text: Consider an infinitely long cylinder of radius  exhibiting torsional oscillation with angular velocity  where  is the frequency. Then the velocity approaches after the initial transient phase to
  where  is the modified Bessel function of the second kind. This solution can be expressed with real argument  as:
  where
   and  are Kelvin functions and is to the dimensionless oscillatory Reynolds number defined as , being  the kinematic viscosity.