Boundary layer model for intruder drag

We propose a boundary layer model for drag on vertical intruders with uniform cross-sections in granular beds. The drag is the surface integral of the stress over a monolayer of particles, where the stress has a simple dependence on depth beneath the surface and angle of the surface normal relative to the direction of flow. This model is in good experimental agreement, accounts for the scale effect and the associated force focusing observed on edges of intruders.