  Suspended particles can significantly alter the fluid properties and, in
particular, can modify the transition from laminar to turbulent flow. We
investigate the effect of heavy particle suspensions on the linear stability of
the Kolmogorov flow by means of a multiple scale expansion of the Eulerian
model originally proposed by Saffman (1962). We find that, while at small
Stokes numbers particles always destabilize the flow (as already predicted by
Saffman in the limit of very thin particles), at sufficiently large Stokes
numbers the effect is non-monotonic in the particle mass fraction and particles
can both stabilize and destabilize the flow. Numerical analysis is used to
validate the analytical predictions. We find that in a region of the parameter
space the multiple-scale expansion overestimates the stability of the flow and
that this is a consequence of the breakdown of the scale separation
assumptions.
