A book to buy and read

[Cd]

I really found this thread interesting.
I found out a couple of things I didn't understand in previous threads, because in the bickering, the information was to the point, and not overly fanciful.
The wording can be really confusing for us folk that aint smart.
Sesquipedalian
Is that the right word ? ( I had to look that up. )

[aerohead]

Originally Posted by freebeard

Have you tried PMing JulianEdgar, in case he had notifications turned on? He still hasn't posted since [Last Activity: 10-14-2020 12:27 PM].

He might appreciate your kind words.

Julian will still have access to out posts, whether or not he chooses to post, so he'll know of certain comments if he's interested.
The crew had just poured the concrete foundation on his new shop, and I presume he has plenty going on in his life.

[aerohead]

Originally Posted by Cd

I really found this thread interesting.
I found out a couple of things I didn't understand in previous threads, because in the bickering, the information was to the point, and not overly fanciful.
The wording can be really confusing for us folk that aint smart.
Sesquipedalian
Is that the right word ? ( I had to look that up. )

Yes. It's perfect in the context of Fachsenfeld's 'Verjungungsverhaltnis'. He just stitched together a string of words to describe a particular geometric, dimensional analysis relationship.
Like ' antidisestablishmentarianism' my middle brother was fond of throwing at people.
My favorite, From Edward's Air Force Base was, ' unsymmetrical dimethylhydrazine.' You'd find it next to the bucket of prop wash.

[aerohead]

Originally Posted by JulianEdgar

I don't think that is right. No air is inviscid. None. Inviscid fluid is imaginary - it doesn't exist.

Inviscid airflow is used only to greatly simplify the model of airflow that can be used. That is, it's a 'model trick'.

Wikipedia puts it well (I can give more formal references if anyone wishes. Anderson's books on aerodynamics are good in this regard):

Inviscid flow is the flow of an inviscid fluid, in which the viscosity of the fluid is equal to zero. When viscous forces are neglected, such as the case of inviscid flow, the Navier-Stokes equation can be simplified to a form known as the Euler equation. Using the Euler equation, many fluid dynamics problems involving low viscosity are easily solved, however, the assumed negligible viscosity is no longer valid in the region of fluid near a solid boundary.

In other words, if we're interested in pressures acting on the surface of the body (ie a solid boundary) - and with car aero, that is all that we are interested in - then using inviscid flow models is of no / very little use to us.

1) If aerodynamic drag is the concern, the two-dimensional, inviscid-flow modeling will provide enough pressure gradient information to the engineer, to be able to establish whether or not, the specific body under investigation will trigger flow separation at any particular location or not.
2) The decision, whether to advance to the viscous effects calculations or not can be determined right then and there. Go NoGo.
3) This would be the primary value of the 2-D modelling.

[aerohead]

Originally Posted by JulianEdgar

I think I skimmed through that video a while ago.

In general it's not worth getting excited about laminar and turbulent flow. Very little airflow on a normal car is technically laminar, and so looking at things in terms of laminar and turbulent flows tends to lead people astray ie create confusion.

I think it's much better to concentrate on separated and attached flows.

1) Most of the 'flow' on an automobile is 'laminar.'
2) Most of the aerodynamic drag of an automobile is pressure drag.
3) Pressure drag is a function of flow separation.
4) Separation creates 'turbulence' and it's associated low pressure which affects base pressure, which is directly associated with pressure drag.
5) The entire premise of automotive streamlining has to do with reducing or eliminating turbulence, via reducing, or eliminating separation.
6) 'Turbulence' is very much worth getting excited about. By default.
7) Confusion will come from not being able to distinguish the distinction between ' boundary layer' conditions, and outer, 'inviscid' flow conditions. Very different animals.
8) So while it IS important to concentrate on 'separated' and 'attached' flows, we do it because of the implications of laminar or turbulent flow.
9) Finally, it's also important to consider the implications of turbulence in connection to 'lift.' Worthy of it's own thread.

[Vman455]

Originally Posted by aerohead

1) Most of the 'flow' on an automobile is 'laminar.'

None of the textbooks on vehicle aerodynamics I have claims this. Quite the opposite, in fact; most of the flow around a car is turbulent (aside from a thin laminar layer next to the body surface), and transitions to a turbulent layer based on Reynolds number, which is proportional to the distance from the leading edge of the body. But this is a good thing, since a turbulent boundary layer will follow curves that a laminar one won't!

The laminar state of the boundary layer flow is stable against disturbances for certain conditions only. At a distance x = xtr from the leading edge of the plate a transition to the so-called turbulent state of the boundary layer takes place. The transition between the two states of the boundary layer flow is largely governed by the value of the Reynolds number.... In general, for medium Reynolds number transition from laminar to turbulent occurs in the region of minimum pressure, and with increasing Reynolds number the transition moves upstream.

...

Turbulent boundary layers can withstand much steeper adverse pressure gradients without separation than laminar boundary layers.

(Hummel, Dietrich, "Some Fundamentals of Fluid Mechanics," in Aerodynamics of Road Vehicles, 4th ed., ed. Hucho [Warrendale: SAE International, 1998], 66-68).

Near the front edge, the air flows smoothly with no turbulent perturbations, and appears to behave rather like a stack of flat sheets or laminae sliding over each other with friction, the outer ones moving faster than the inner ones. This type of flow is therefore called laminar flow. Further along, as indicated in Fig. 1.7, there is a sudden change or transition to a turbulent type in which random motion is superimposed on the average flow.

(Barnard, R.H., Road Vehicle Aerodynamic Design, 3rd ed., [St. Albans: MechAero, 2009], 9-10).

Further, researchers recognized as far back as the 1970s that there is an additional problem of cars operating in a turbulent atmosphere, i.e. the free stream flow, rather than being laminar as in a wind tunnel, is in fact turbulent. P.W. Bearman outlined the problem and attempted to begin to address it in his paper "Some Effects of Free-Stream Turbulence and the Presence of the Ground on the Flow Around Bluff Bodies" at the 1976 GM conference:

Road vehicles are exposed to turbulence. generated both by the natural wind and by other vehicles, and under some conditions this may affect their mean-drag characteristics. It is shown that the free stream turbulence can change separation and reattachment positions on bluff bodies by modifying the boundary layer, free shear layer and wake development.

(Bearman, P.W. "Some Effects of Free-Stream Turbulence and the Presence of the Ground on the Flow Around Bluff Bodies," in Aerodynamic Drag Mechanisms of Bluff Bodies and Road Vehicles, ed. Sovran et al, [New York: Plenum Press, 1978], 112).

But it is a problem that still vexes engineers. How do we model dynamic atmospheric turbulence--which by its nature is random--in a systematic manner? Various turbulence models are used in CFD analysis, but these are approximations. I don't know of any wind tunnels that have some device for creating free stream turbulence, but perhaps there are some.

Maintaining laminar flow as far back from the front of the car as possible by smoothly rounding the front end is one strategy to achieve lower drag, where laminar flow can be encouraged (given acceptable environmental conditions) before the inevitable transition to turbulence:

Using the techniques of computational fluid dynamics described in the final chapter, it is possible to design a front-end profile that provides a steadily decreasing pressure almost up to the front screen. This not only inhibits separation, but produces a low rate of boundary layer growth, and provides the possibility of producing a significant area of low-drag laminar boundary layer.

(Barnard, R.H., Road Vehicle Aerodynamic Design, 3rd ed., [St. Albans: MechAero, 2009], 80).

Lastly, it is quite easy to observe on the road whether the flow at any point on a car is laminar or turbulent: simply tape wool tufts onto the body surface and then observe their behavior. If a tuft points perfectly in one direction with no movement, the flow is laminar; in laminar flow, pathline = streamline = streakline, and there is no mixing. If it moves, vibrates or fluctuates at all, the flow is turbulent. In fact, now that I think about it, it may be possible to observe the point of transition at the front of the car on a calm day--but you would need a road completely free of other traffic and a day with absolutely no wind.

[aerohead]

Originally Posted by Vman455

None of the textbooks on vehicle aerodynamics I have claims this. Quite the opposite, in fact; most of the flow around a car is turbulent (aside from a thin laminar layer next to the body surface), and transitions to a turbulent layer based on Reynolds number, which is proportional to the distance from the leading edge of the body. But this is a good thing, since a turbulent boundary layer will follow curves that a laminar one won't!


(Hummel, Dietrich, "Some Fundamentals of Fluid Mechanics," in Aerodynamics of Road Vehicles, 4th ed., ed. Hucho [Warrendale: SAE International, 1998], 66-68).


(Barnard, R.H., Road Vehicle Aerodynamic Design, 3rd ed., [St. Albans: MechAero, 2009], 9-10).

Further, researchers recognized as far back as the 1970s that there is an additional problem of cars operating in a turbulent atmosphere, i.e. the free stream flow, rather than being laminar as in a wind tunnel, is in fact turbulent. P.W. Bearman outlined the problem and attempted to begin to address it in his paper "Some Effects of Free-Stream Turbulence and the Presence of the Ground on the Flow Around Bluff Bodies" at the 1976 GM conference:


(Bearman, P.W. "Some Effects of Free-Stream Turbulence and the Presence of the Ground on the Flow Around Bluff Bodies," in Aerodynamic Drag Mechanisms of Bluff Bodies and Road Vehicles, ed. Sovran et al, [New York: Plenum Press, 1978], 112).

But it is a problem that still vexes engineers. How do we model dynamic atmospheric turbulence--which by its nature is random--in a systematic manner? Various turbulence models are used in CFD analysis, but these are approximations. I don't know of any wind tunnels that have some device for creating free stream turbulence, but perhaps there are some.

Maintaining laminar flow as far back from the front of the car as possible by smoothly rounding the front end is one strategy to achieve lower drag, where laminar flow can be encouraged (given acceptable environmental conditions) before the inevitable transition to turbulence:


(Barnard, R.H., Road Vehicle Aerodynamic Design, 3rd ed., [St. Albans: MechAero, 2009], 80).

Lastly, it is quite easy to observe on the road whether the flow at any point on a car is laminar or turbulent: simply tape wool tufts onto the body surface and then observe their behavior. If a tuft points perfectly in one direction with no movement, the flow is laminar; in laminar flow, pathline = streamline = streakline, and there is no mixing. If it moves, vibrates or fluctuates at all, the flow is turbulent. In fact, now that I think about it, it may be possible to observe the point of transition at the front of the car on a calm day--but you would need a road completely free of other traffic and a day with absolutely no wind.

* any mention of the boundary layer taken within the context of laminar flow, would have to do with going out of bounds on what the turbulent boundary could tolerate as to pressure rise in the flow direction of the aft-body.
* Due to the 'size' of an automobile, once to 20-mph, it's going to be fully immersed in a turbulent boundary layer. And it's Cd constant up to 250-mph.
* it's technically impossible to have a laminar boundary layer for more than an inch or so on a production passenger car, even on a perfectly calm day.
* At the time of Hucho's 2nd-Edition publication in 1987, forebody separation was basically, already a thing of the past. Minimum edge radii were already sufficient to provide attached laminar flow up to the A-pillars and windshield
header.
* mirror turbulence has always been an issue. That's why the DOT has been petitioned to allow carmakers to go to camera systems.
* recessed window glass triggers separation
* wheel openings
* pseudo-Jaray 'fastbacks'
* you'll see that the 2011 Audi A7 Sportback had nearly full rear separation over the car.
* notchbacks
* It was from the A-pillars aft that's the issue, however plenty of cars maintain attached, laminar flow all the way to the rear separation edges.
* Tufts can reveal some aspects of attached flow, however cannot differentiate vortex flow and downwash flow. Typically, it takes smoke flow visualization or tuft screens behind the vehicle to discern that.
--------------------------------------------------------------------------------------
Automotive aerodynamics has to do with reducing or eliminating separation. Separation is caused by the body profile ( hence 'profile drag').
And by definition, if the body profile is not 'streamlined' it will create an unfavorable pressure gradient in the direction of flow which will cause the TBL to detach under the conditions illustrated in boundary layer theory.
Once separated, all kinetic energy that might have been harvested with a 'streamlined' body will be lost to the atmospheric heating of turbulence.
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Any vehicle with a drag coefficient above Cd 0.09 is demonstrating drag due to separation, linked to the turbulence it creates, which cannot recover kinetic energy which IS recovered with a 'streamlined' body.
Energy lost to the turbulent boundary layer is a non-negotiable consequence of air viscosity.
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We have no control over atmospheric turbulence, crosswinds, or gust. The natural 7-mph crosswind yaw effects, statistically experienced by all vehicles, is measured in CFD or wind tunnel for EPA certification purposes.
Turbulence 'screens' can be introduced in a wind tunnel. CFD uses an accepted Kappa- Epsilon (K-E) turbulence model as part of its makeup last I looked.

[aerohead]

I dug out what I have on P.W. Bearman and brought it with me. I've had Hummel with me for the last 9-months or so.
We can better discuss it now if you like, however you may regret it. It's very pro-template, and extremely counterfactual to some of your 'modern' contemporary aerodynamic assertions.

[freebeard]

We can better discuss it now if you like, however you may regret it.

Loaded for bear, man.

Loaded For Bear | Definition of Loaded For Bear by Merriam ...
https://www.merriam-webster.com/dictionary/loaded for bear
Loaded for bear definition is - prepared to deal with attacks or criticism : prepared to fight or argue. How to use loaded for bear in a sentence.

[Piotrsko]

I was under the possibly mistaken thought that bears were known for ignoring small caliber injuries so therefore loaded for bear meant sufficient rapid fire capabilities of calibers exceeding .4

Getting popcorn.