MEASUREMENT OF TURBULENCE NEAR SHEAR-FREE DENSITY INTERFACES

The results of an experimental study carried out to investigate the st ructure of turbulence near a shear-free density interface are presente d. The experimental configuration consisted of a two-layer fluid mediu m in which the lower layer was maintained in a turbulent state by an o scillating 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 introd uction 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 dampe d. The modification of r.m.s velocities is essentially limited to dist ances smaller than about an integral lengthscale. Inspection of spectr a shows that these distortions are felt only at small wavenumbers of t he order of the integral scale and a range of low-wavenumbers of the i nertial subrange; the distortions become pronounced as the interface i s approached. Comparison of the horizontal velocity data with the rapi d distortion theory (RDT) analyses of Hunt & Graham (1978) and Hunt (1 984) showed a qualitative agreement near the interface and a quantitat ive agreement away from the interface. On the other hand, the RDT pred ictions for the vertical component were in general agreement with the data. The near-interface horizontal velocity data, however, showed qua ntitative agreement with a model proposed by Hunt (1984) based on nonl inear vortex dynamics near the interface. The effects due to interfaci al waves appear to be important for distances less than about 10% of t he integral lengthscale. As a consequence of the non-zero energy flux divergence, the introduction of a density interface to oscillating gri d turbulence increases the rate of dissipation in the turbulent layer except near the interface, where a sharp drop occurs. The present meas urements 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 .