  Knowledge of turbulent flows over non-flat surfaces is of major practical
interest in diverse applications. Significant work continues to be reported in
the roughness regime at high Reynolds numbers where the cumulative effect of
surface undulations on the averaged and integrated turbulence quantities is
well documented. Even for such cases, the surface topology plays an important
role for transitional roughness Reynolds numbers that is hard to characterize.
In this work, we attempt to develop a bottom up understanding of the mechanisms
underlying turbulence generation and transport, particularly within the region
of the turbulent boundary layer (TBL) affected by the surface. We relate
surface characteristics with turbulence generation mechanisms, Reynolds stress
transport and the resulting drag increase. To this end, we perform a suite of
direct numerical simulations of fully developed turbulent flow between two
infinitely wide, two-dimensional sinusoidally wavy surfaces at a friction
Reynolds number, $Re_{\tau}=180$, with different mean surface slopes, $\zeta$
(and fixed inner-scaled undulation height, $a^+=13$) corresponding to the
`waviness' regime. The increase in wave slope enhances near surface turbulent
mixing resulting in increased total drag, higher fraction of form drag, faster
approach to isotropy and thereby, modulation of the buffer layer. The primary
near-surface streamwise and vertical turbulence generation occurs in the
leeward and windward side of the wave. In contrast, the spanwise variance is
produced through pressure-rate-of-strain mechanism, primarily, along the
windward side of the wave for flows with very little flow separation. We also
observe significant dispersion effects in the production structure.
