  We investigated the stability of the bottom boundary layer (BBL) beneath
periodic internal solitary waves (ISWs) of depression over a flat bottom
through two-dimensional direct numerical simulations. We explored the effects
of variation in wave Reynolds number $Re_{ISW}$ and wave period on the nature
of the instability, and energy production in the separated BBL. The instability
characteristics and rate of vortex shedding of the BBL were strongly dependent
on $Re_{ISW}$. The BBL was laminar and convectively unstable at $Re_{ISW}$ 90
and 300, respectively. At $Re_{ISW}=300$, the convective wave packet was
periodically amplified by each successive ISW, until vortex-shedding occurred.
This implies noise-amplification behavior and suggests that the discrepancies
in the critical $Re_{ISW}$, for vortex shedding between lab and different
numerical simulations, are due to differences in background seed noise.
Instability energy decreased under the front shoulder of the ISW, analogous to
flow relaminarization under a favourable pressure gradient. At larger
$Re_{ISW}=900$, the BBL was initially convectively unstable, and then the
instability tracked with the ISW, characteristic of global instability,
regardless of the ISW periodicity. The simulated initial convective instability
at both $Re_{ISW}$ 300 and 900 is in agreement with local linear stability
analysis which predicts that the instability group speed is always lower than
the ISW celerity. Increased free-stream perturbations and larger $Re_{ISW}$
shift the location of vortex shedding (and enhanced bed shear stress) closer to
the ISW trough, thereby potentially changing the location of maximum sediment
resuspension from the ISW, in agreement with field observations at higher
$Re_{ISW}$.
