Why does Stoke's Law (sedimentation) not work for measuring sand-size fractions?

Your question and details don’t exactly jibe: Are you specifically asking about “sand” or “sand-size fractions” (which could be nearly anything)?

Also, while sand crystals are not typically spherical, perhaps the lab experiments assume a low enough Reynold’s Number is at play to simplify the math?

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< < has just used up the final dregs of what he eked out Fluid Mechanics & Dynamics class back in school.

The question says “sand-sized fractions”, though I admit I wasn’t really sure what that meant and assumed he was talking about sand. I honestly can’t figure out why you can’t use Stoke’s Law for this.

Oh! I just figured it out. One of the rules of Stoke’s Law is that “the flow of the fluid around the particles is laminar – no particle exceeds the critical velocity for the onset of turbulence (<0.05mm)”. A “sand fraction” in the texture class is >0.05mm.