DEM_wheelDP_simplifed_low DP forces at 50%slip

[sumaya]

I am currently trying to prevent wheel sinkage by modifying the wheelDP script with a new set of parameters. I have attempted to set the wheel's Z-position to match the existing script; however, the wheel still sinks as soon as the load is applied.

For this simulation, I am using a 2-sphere clump to model the soil particles, and my reference max_z is slightly higher. Most importantly, I am observing low drawbar pull (DP) forces, including negative DP forces at 50% slip, which I believe is unexpected for a 50% controlled slip scenario. I have attahced some important plots to examine.

Given your expertise, I would greatly appreciate it if you could  provide some feedback or suggestions. I have attached the script for your reference.

Thank you for your time and assistance.

1. Chrono existing wheelDPSimplified plot (DP force vs simulation time of 6 sec at 50% slip)---->Attached image named Chrono

2. Modified wheelDPSimplified plot (DP force vs simulation time of around 6 sec at 50% slip)

3.Paraview version modified wheelDP version

4.Code: modified, wheel parameters, x,y world dimensions, scaled down the two clumps to get a specific radius, added two extra families(goal to settle the wheel gently). The z world size has stayed the same, modified the mesh file for the wheel.

Sincerely,

Sumaya

[sumaya]

Also can you please explicitly explain the max_z parameter used in the wheelDPSimplified script. Thanks.

[Ruochun Zhang]

Hi Sumaya,

I don't think this is that unexpected based on your description. 

About wheel sinking: It seems that neither your script nor the demo DPSimplified enforces the z coordinate of the wheel, so sinking is expected. It seems that you thought that setting the coordinate using the tracker would fix the wheel's location forever, and that's definitely not the case. This set was done once to put it at the correct initial location, and then the wheel is still affected by the physics, and it should indeed sink when the load is applied.

About drawbar pull: I'd rather conclude from the chart that when the simulation stabilizes, the average DP force is low and close to zero. To me it's quite expected for a smooth cylindrical wheel. I usually imagine the physical scene represented by this simulation as a smooth, metal cylinder rolling on a pile of also smooth and spherical glass beads with high slip, and the propulsion you'd expect to get from them is low anyway. If not for the enforced slip I'd imagine the wheel should just spin in place. The demo, on the other hand, uses a wheel with grousers and that is very different and should give significant propulsion. In this type of simulations usually the geometric locking contributes to the force much more than other effects. That is not to say you should arbitrarily add some sources of geometric locking to "resolve the problem"; you should still reproduce the actual physics you are after.

The variable max_z simply records the max z coordinate of the granular material, so that when we set the wheel's initial location, we put the wheel just above the terrain and do not let the wheel have initial penetration with the terrain. After that, the wheel drops to the material and sinks, and max_z has nothing to do with all these.

Thank you,

Ruochun