The Fastback bloc

[JulianEdgar]

Originally Posted by aerohead

Hucho wrote about spoilers. And he said that simply lofting the rear of the body contour, up to the height of where someone intended to end the spoiler, would not only be better for spoiling lift, it would be better for drag as well.
I gave you the exact page number many months ago.
You're like my cat. Once she learned to speak, all she wanted to do from then on is to argue.

More personal belittlement.

Note the change in Aerohead's proposition - now Hucho isn't talking about the Taycan, he is talking generically about spoilers.

Hey, all one and the same, isn't it?

A good example of Aerohead misquoting a source.

[JulianEdgar]

Originally Posted by aerohead

You're confusing downwash for flow attachment. They are completely different animals. The vortices which are inducing the downwash cannot be visualized with tuft testing.
Again, all provided to you specifically, months ago, yet you cling to your folk knowledge.

No I am sorry, flow attachment caused by downwash is still flow attachment.

In fact, that was said to me by no less authority than the head of Jaguar aerodynamics.

But hell, what would he know - he should have consulted Aerohead!

[JulianEdgar]

Originally Posted by freebeard

Porsche exploited downwash for air intake.

edit:
I know, not a good example. The firewall is modeled incorrectly and they took 'boxer' engine too literally.



Better? Smaller? It's all I've got.

No lift in the rear, all it needs is a rake.

I am sorry, but crap CFD and a hand-drawn picture of what someone assumes is happening doesn't cut it in 2020 when we have techniques that allows us to show where there is attached and separated flow, and exactly how much lift is being created, all by some simple testing on the road.

(No lift in the rear? Not a snowflake's chance in hell.)

[freebeard]

I was sorry first.

[kach22i]

I'm assuming downwash and or upwash is not equal to attached flow because of what the surrounding air is doing.

That is to say: if the downwash attachment is being caused by a giant drag causing vortex, then you do NOT have clean pressure or evidence of attached flow.

Yes or no on the assumption and description above.

Pressure attachment Vs Flow attachment.

Is there an article on this?

........................part II of post.

In general the objections I have about template use is the it's shadow obscuring adjacent subjects.

Sure we take time to discuss air curtains, air tabs, diffusers and testing methods, but that shadow hovers overhead.

We need not destroy or perforate the template in order to shine light on other topics.

However, sometimes we have to shove it off to the side so we can look around and see what the real world is doing. And they are doing some cool things with air management that were never dreamed of in the 1930's because they didn't have the tools the industry has today.

Us novice type folks and Ecomodders do not have these industry tools.

[kach22i]

Originally Posted by freebeard

Porsche exploited downwash for air intake

Interesting but not 100 percent convincing images.

I know that we have both longed for more on this topic, thank you for sharing.

Perhaps those diagrams would be closer to reality if the aft roof and rear window had a diffuser on it similar to a rear underbelly diffuser just upside down.

There are images of drag racers and salt flat racers (VW-Beetle) with such fins on them.

I'm just projecting/predicting air rolls off sideways at a diagonal angle because of the compound curves.

[freebeard]

Interesting but not 100 percent convincing images.

I am truly sorry... that I have falsified Cunninghams Law. I posted a bad illustration and nobody has posted a better one.

I think drag racers call it a dog house:

https://trulyvintagephototutorials.f...1/dsc_1573.jpg

I tried to model the vortexii in Wings 3D years ago, but they looked like corkscrews. It would be better in Blender with it's particle system, but it still would be a[n unconvincing] cartoon.

[JulianEdgar]

Originally Posted by kach22i

I'm assuming downwash and or upwash is not equal to attached flow because of what the surrounding air is doing.

That is to say: if the downwash attachment is being caused by a giant drag causing vortex, then you do NOT have clean pressure or evidence of attached flow.

Yes or no on the assumption and description above.

Pressure attachment Vs Flow attachment.

Is there an article on this?

1. Tufting shows separated or attached flow very clearly.

Attached flow is when the airflow is being guided by the shape over which it is flowing.

Separated flow is when the air has gone its own way, and is not being guided by the car's shape.

It's that simple - note how there is nothing to do with lift, drag or anything else in that definition.

2. Both attached and separated flows can cause drag and lift.

An example of separated flow causing drag is in the wake behind the car. The air in the separated flow is of low pressure and is causing a backwards pull on the car.

An example of attached flow causing drag is where air passes from the roof down onto the rear window of a modern sedan. The vector force created by this flow has backwards and upwards components - ie an arrow showing this force points up and backwards.

Note that both types of flows can create low pressures (and so lift and/or drag, depending on the force direction). However, the force developed by low pressure attached flow can be much higher than the force developed by low pressure separated flow. (Easy to remember - an aircraft wing stalls (decreases lift) when airflow separates.)

3. Trailing vortices create low pressures and so drag and/or lift. They are typically shed from angled pillars eg A pillar and C pillars. Viewed from behind, they rotate clockwise on the left and anticlockwise on the right, if the car is developing lift. The 'final pillar' (ie C or D) can add a downwash component on the rear section of the car, giving attached flow where normally separated flow would have occurred.

To directly quote Adrian Gaylard, head of Jaguar aero:

Typically, for lowest drag, the EBLA (effective backlight angle in a sedan) is around 12 degrees. As this increases, so does drag, until around 30 degrees where the rear-pillar vortices burst and the rear flow fully separates. With an effective backlight angle approaching 30 degrees, it’s often better to separate it as the drag can be lower for a fully separated rear flow, compared to one where rear pillar vortices are keeping the rear screen flow attached on a high screen angle.

Note a few things from this quote that support what I have said:

1) The flow is classed as attached, even if that is caused by the action of trailing vortices.

2) Attached flow can cause greater drag than separated flow. (And so, attached flow can also cause greater lift than separated flow.)

All of this is in my book, that was reviewed by real experts prior to publication. It's also supported by dozens of measurements on different cars.

[JulianEdgar]

Originally Posted by kach22i

Us novice type folks and Ecomodders do not have these industry tools.

It's now 2020, not 1980. We have lot of tools at our disposal to measure nearly every aspect of car aero on the road. Here's a few:

• locations of separated and attached flow
• panel pressures anywhere on the car
• relative changes in drag
• lift and downforce, both front and rear
  

About the only one that we cannot currently directly measure is the strength of trailing vortices, and I have a few ideas on that one that I'll get around to eventually.

[freebeard]

Umbrella on a spring scale?