  Hard X-ray (HXR) sources are frequently observed near the top of solar flare
loops, and the emission is widely ascribed to bremsstrahlung. We here revisit
an alternative scenario which stresses the importance of inverse Compton
processes and the Kelvin- Helmholtz instability (KHI) proposed by Fang et al.
(2016). This scenario adds a novel ingredient to the standard flare model,
where evaporation flows from flare-impacted chromospheric foot-points interact
with each other near the loop top and produce turbulence via KHI. The
turbulence can act as a trapping region and as an efficient accelerator to
provide energetic electrons, which scatter soft X-ray (SXR) photons to HXR
photons via the inverse Compton mechanism. This paper focuses on the trigger of
the KHI and the resulting turbulence in this new scenario. We perform a
parameter survey to investigate the necessary ingredients to obtain KHI through
interaction of chromospheric evaporation flows. When turbulence is produced in
the loop apex, an index of -5/3 can be found in the spectra of velocity and
magnetic field fluctuations. The KHI development and the generation of
turbulence are controlled by the amount of energy deposited in the
chromospheric foot-points and the time scale of its energy deposition, but
typical values for M class flares show the KHI development routinely. Asymmetry
of energy deposition determines the location where the turbulence is produced,
and the synthesized SXR light curve shows a clear periodic signal related to
the sloshing motion of the vortex pattern created by the KHI.
