The second term that contains the kinematic phenomenological viscosity νphen (in
m2 s−1) is necessary to get a threshold for Faraday instability and to operate close
to but below the instability region. This form of the damping term mimics the
internal dissipation from the Navier–Stokes equation in this ODE. Though simple,
this approach provides a sufficient description for modes and threshold (see Kumar &
Information stored in Faraday waves: the origin of a path memory 17
Tuckerman 1994 for a critical analysis). Finally, the initial conditions are given by
the coefficients am(tinit ) evaluated by a discrete Hankel transform of function (6.1).
Similarly, an inverse Hankel transform gets back the surface at time t from the
coefficients am(t).