Stokes' law (Drag at low velocity)

The force required to move a sphere through a continuous quiescent fluid under conditions of laminar flow is proportional to the velocity, the radius of a sphere, and the viscosity of the fluid. Stokes' drag force equation is ... f = 6ph rv, ... where r is the sphere radius, h (eta) is the viscosity, and vT is the terminal velocity, given for a smooth sphere by....

... where r1 and r2 are the densities of the sphere and the fluid respectively.

Stokes' work was refined by Gibson and Jacobs in 1920 to include wall and end effect corrections, based on the ratio between the sphere diameter (d) and the inner diameter of a cylinder (D). The first order correction for the larger wall effect, with no higher powers of (d/D), in terms of (vT) above, includes the measured terminal velocity (v) in the expression....

This equation applies in laminar flow.

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