| Popis: |
The stratified turbulent shear flow established by a cold wall jet issuing into an ambiant heated stream has been investigated experimentally. Conditions were such that buoyancy remained negligible in the jet establishment zone but became important further downstream. It was found previously that entrainment ceases when the “difference” Richardson number Ri, based on the scales of the free mixing zone of the wall jet, increased to a value of 1/3 and the same value has also been reported for the stop of the growth of a mixing layer. Flow visualisations carried out in a mixing layer with a Reynolds number close to that of the jet (≈103) _showed that it is the pairing process which is inhibited when Ri ≽ 1/3. From the results obtained in the jet it is inferred that a pairing process is also fundamental to the jet growth, even though no two-dimensional spanwise rolls exist in the jet and the spanwise temperature variation seems to indicate streamwise rolls. The turbulence becomes increasingly more anisotropic with increasing Ti and the vertical heat flux decreases relative to the longitudinal heat flux or the vertical momentum flux. These variations are explained using time scale arguments. When entrainment ceases the turbulence intensity was found to be still high and the Reynolds stress still appreciable, a result which is somewhat unexpected. |
