  Shear flows are ubiquitous in astrophysical objects including planetary and
stellar interiors, where their dynamics can have significant impact on
thermo-chemical processes. Investigating the complex dynamics of shear flows
requires numerical calculations that provide a long time evolution of the
system. To achieve a sufficiently long lifetime in a local numerical model the
system has to be forced externally. However, at present, there exist several
different forcing methods to sustain large-scale shear flows in local models.
In this paper we examine and compare various methods used in the literature in
order to resolve their respective applicability and limitations. These
techniques are compared during the exponential growth phase of a shear flow
instability, such as the Kelvin-Helmholtz (KH) instability, and some are
examined during the subsequent non-linear evolution. A linear stability
analysis provides reference for the growth rate of the most unstable modes in
the system and a detailed analysis of the energetics provides a comprehensive
understanding of the energy exchange during the system's evolution. Finally, we
discuss the pros and cons of each forcing method and their relation with
natural mechanisms generating shear flows.
