[pilot.john]

Hello everyone

I am currently investigating the effect of slant angle changes on the drag coefficient of an Ahmed Body using Simcenter Star-CCM+.

At this stage I have to validate my sims with the original paper on the Ahmed Body (linked below).
My problem is that my Drag cd values are quite high compared to the experimental value of 0.290 for a slant angle of 25deg according to Ahmed’s paper.
I get for k-e 0.377-0.368 and 0.38 for K-omega. I aim to get to 10% error.

I am a CFD beginner and I also somewhat limited by PC’s 4 cores, so I only use a model split in half.
One of the problems I have is that parameters such as temp are not stated in the paper, so I assume 1.225 kg/m^3 and 1.79E-5 Pa-s for dynamic viscosity in order to match the paper’s Reynold’s number of 4.29 million. I use the same wind tunnel dimensions as Ahmed’s paper, using volumetric controls I have made a finer mesh around the AB where most of the interesting features happen with a 4mm custom size.
I have included my sims and some photos of the mesh (using hexahedron mesh).
https://drive.google.com/drive/folde...T1?usp=sharing

If you could help me or give me some suggestions I would be very thankful.


Some Salient Features of the Time -Averaged Ground Vehicle Wake
https://www.jstor.org/stable/44434262?seq=1

[lvl99]

Originally Posted by pilot.john

Hello everyone

I am currently investigating the effect of slant angle changes on the drag coefficient of an Ahmed Body using Simcenter Star-CCM+.

At this stage I have to validate my sims with the original paper on the Ahmed Body (linked below).
My problem is that my Drag cd values are quite high compared to the experimental value of 0.290 for a slant angle of 25deg according to Ahmed’s paper.
I get for k-e 0.377-0.368 and 0.38 for K-omega. I aim to get to 10% error.

I am a CFD beginner and I also somewhat limited by PC’s 4 cores, so I only use a model split in half.
One of the problems I have is that parameters such as temp are not stated in the paper, so I assume 1.225 kg/m^3 and 1.79E-5 Pa-s for dynamic viscosity in order to match the paper’s Reynold’s number of 4.29 million. I use the same wind tunnel dimensions as Ahmed’s paper, using volumetric controls I have made a finer mesh around the AB where most of the interesting features happen with a 4mm custom size.
I have included my sims and some photos of the mesh (using hexahedron mesh).
https://drive.google.com/drive/folde...T1?usp=sharing

If you could help me or give me some suggestions I would be very thankful.


Some Salient Features of the Time -Averaged Ground Vehicle Wake
https://www.jstor.org/stable/44434262?seq=1

Hi,

What do you mean by splitting the model in half? If you are cutting the geometry it will change the drag coeffiecient even if the geometry is symmetrical, maybe the problem comes from there

[CFDfan]

Originally Posted by lvl99

Hi,

What do you mean by splitting the model in half? If you are cutting the geometry it will change the drag coeffiecient even if the geometry is symmetrical, maybe the problem comes from there

Yes I wouldn't use symmetry and would extend the computational domain behind the solid body at least twice.

P.S. What is the inlet air-velocity in your model (20m/s, 40m/s, 60m/s, etc)?

[pilot.john]

Originally Posted by lvl99

Hi,

What do you mean by splitting the model in half? If you are cutting the geometry it will change the drag coeffiecient even if the geometry is symmetrical, maybe the problem comes from there

Thank you for your reply.

So I sketch the model in the built in starccm 3d-cad and then I create a plane through the middle of the geometry and the cut the model in half.
This plane is then used as a wall for the wind tunnel and is turned into a symmetry plane. I would have included a photo but unfortunately Im currently running a sim.

Since I am investigating the drag coefficient and I am simulating a symmetrical half body, how would that change the coefficient?

[pilot.john]

Originally Posted by CFDfan

Yes I wouldn't use symmetry and would extend the computational domain behind the solid body at least twice.

P.S. What is the inlet air-velocity in your model (20m/s, 40m/s, 60m/s, etc)?

Thank you for your reply.


I don't understand how the symmetry would affect the drag coefficient. This was done to reduce the computing power required for the sims. I'll give it a try though.

Why do you think that the domain isn't long enough? The dimensions are the same as the experimental wind tunnel from Ahmed. From the the scalar and vector scenes it looks like the length is enough to capture the flow features.

What I did find though is that my domain wasn't wide enough and would cause a lot of pressure in the front of the model.

My inlet velocity is 60 m/s.

I was thinking of using tetrahedron (3d triangle) mesh. Do you think that would improve the results compared to the current hexahedron (square) mesh?

[lvl99]

Originally Posted by pilot.john

Thank you for your reply.

So I sketch the model in the built in starccm 3d-cad and then I create a plane through the middle of the geometry and the cut the model in half.
This plane is then used as a wall for the wind tunnel and is turned into a symmetry plane. I would have included a photo but unfortunately Im currently running a sim.

Since I am investigating the drag coefficient and I am simulating a symmetrical half body, how would that change the coefficient?

I thought about it and you are right, if you are using RANS the symmetry plane should work fine I think, I posted my last comment because I had a problem using symmetry in a similar simulation, but I was using LES in that case.

[CFDfan]

Originally Posted by pilot.john

Hello everyone

I am currently investigating the effect of slant angle changes on the drag coefficient of an Ahmed Body using Simcenter Star-CCM+.

At this stage I have to validate my sims with the original paper on the Ahmed Body (linked below).
My problem is that my Drag cd values are quite high compared to the experimental value of 0.290 for a slant angle of 25deg according to Ahmed’s paper.
I get for k-e 0.377-0.368 and 0.38 for K-omega. I aim to get to 10% error.

I am a CFD beginner and I also somewhat limited by PC’s 4 cores, so I only use a model split in half.
One of the problems I have is that parameters such as temp are not stated in the paper, so I assume 1.225 kg/m^3 and 1.79E-5 Pa-s for dynamic viscosity in order to match the paper’s Reynold’s number of 4.29 million. I use the same wind tunnel dimensions as Ahmed’s paper, using volumetric controls I have made a finer mesh around the AB where most of the interesting features happen with a 4mm custom size.
I have included my sims and some photos of the mesh (using hexahedron mesh).
https://drive.google.com/drive/folde...T1?usp=sharing

If you could help me or give me some suggestions I would be very thankful.


Some Salient Features of the Time -Averaged Ground Vehicle Wake
https://www.jstor.org/stable/44434262?seq=1

I couldn't get Ahmed's paper from your link. What were the experimental values (Cd and Cl) for 60m/s?

[CFDfan]

Originally Posted by pilot.john

Thank you for your reply.


I don't understand how the symmetry would affect the drag coefficient. This was done to reduce the computing power required for the sims. I'll give it a try though.

Why do you think that the domain isn't long enough? The dimensions are the same as the experimental wind tunnel from Ahmed. From the the scalar and vector scenes it looks like the length is enough to capture the flow features.

What I did find though is that my domain wasn't wide enough and would cause a lot of pressure in the front of the model.

My inlet velocity is 60 m/s.

I was thinking of using tetrahedron (3d triangle) mesh. Do you think that would improve the results compared to the current hexahedron (square) mesh?

please see this investigation:

https://www.linkedin.com/pulse/effec...20the%20center.

about the impact of the CD size