Calculate Tangential Rotational Force of Sphere in Flow

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Homework Statement


Considering a unsteady, incompressible flow past a sphere placed in a channel. The sphere is offset somewhat from the center of the flow to destabilize what otherwise would be steady-state symmetrical flow. Calculate the tangencial rotational force at the sphere.

Homework Equations


The Attempt at a Solution


My attempt is to make use of the Stoke's friction law for rotational motion of a sphere:
F=-8 \pi \eta_0 a^3 \Omega
with \eta_0 the viscosity, a sphere radius, \Omega sphere velocity.
But this is the total force and I have to calculate only the tangencial.
 
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The friction force is a tangential force, then if I have to calculate the tangential rotational force, then I have only to calculate the friction force written above?
 

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