Inconsistency in surface tangential stress and its X-averaged value at t=0

[JYonKim]

Hello,

I'm using PyBaMM with the SPMe model and the OKane2022 parameter set, and I've encountered a discrepancy in the computed surface tangential stress.

Let’s assume the solution object is sol1. Then the following two variables return different values at t = 0:

stress_t_surf_avg = sol1.cycles[0].steps[0]['X-averaged negative particle surface tangential stress [Pa]'].entries
stress_t_surf = sol1.cycles[0].steps[0]['Negative particle surface tangential stress [Pa]'].entries.T

The X-averaged value(stress_t_surf_avg) is consistently lower than the uniform local value, despite all entries in stress_t_surf[0] being identical.

To understand the cause, I inspected the expression behind the variable "X-averaged negative particle surface tangential stress [Pa]". It appears that this variable is computed using X-averaged concentration, rather than by directly averaging the local surface stress values across the electrode.

However, at t = 0, both the local and X-averaged concentrations should be equal to the initial value throughout the domain, so both stress variables should produce the same result.

I also checked the spatial discretization:

sol1.cycles[0].steps[0]['x_n [m]'].entries.T[0][1:] - sol1.cycles[0].steps[0]['x_n [m]'].entries.T[0][:-1]

The mesh is uniform (dx is constant), so integration weights should not introduce any error.

Given these conditions, why is there a discrepancy between the X-averaged and local surface stress values at the initial time? I’d appreciate any clarification on how these variables are computed under the hood.

Thanks in advance for your help!