New (free) tech paper

[JulianEdgar]

This has just been published: https://www.mdpi.com/2311-5521/5/4/236/htm

It's probably the best comparison of flows / pressures / drag / lift on hatchbacks / /notchbacks / fastbacks that I've seen.

Warning: the pdf download is a large file, but the paper can be viewed in web browser too.

Lots of pretty pictures for people who don't want to read a lot of text.

In order from lowest to highest:

Drag: Fastback, notchback, squareback

Lift: Fastback, notchback, squareback

There's a lot in this paper that contradicts what has been said here over the last decade.

[freebeard]

Is seem very thorough, they're modeling the suspension and radiator.

I don't see much variation between the fastback and notchback, compared to my standard, the VW Type III. Another easy comparison would be the Nissan Pulsar NX, but it only came with Sportbak (squareback) and coupe (notchback) variants. A lift-off fastback could be fabricated.

[JulianEdgar]

I've now read the paper carefully and I think it is very good indeed. Why?

1) Despite being based on models rather than full-size shapes, the models are detailed and the wind tunnel and authors highly reputable.

2) The shapes are measured very well, with pressure taps and Particle Image Velocimetry (PIV) ie photographing real airflows.

3) The three shapes - squareback, fastback and notchback - match modern shape vehicles (so making, Freebeard, the old VW Type III data utterly irrelevant - unless you are working on a VW Type III of course).

4) There is no CFD involved - in fact, the data is of such high quality that it is designed to validate CFD!

5) Pressure and particle velocities are shown for the three different shapes, not only on centrelines but also to each side of the centrelines. That works really well eg the separation bubble in the middle / bottom of a notchback's rear window can be clearly seen, as can the different vortex behaviour of the different shapes. This is one of the very few papers on rear-end airflow where everything matches what I have seen and measured. (Not trying to be the arbiter of experts, but it gives me confidence when I see this.)

6) Overall CD and CLf and CLr values are shown for the different shapes. I don't think I have ever seen that so clearly done - same wind tunnel, same measuring approaches, modern shape car models.

For people who don't want to go into the nitty gritty of the paper (and it is complex in parts), I'll be doing a video on it. The key visual elements are Table 3 and Figures 13 (actually Fig 11 is probably more useful) and 14.

[Vman455]

Very interesting!

I would argue, however, that Fig. 11 is probably better for people here to look at than Fig. 13; Fig. 13 shows the standard deviation (variability) in pressure measurements, but Fig. 11 shows the mean pressures (cP) over the three body types. (You can see from Fig. 13 that there is more variability in the pressure over the notchback and fastback shapes, but it doesn't tell you anything about the relative difference in average pressures between the shapes).

Edited to add: Also, notice the drastic difference in front lift between the squareback and notchback/fastback! This is an excellent illustration of shape changes at the rear affecting flow at the front.

2nd edit: Ha, just realized I'm an idiot--you probably meant the whole page of illustrations that includes Fig. 11-Fig. 13. Disregard!

[JulianEdgar]

Originally Posted by Vman455

Very interesting!

I would argue, however, that Fig. 11 is probably better for people here to look at than Fig. 13; Fig. 13 shows the standard deviation (variability) in pressure measurements, but Fig. 11 shows the mean pressures (cP) over the three body types. (You can see from Fig. 13 that there is more variability in the pressure over the notchback and fastback shapes, but it doesn't tell you anything about the relative difference in average pressures between the shapes).

Edited to add: Also, notice the drastic difference in front lift between the squareback and notchback/fastback! This is an excellent illustration of shape changes at the rear affecting flow at the front.

2nd edit: Ha, just realized I'm an idiot--you probably meant the whole page of illustrations that includes Fig. 11-Fig. 13. Disregard!

No you're right - I did mean Figure 11 not Figure 13.

[JulianEdgar]

https://www.youtube.com/watch?v=n7EYKVEL7Ds

[aerohead]

None of the images represent a true 'fastback.' They all have a 'deck' behind where the fastback would terminate. Technically a 'spoiler.'

[JulianEdgar]

Originally Posted by aerohead

None of the images represent a true 'fastback.' They all have a 'deck' behind where the fastback would terminate. Technically a 'spoiler.'

Yes, then it would have even more lift.

[aerohead]

Originally Posted by JulianEdgar

Yes, then it would have even more lift.

1) I totally agree, within the context that, the depictions are those of 'pseudo-Jaray' rooflines, prone to separation.

[JulianEdgar]

Originally Posted by aerohead

1) I totally agree, within the context that, the depictions are those of 'pseudo-Jaray' rooflines, prone to separation.

Little to do with separation - Page 10 of the cited paper.