University of Manitoba
Winnipeg, Manitoba, Canada
The entrainment rate across a stratified interface subjected to a tilted, impinging jet is measured in laboratory experiments. According to earlier work, the entrainment rate from a vertical jet is proportional to Ri$^{-1/2}$. In sharp contrast, tilting the jet only 15 degrees yields an entrainment rate proportional to Ri$^{-3/2}$, identical to that of a horizontal jet of 90 degree tilt. In the present experiments, this sharp transition is explored at tilt angles of five and ten degrees. Remarkably, the dimensionless entrainment rates at these two angles are identical and are proportional to Ri$^{-1/2}$, but they are reduced from that of the vertical jet by about a factor of four. For a vertical jet, a persistent lateral vortex surrounding the impingement dome is responsible for the entrainment. An hypothesis for the reduced entrainment of the tilted jet is that the persistence of this lateral vortex is destroyed over much of its length by the tilt. The surviving segment of the vortex seems to be independent of the tilt angle, until the tilt angle becomes about equal to half the spreading angle of the turbulent jet. Thus modest tilt reduces the entrainment rate while preserving the entrainment regime.