Effect of P\'{e}clet number on miscible rectilinear displacement in a Hele-Shaw cell

TL;DR

This study investigates how the Péclet number influences the stability and finger formation in miscible fluid displacement within a Hele-Shaw cell, revealing that higher Pe generally increases instability but can also reduce growth rates due to Korteweg stresses.

Contribution

It introduces a comprehensive analysis of Péclet number effects on miscible displacement instability, combining linear stability analysis and nonlinear simulations, including anisotropic dispersion and Korteweg stresses.

Findings

Higher Pe leads to earlier instability onset.

Unstable wave numbers increase with Pe.

Large transverse diffusivity results in coarser fingers.

Abstract

Influence of fluid dispersion on the Saffman-Taylor instability in miscible fluids has been investigated both in the linear and nonlinear regimes. The convective characteristic scales are used for the dimensionless formulation that incorporates P\'{e}clet number (Pe) into the governing equations as a measure for the fluid dispersion. A linear stability analysis (LSA) has been performed in a similarity transformation domain using the quasi-steady-state approximation. LSA results show that systems with large Pe become more unstable and the onset of instability occurs earlier compared to the case when Pe is smaller. Variations of the most unstable wave number and the cut-off wave number with Pe have been analyzed. Fourier spectral method has been used for the numerical simulations of the fully nonlinear system. The results indicate that the wave numbers of the unstable modes increase with Pe. Influence of the anisotropic dispersion on the onset in both the linear and nonlinear regimes has been analyzed. Large transverse diffusivity increases the length scale of the fingers quickly and merges the fingers to generate coarser fingers. Finally the combined effect of the Korteweg stress and Pe in the linear regime has been perused. In the presence of the Korteweg stresses and depending upon various flow parameters, a fluid system with larger Pe exhibits smaller instantaneous growth rate than with smaller Pe, which is a counter-intuitive result.