  Early to intermediate time behaviour of the planar Richtmyer--Meshkov
instability (RMI) is investigated through direct numerical simulation (DNS) of
the evolution of a deterministic interfacial perturbation initiated by a
$\textrm{Ma}=1.84$ shock. The model problem is the well studied initial
condition from the recent $\theta$-group collaboration [Phys. Fluids. 29 (2017)
105107]. A grid convergence study demonstrates that the Kolmogorov microscales
are resolved by the finest grid for the entire duration of the simulation, and
that both integral and spectral quantities of interest are converged.
Comparisons are made with implicit large eddy simulation (ILES) results from
the $\theta$-group collaboration, generated using the same numerical algorithm.
The total amount of turbulent kinetic energy (TKE) is shown to be decreased in
the DNS compared to the ILES, particularly in the transverse directions, giving
rise to a greater level of anisotropy in the flow (70\% vs. 40\% more TKE in
the shock parallel direction at the latest time considered). This decrease in
transfer of TKE to the transverse components is shown to be due to the viscous
suppression of secondary instabilities. Overall the agreement in the large
scales between the DNS and ILES is very good and hence the mixing width and
growth rate exponent $\theta$ are very similar. There are substantial
differences in the small scale behaviour however, with a 38\% difference
observed in the minimum values obtained for the mixing fractions $\Theta$ and
$\Xi$. Differences in the late time decay of TKE are also observed, with decay
rates calculated to be $\tau^{-1.41}$ and $\tau^{-1.25}$ for the DNS and ILES
respectively.
