Abstract
In order to reveal the entrainment mechanisum of ambient water and the transition mechanism into fully developed turbulent jet, it is necessary to investigate the processes of the vortex formation, growth, coalescence and breakdown in near-field of jet by means of both visual and anemometry measurements. We(1980)1) have examined visually the effects of the Reynolds number and the nozzle length, i.e.initial boundary-layer thickness, on the coherent vortex motions in a two-dimensional free jet, and observed two kinds of vortex patterns; one is 'varicose' pattern and the other is 'sinuous' pattern.
The present paper is firstly to examine the effects of the bottom wall(Fig.1) and the free surface (Fig.2) on the coherent vortices in a horizontal plane jet, by making use of hydrogen-bubble method. Although the vortex production may be not strongly affected by these boundary conditions(fig.3), the vortex growth is fairly restrained due to the bent flow-axis(Coanda effect). A noticeable feature is that there exists a pair of coherent vortices even in the wall jet(Fig.4), in spite of perfect suppression of the wall-side entrainment by the bottom wall.
Next, we have tried to analyze the velocity fluctuations of vortices by means of simultaneous visual and hot-film mesurements. The first spectral peak frequency on the flow axis coincides well with the vortex frequency obtained visually(Fig.9). However, this peak frequency in the coalescence region is lower than the vortex frequency(Figs.10 & 11), because the interaction between the pairing vortices may be so strong as to appear in the hot-film signals(Fig.12). Thus, we examined the probability of the vortex period on the flow axis, by using a conditional sampling method (u/u' [greater than or equal] H)(Fig.14). These results (Fig.15) also coincides well with visual data (Fig.7).
