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Old 09-16-2020, 04:03 PM   #11 (permalink)
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context of 'inviscid'

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Originally Posted by freebeard View Post
I found interest in the Moody diagram and breakdown of CFD (DNS/LES/RANS)

Something fishy about this grid though.

Viscid/inviscid isn't black and white, it's a gradient.
The use of the word is contextual and dangerous if used loosely.
1) inviscid can describe an imaginary, frictionless fluid, incapable of supporting shear, or rotation, used in numerical models to establish streamline positions.
2) then the Bernoulli theorem is employed to establish the pressure distribution over the 'model' ,minus viscous effects.
3) then viscosity effects are incorporated to flesh out the total.
--------------------------------------------------------------------------------------
4) inviscid is also used to describe actual, ' real' fluid, outside the boundary layer, which telegraphs pressure through the boundary layer to the lowest strata of air, adjacent to the surface of the model, measured as local static pressure.
5) all viscous effects are limited to the boundary layer.

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Old 09-16-2020, 06:18 PM   #12 (permalink)
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Quote:
Originally Posted by aerohead View Post
4) inviscid is also used to describe actual, ' real' fluid, outside the boundary layer, which telegraphs pressure through the boundary layer to the lowest strata of air, adjacent to the surface of the model, measured as local static pressure.
Can you provide me with a reference for that? I've not seen 'inviscid' used to mean that (except in what you write).
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Old 09-17-2020, 01:04 AM   #13 (permalink)
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Quote:
Originally Posted by aerohead View Post
who cares?
That's an odd thing to write.

Who cares that there's a good book, written by a professional aerodynamicist, that is available cheaply - and which would help counter many of the misconceptions about car aero that have been spread in this group?

I care - and I would have thought you would too.


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Old 09-17-2020, 01:23 PM   #14 (permalink)
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For those interested in buying this (those who care, I suppose), it looks like there are at least three editions--1996, 2001, and 2009. I was only able to find the 3rd ed. on eBay, which I just bought, but the 1st and 2nd are available through the Abe Books link JulianEdgar posted or on Amazon.
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Old 09-18-2020, 05:15 AM   #15 (permalink)
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Quote:
Originally Posted by Vman455 View Post
For those interested in buying this (those who care, I suppose), it looks like there are at least three editions--1996, 2001, and 2009. I was only able to find the 3rd ed. on eBay, which I just bought, but the 1st and 2nd are available through the Abe Books link JulianEdgar posted or on Amazon.
I think you'll enjoy it.

I read the book a very long time after I first got interested in car aero, and (in the context of all the other aero books I'd already read) I thought: wow, this guy writes so clearly!

I also like the 'key points' approach that he takes at the end of each chapter.

I guess it says it all that he was my first pick to act as a tech consultant when I wrote my aero book. I was absolutely delighted when he agreed*, and I have never regretted that decision.

The feedback from other professional aerodynamicists (feedback that I have received since my book has been published) is that Dick Barnard is also well-known and well-respected in the profession.

As I said, the info in his book is much more accessible than most SAE tech papers (and books like Hucho), so in the real world, it's likely to be far more useful to people modifying their cars.

* Interesting sidelight for people interested in writing books, etc. Before Dick agreed to work with me, he requested copies of some of my published material. He read the material, then proceeded to pull it apart very precisely. (For example: a stagnation point - as I had described it - is on a car, not a point - it is a zone. Airflow on a car with a rough underbody doesn't run into things, it causes a change in behaviour that results in positive pressure. And so on. All very good!)
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Old 09-18-2020, 02:14 PM   #16 (permalink)
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inviscid

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Originally Posted by JulianEdgar View Post
Can you provide me with a reference for that? I've not seen 'inviscid' used to mean that (except in what you write).
Neither 'inviscid' nor 'ideal fluid' can be found in the INDEX, page 560, of Hucho's 2nd-Ed,
I did find inviscid used in Hucho's 2nd-Ed at:
page-49,59, 50, 51, 52.
The term ' ideal inviscid' occurs only on page 52.
p. 49, ' Provided no flow separation takes place ( the template ), 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.' Professor Dietrich Hummel.
-------------------------------------------------------------------------------------
From Daugherty and Franzini:
* page 279, ( Re boundary layer) ' outside of this layer the fluid can be considered as frictionless or ideal... allowing that the mathematical theory of ideal fluid flow can be used to determine the streamlines in the real fluid at a short distance away from a solid wall / boundary / surface. ( qualitative! )
* page 87, Bernoulli theorem, for a frictionless incompressible fluids with good results in situations where frictional effects are very small ( template ).
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Old 09-18-2020, 02:25 PM   #17 (permalink)
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odd thing

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Originally Posted by JulianEdgar View Post
That's an odd thing to write.

Who cares that there's a good book, written by a professional aerodynamicist, that is available cheaply - and which would help counter many of the misconceptions about car aero that have been spread in this group?

I care - and I would have thought you would too.

I actually addressed this one today on another thread, but I'll repeat what I commented.
I didn't mean no interest in the book, just not really interested in 'lift' issues.
Since no caveats or conditions were volunteered in the 5th-Ed Aerodynamics of Road Vehicles, my only explanation for this relatively new interest in lift would have to do with ( in the USA anyway) increased posted speed limits since 1995, and the plethora of sales in higher profile vehicles such as pickups, SUVs, VANs, and CUVs.
Sports cars are already compensating.
I'm pinching pennies, saving for a BEV, and home improvements . Any book purchases are off the table for me.
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Old 09-18-2020, 05:58 PM   #18 (permalink)
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Quote:
Originally Posted by aerohead View Post
Neither 'inviscid' nor 'ideal fluid' can be found in the INDEX, page 560, of Hucho's 2nd-Ed,
I did find inviscid used in Hucho's 2nd-Ed at:
page-49,59, 50, 51, 52.
The term ' ideal inviscid' occurs only on page 52.
p. 49, ' Provided no flow separation takes place ( the template ), 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.' Professor Dietrich Hummel.
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.
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Old 09-18-2020, 06:16 PM   #19 (permalink)
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Isn't that "inviscid" airflow?
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Old 09-18-2020, 06:29 PM   #20 (permalink)
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Quote:
Originally Posted by freebeard View Post
Isn't that "inviscid" airflow?
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.

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