[CIG-SHORT] Mesh interpenetration

[Brad Aagaard]

On 11/01/2017 12:59 PM, Li, Teng wrote:
> Hi Brad,
> 
> Thank you. Actually, I asked the similar questions in the last month. I find normal traction become zero in the junction area. And also shear traction become zero in this area. After analyzing HDF5 file, I find there are both positive and negative normal slip occur in different time steps. Last time, we thought it was because the inappropriate parameters chosen for my problem. For my problem, the mesh size is 12.5m. And I first choose nordimensinalizer: normalizer.shear_wave_speed = 1*km/s,  normalizer.wave_period = 1*s. Now I change them to 0.1*km/s and 0.1s. Furthermore, I add formulation.norm_viscosity = 0.4. And I still see the both positive and negative normal slips. The magnitudes for both are about 0.005m.
> 
> Is it because the rounding error brought by Matlab? Or is it because the mesh? I use Trellis to mesh all the domain.

Teng,

I went back to our email exchange in Aug and looked at your JSON 
parameter file that you provided then. It looks like you are running a 
dynamic simulation with multiple intersecting faults with a friction 
interface.

The JSON parameter file doesn't contain your spatial databases so I 
don't know what sort of tractions you have on the fault or what your 
material properties are. What information are you using to select your 
new nondimensionalization scales? The shear wave speed should be close 
to your minimum shear wave speed and the wave period should be close to 
the minimum wave period you can resolve with your discretization (a 
wavelength of usually 10-12 times your cell size). You updated 
nondimensionalization scales correspond to a wavelength (length scale) 
of 0.01 km or 10m. I suspect that the wavespeed you use for 
nondimensionalization should be on the order of 1 km/s not 0.1 km/s.

With a geometric incompatibility, usually you can keep the fault closed 
with an appropriate normal traction. That is, if you have a small amount 
of slip relative to your cell size, then the local transient normal 
tractions are usually much smaller than the compressive normal tractions 
arising from the overburden pressure.

If you need further help, I suggest sending a diagram of what you are 
trying to do along with a description (include approximate values for 
your materials and BC) and the JSON parameter file.

Regards,
Brad