FREQUENCY-SPECTRA OF SCALAR FLUCTUATIONS AT ENTRAINING STRATIFIED INTERFACES

Observations of frequency (omega) spectra of buoyancy fluctuations in entraining stratified fluids are presented. A two-fluid system, with t urbulence in the dense layer, was used for this purpose and the spectr a were measured at different distances (z) from the interface. As z in creases, interesting transitions were found to occur in spectral shape s. Near the interface where the buoyancy effects are still important, z < 0.2L(H) (L(H) is the integral length scale measured in homogeneous turbulence), the spectra were dominated by omega(-3) form which gradu ally transitioned into the omega(-5/3) shape at z approximate to 0.5L( H). At z approximate to 0.7L(H), the spectra appeared to contain both omega(-5/3) and omega(-1) shapes, and away from the interface at z > 0 .8L(H) the omega(-5/3) form disappeared leaving the omega(-1) form. Th ese changes of the spectral form of scalar fluctuations evidence the f ate of dense fluid particles that are entrained at the density interfa ce and then break up into smaller scales and lose buoyancy as they are advected by large-scale eddies.