Initial turbulence effect on jet evolution with and without tonal excitation

Abstract

The effect of the initial turbulence level on the development of an axisymmetric jet was experimentally investigated. The turbulence intensity at the jet exit was varied, over the range 0.15%–5%, by using screens and grids placed upstream of an 8.8 cm diam nozzle. Top-hat initial mean velocity profiles with approximately identical boundary layer states were ensured in all cases; the turbulence was homogeneous and the spectra were broadband. It was found, contrary to earlier reports, that the natural jet evolution remained essentially unchanged for varying initial turbulence intensities. The response of the jet to single frequency, plane wave excitation was then studied over the full range of initial turbulence intensities. It was found that for even the highest turbulence (5%), the jet was quite excitable and could be influenced measurably by a tone of small amplitude. However, the jet ‘‘excitability,’’ as assessed from the variations of the mean velocity as well as the total and fundamental rms amplitudes on the axis, diminished with increasing initial turbulence. As the initial turbulence was increased, the amplitude of the discrete tone had to be increased in order to achieve the same excited state of the jet. It was also found that there existed a limit to the jet excitability, beyond which no additional effect could be achieved, as the amplitude of the discrete tone was increased. Results are also shown for a case having no grid or boundary layer trip, yielding a nominally laminar boundary layer at the jet exit. This case illustrates the profound effect of the initial boundary layer state on the jet evolution and excitability. This jet decayed the fastest naturally, and consequently, it was the least excitable, in spite of very low initial turbulence intensity (0.15%).