  We study the experimental properties of exchange flows in a stratified
inclined duct (SID), which are simultaneously turbulent, strongly stratified by
a mean vertical density gradient, driven by a mean vertical shear, and
continuously forced by gravity. We focus on the core shear layer away from the
duct walls, where these flows are excellent experimentally-realisable
approximations of canonical hyperbolic-tangent stratified shear layers, whose
forcing allows mean and turbulent properties to reach quasi steady states. We
analyse state-of-the-art data sets of the time-resolved density and velocity in
three-dimensional sub-volumes of the duct in 16 experiments covering a range of
flow regimes (Holmboe waves, intermittent turbulence, full turbulence). In this
Part 1 we first reveal the permissible regions in the multi-dimensional
parameter space (Reynolds number, bulk Richardson number, velocity-to-density
layer thickness ratio), and their link to experimentally-controllable
parameters. Reynolds-averaged balances then reveal the subtle momentum forcing
and dissipation mechanisms in each layer, the broadening or sharpening of the
density interface, and the importance of the streamwise non-periodicity of
these flows. Mean flows suggest a tendency towards self-similarity of the
velocity and density profiles with increasing turbulence, and gradient
Richardson number statistics support prior internal mixing theories of
equilibrium Richardson number, marginal stability and self-organised
criticality. Turbulent volume fractions based on enstrophy and overturn
thresholds quantify the nature of turbulence between different regimes, while
highlighting the challenges of obtaining representative statistics in
spatio-temporally intermittent flows. These insights may stimulate and assist
the development of numerical simulations with a higher degree of experimental
realism.
