The results of an experimental study carried out to investigate
the
structure of turbulence near a shear-free density interface are presented.
The
experimental configuration
consisted of a two-layer fluid medium in which the lower layer was maintained
in
a turbulent state by an oscillating grid. The measurements included the
root-mean-square (r.m.s.) turbulent velocities, wavenumber spectra, dissipation
of
turbulent kinetic energy and integral lengthscales. It was found that the
introduction of a density interface to a turbulent flow can strongly distort
the
structure of turbulence near
the interface wherein the horizontal velocity components are amplified
and the
vertical component is damped. The modification of r.m.s velocities is essentially
limited to distances smaller than about an integral lengthscale. Inspection
of
spectra shows that these distortions are felt only at small wavenumbers
of the
order of the integral scale
and a range of low-wavenumbers of the inertial subrange; the distortions
become
pronounced as the interface is approached. Comparison of the horizontal
velocity
data with the rapid distortion theory (RDT) analyses of Hunt & Graham
(1978) and
Hunt (1984) showed a qualitative agreement near the interface and a quantitative
agreement away from the interface. On the other hand, the RDT predictions
for the
vertical component were in general agreement with the data. The near-interface
horizontal velocity data, however, showed quantitative agreement with a
model
proposed by Hunt (1984) based on nonlinear vortex dynamics near the interface.
The effects due to interfacial waves appear to be important for distances
less
than about 10% of the integral lengthscale. As a consequence of the non-zero
energy flux divergence, the introduction of a density interface to oscillating
grid turbulence increases the rate of dissipation in the turbulent layer
except
near the interface, where a sharp drop occurs.
The present measurements provide useful information on the structure of
turbulence
in shear-free boundary layers, such as atmospheric and oceanic convective
boundary
layers, thus improving modelling capabilities for such flows.