Abstract:
The influence of fluid dispersion on the Saffman-Taylor instability in miscible fluids has been investigated in
both the linear and the nonlinear regimes. The convective characteristic scales are used for the dimensionless
formulation that incorporates the Peclet number (Pe) into the governing equations as a measure for the fluid ´
dispersion. A linear stability analysis (LSA) is performed in a similarity transformation domain using the
quasi-steady-state approximation. LSA results confirm that a flow with a large Pe has a higher growth rate than
a flow with a small Pe. The critical Peclet number (Pe ´ c) for the onset of instability for all possible wave numbers
and also a power-law relation of the onset time and most unstable wave number with Pe are observed. Unlike
the radial source flow, Pec is found to vary with t0. A Fourier spectral method is used for direct numerical
simulations (DNS) of the fully nonlinear system. The power-law dependence of the onset of instability ton on
Pe is obtained from the DNS and found to be inversely proportional to Pe and it is in good agreement with
that obtained from the LSA. The influence of the anisotropic dispersion is analyzed in both the linear and the
nonlinear regimes. The results obtained confirm that for a weak transverse dispersion merging happens slowly
and hence the small wave perturbations become unstable. We also observ that the onset of instability sets in
early when the transverse dispersion is weak and varies with the anisotropic dispersion coefficient, , as ∼√,
in compliance with the LSA. The combined effect of the Korteweg stress and Pe in the linear regime is pursued.
It is observed that, depending on various flow parameters, a fluid system with a larger Pe exhibits a lower
instantaneous growth rate than a system with a smaller Pe, which is contrary to the results when such stresses are
absent.
