A book to buy and read

[freebeard]

Simplified models make the world go round. Ferinstance, the mirrored half-body.

[JulianEdgar]

Originally Posted by aerohead

I'm pinching pennies, saving for a BEV, and home improvements . Any book purchases are off the table for me.

And that's fine. But the trouble is, you refuse to acknowledge when people quote from more recent references - let alone learn from them.

So we're stuck in an era where:

• full wheel covers always gave the lowest drag
• lift was caused by separation
• airflow direction can be predicted by The Template
• rear spoilers 'reach up' to flow
• airflow separates at the end of the roof of notchbacks

.... and so on - all of which are wrong for any cars of the last 30 years.

[freebeard]

OTOH, every house in the last 30 years is wrong, excepting the hemisphere with oculus.



(my parents', not the one I lived in)

[JulianEdgar]

Originally Posted by freebeard

OTOH, every house in the last 30 years is wrong, excepting the hemisphere with oculus.



(my parents', not the one I lived in)

Sorry, I don't really see it.

I am not saying the cars (ie the house) was/were wrong.

I am saying that analysing modern aerodynamics on the basis of car shapes that are often 60+ years old is not a good idea.

Nor is pretending we have learnt nothing about car aero since the 1930s/1950s/1960s/1980s.

It would be a bit like taking the contemporaneous 1960s state of the art analysis of the house that you show - and applying it today. So no IR temperature analysis, no air leakage tests, no modern insulating materials or selective coatings on solar heaters, etc. Let alone discussions of embodied energy, etc.

I have absolutely no issue with Aerohead choosing not to keep up with current car aero thinking. But I have major issues with his continually misleading people on the basis of his outdated ideas and understandings.

If he qualified his comments ("This is what I read 40 years ago and I am not sure it is still current") then that would be fine. But in fact he actively rejects any more recent references and/or tests.

And that isn't fine.

[freebeard]

It would be a bit like taking the contemporaneous 1960s state of the art analysis of the house that you show - and applying it today. So no IR temperature analysis, no air leakage tests, no modern insulating materials or selective coatings on solar heaters, etc. Let alone discussions of embodied energy, etc.

I understand it is an analogy. You describe an analysis i'd love to see applied.

The design is from first principles, promoting a toroidal air flow inside and low surface area outside. A 1950s design built in 1980. If I were to design it today, instead of plywood and cedar shakes, it would be stainless steel. With Gorilla glass/electrochromic windows.

Still, my parents were somewhat ahead of their peers. It had a passive solar hot water system, but their subcontractor (he drove down from Dallas, OR, and had a cowboy hat with a feather rosette on the headband. Never trust a contractor in a cowboy hat) built a flawed system and the 2nd owners took it out. You can see the shingles instead of hand-split shakes on the south-facing gable.

[JulianEdgar]

Originally Posted by freebeard

I understand it is an analogy. You describe an analysis i'd love to see applied.

The design is from first principles, promoting a toroidal air flow inside and low surface area outside. A 1950s design built in 1980. If I were to design it today, instead of plywood and cedar shakes, it would be stainless steel. With Gorilla glass/electrochromic windows.

Still, my parents were somewhat ahead of their peers. It had a passive solar hot water system, but their subcontractor (he drove down from Dallas, OR, and had a cowboy hat with a feather rosette on the headband. Never trust a contractor in a cowboy hat) built a flawed system and the 2nd owners took it out. You can see the shingles instead of hand-split shakes on the south-facing gable.

I am designing our solar house as we speak.

I've been interested in solar design for about 40+ years, and have been playing with solar things that long. Unfortunately, though, Covid has just absolutely killed our business (face-to-face training in high-level writing skills, but no face-to-face training now...) and so everything has been put on the back burner.

Still, we have the land (next door to where we currently live), and the concrete slab for my new workshop went in last week... so not all bad news!

Off-topic, but, here's the workshop slab:

[aerohead]

Originally Posted by JulianEdgar

So, the diagram on Page 49 shows flow nothing like that on any modern car. Nor does the quote in Hucho actually mention the template, nor any shape like it. You are yet again misquoting.

You didn't bother quoting Hucho Page 52, that talks about the "considerable difference" between an idealised model using inviscid fluid and the real world.

I can't see any mention of inviscid on page 59.

Aerodynamics of Road Vehicles (5th edition) has a major section on inviscid flow - much too big to summarise here.

However, a key point made early in the section (Page 94) is that where airflow wraps around a curve ("where the streamlines are curved") a force is developed that acts outwardly (an "outward oriented centrifugal force").

So yes, I have learnt something - inviscid flow is responsible for the low pressures we can measure where attached airflow wraps around curves.

It seems like miscomprehension on your part:
1) the figure is probably the only one in the book to address a 'streamlined ' shape and lift, of which you've expressed interest.
2) the streamlines depicted are exactly as they'd be in the real world.
3) it is the pressure distribution which would be 'qualitative', however perfectly accurate for the first 92% of body length, and within 90% of what's depicted for the last 8% of body, ads it's a 'streamlined body we're discussing.
4) there is no considerable difference between an idealized model and a streamlined body in the real world and Hucho knows that. You may come to learn that.
5) I can't address page 59 until Friday. Sorry! I left both your book and Hucho's by my front door when I left for town this morning.
6) as to 'inviscid' I re-read every word of Hucho's book since last Friday. Word for word. I'll be posting an addendum to the 'Subject index' on Friday. I found dozens of critical terms that never made it into the index. One would have to read the entire book, hoping to stumble upon a topic of particular interest. Very disappointing!
7) air is a gas and has mass. It has inertia. It has momentum when it's moving. It should come as no surprise that it would exhibit centripetal / centrifugal force when moving in any acceleration, which it would be within a 'curve,'
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* As to inviscid flow / pressure, you're off base. Here's a checklist:
1) the body determines the streamlines.
2) streamlines determine local velocity at any point.
3) velocity is responsible for pressure.
4) pressure determines whether the boundary layer will be attached or not.
5) even a turbulent boundary layer can sustain only so much pressure rise in the direction of flow.
6) if that threshold is crossed, the boundary layer will separate from the contour, beginning the turbulent wake.
7) pressure behind the separation line will be the pressure at which separation occurred.
8) the closer the separation line is to the suction peak, the lower the pressure over the aft-body.
9) if airflow 'wrapped' over a body, the streamlines would diverge, velocity slow, and pressure increase. You'd have the highest pressure attainable. Your Porsche 911 would have Cd 0.303, not 0.40, just by having a 'template' roofline. Dig Kamm or Fachsenfeld up and ask them. Your general theory of 'wrapped' flow is riddled with physical inconsistencies, and is not supported by scientific observation.
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There is a 'CONTEXT' about your 'lift' concept, and we will examine this. I'm compiling a list of spoiler'd cars which fit the conditions of your logic. You're so close to 'getting it' that I hope you'll hang on for the ride. The only hitch right now is your need of a universal, absolute with respect to lift. It doesn't exist. There's no one-size-fits-all, generalized rule governing lift. Hucho expresses this specifically.

[aerohead]

Originally Posted by freebeard

Isn't that "inviscid" airflow?

Hang on until Friday, I'll have Hucho's book with me then, and I'll be able to pick apart the 'language.'
I hope the more recent editions have been corrected. If not, they'll be nearly impossible to navigate.

[aerohead]

Originally Posted by JulianEdgar

Well, I think we're all a bit right on this one.

Aerodynamics of Road Vehicles (5th ed) makes the point that inviscid flow is not real, but that using the model of inviscid flow works for flows away from boundaries, and allows us to picture pressures from streamlines.

Aerodynamics of Road Vehicles (2nd ed): Provided no flow separation takes place, the viscous effects in the fluid are restricted to a thin layer of a few millimeters thickness, called the boundary layer. Beyond this layer the flow is inviscid and its pressure is imposed on the boundary layer.

Two points:

1. I've corrected Aerohead's misquote

2. Obviously there isn't a car made where flow separation doesn't take place.

So with the very important point kept in mind that inviscid flow is just a simplified model, we can use it when considering pressures where there is attached flow.

1) you haven't corrected a thing.
2) 'a car made' is not germane to streamlined cars.
3) and keep in mind that 'downwash' and attached longitudinal vortices' do not constitute 'attached flow.'

[JulianEdgar]

Originally Posted by aerohead

------------------------------------------------------------------------------------
* As to inviscid flow / pressure, you're off base. Here's a checklist:
1) the body determines the streamlines.
2) streamlines determine local velocity at any point.
3) velocity is responsible for pressure.
4) pressure determines whether the boundary layer will be attached or not.
5) even a turbulent boundary layer can sustain only so much pressure rise in the direction of flow.
6) if that threshold is crossed, the boundary layer will separate from the contour, beginning the turbulent wake.

All correct!

9) if airflow 'wrapped' over a body, the streamlines would diverge, velocity slow, and pressure increase. You'd have the highest pressure attainable.

Incorrect - as about a million streamline pics show. Here's one:



Note the airflow wrapping around those upper curves. Note how the streamlines get closer together. Note how this indicates higher airflow speed and lower pressure.

How you can seamlessly segue from correct material to absolute rubbish is beyond me.