car to fly. Maybe read pages 180 and 181 of my book that reference the SAE papers showing how small a lift force needs to occur before it impacts car stability?

7) Yes, lift coefficients have dropped considerably since 1969. What is your point?

8) Page reference please for that specific statement? I am not aware of any such statement in the book.

9) and 10) See above.

11) Lift would in fact be more with the template - repeatedly saying something doesn't make it true.
1) I'm not interested.If you are, do your own work.You talk of wings. For wings, the plan-view area of chord and span is used.
2-3) The template was created to serve as a Go NoGo. It's derived from technology developed in the 1920s by NACA ( NASA). Its contour is incapable of producing flow separation ( that's why you'd want to use it). If you go ahead and revolve the contour you get a half-body of revolution. The VSPORT is close to the contour. Its profile is a bit mutilated, but flow perturbation would be limited to a very small area. Your tuft study does not have scientific rigor. It's fraught with shortcomings. Automakers don't use it.
4-5-6) Sometimes you appear to be obsessed with lift issues. Given the lift values reported for the Cadillac, Hucho's non-concern with lift was only reinforced. If you're a Princess and can feel a pea under a stack of mattresses, that's your problem. Lift is not associated with attached flow.
* revisit your polar diagrams for wing sections.( A. Silverstein, NACA Report 502, p. 15, 1934 will be a revelation ).
* Hucho, p. 122 ( aspect ratio )
* Hucho, p. 151, Fig. 4.54 (attached flow= lower lift)
* Hucho, p. 217, Fig. 5.4, RAE 101 aerofoil @ 4-degrees offset flow pressure distribution, both sides.
* Hucho, p. 282, Fig. 7.34 ( attached flow = lower lift )
7) what is your point?
8) I can't exit this post to see what I posted in order to reply.
9-10 ) ditto
11) you have measured a half-body in a wind tunnel? If a wing produces zero lift, what would make a half-body produce lift? Hucho presented data for only full bodies of revolution being tested, which is not germane to half-bodies. The virtual aspect ratio for a half-body isn't even 0.3, due to transverse curvature. It would be best characterized as two wingtips joined together. Wing circulation strength is function of shape, velocity, and 'orientation' ( angle-of-attack ). You are speculating about the template's ability to generate lift. An extremely contextual subject.
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Old 09-09-2020, 12:49 PM   #20 (permalink)
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Figure 2.4

Quote:
Originally Posted by JulianEdgar View Post
Normal mix of Aerohead irrelevancies and the justifiable of the unjustifiable.

Fact 1: Aerohead cited the diagram and said of it:

The Cadillac poses only a small perturbation. It's virtual lack of rear lift is a testament to the pressure-producing capability of a streamlined shape.
If you'll revisit Figure 2.4, page 51 of Hucho, you can see how,over the last 14.5% of the body, local pressure rises all the way back to local barometric pressure. Depending on rear overhang, and low pressure under the body, due to a diffuser, rear lift can be zero, like the VSPORT's


Fact 2: The diagram is for an imaginary, viscous-free (inviscid) fluid. The accompanying body text says of the diagram:

On the rear part of the vehicle's upper surface a steep pressure rise occurs, and it is in this region where considerable differences exist between the real flow of a viscous fluid and the inviscid flow shown here.

Fact 3: Aerohead's description of what the diagram shows is completely contradicted by the text, which makes the clear point that the diagram does not apply to real cars, especially in terms of pressures on the rear half (exactly the area Aerohead references).
I posted a new thread dedicated to Hummel's Figure 2.4. Please check it out.
I agree with everything you've posted, however there's a context associated with everything I've posted. I'm glad you've brought this to the attention of the forum.

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