Calculation of cohesive force between the clump particle

Hi, I have some questions about the calculation method of cohesive force between the clump particles.

In this simulation model, the particle motion was described using the Hertz‐Mindlin with the DMT model to simulate the Hall flowmeter test.

In the simulation result, I found that the spherical powder was changed to a clump model, the cohesive force could be decreased based on the attached powder quantity on the funnel surface.
The cohesive force by DMT model can be expressed by:
F(DMT) = 􀀀 4*pi*a*γ, a: equivalent radius, γ: surface energy

In my speculation, the equivalent radius between the contact particle seems to be calculated by contacted particles between the clump particles.

I'd like to search some detailed equations for the cohesive force between clump particles, but I cannot find it.

I'd like to receive some advice about this.

Best regards,
Yim