Strange values of Y+ on a Ahmed body
 

Hi everyone,

Im a beginner in CFD. For the last assignment in my university i have to create a reliable Ahmed body model, using SST model. This means that the y+ should be around 1 as much as possible. After few iterations on the first cell thickness, the distribution of y+ is pretty much ok, but there is two really specific regions where the y+ goes to 0, or very close to, everytime, and i didn't figured out why.
If someone has a clue of what is happening i would really appreciate it !
Here is what i am talking about : https://www.casimages.com/i/200308014028682006.png.html

Thanks in advance for all the people taking the time to answer me.

This is due to the low velocity (air flow) at areas of re-circulation....similar phenomenon as on the sloped rear end.

Further, low Y+ values can be seen at the flat front face of the Ahmed body due to the fact that the air flow has practically come to a stop.

Regards

As mentioned, it appears to be localised recirculation or BL separation.

Its been years since I've looked at Ahmed so could be very wrong here, but I don't recall there being separated zones ahead of the sloped rear - the whole point of the model was to examine that aspect of turbulence models.

(i) Are you running at a very subsonic air speed
(ii) What is your surface roughness?
(iii) Is the nose definitely fully blended to the main body (no sharp curvature changes)?

Hi,

We are running the simulations with a Reynolds of 30 000 with water (imposed), so with the frontal area of the car as characteristic length the average velocity is around 0.3 m/s so yes, that is a very subsonic air speed.

I honestly have no idea about the surface roughness, but we have imposed a no slip condition on all surfaces of the car (maybe is that a bad idea ?).

Finally we made the geometry with a simple sketch extruded so yes the nose should be fully blended to the main body.

Im wondering if these low y+ are really that dramatic, because i can't see how i can fix it.

Thanks again for your answers.

No, no - no-slip is fine.

Looks like your fine as your running in water:

https://www.researchgate.net/publica...Porous_Devices

This is run in water at Re 30000, and there are circulation zones in around the areas you see the Y+ deviations.


Now - obviously that is still just comparing numerical to numerical rather than experimentals - but it gives some confidence in what your doing. But I know Bordeaux have good water tunnels with experienced people running them, so its unlikely they would not have some good internal data to compare against.

If you use k-w or k-w SST turbulence models then you don’t have min y+ bound. There are no sudden flow changes at y+ <1. At range 0<y+<5 flow is assumed to be laminar.
https://en.wikipedia.org/wiki/Law_of_the_wall
https://www.youtube.com/watch?v=fJDYtEGMgzs
https://upload.wikimedia.org/wikiped...ish%29.svg.png

Hi,

Indeed it seemed to be caused by water and the re circulation areas, which i can not do anything about it.

Thanks to everyone who took the time to answer and help, you guys are awesome !