High stability of spiral jet issued from a nozzle, which was designed with an annular slit connected to a conical cylinder, has been clarified by measuring the radial turbulence level and analyzing linearly the amplitude of radial perturbation. The measurement by using laser doppler annemometer indicated that the level of spiral jet was much smaller than that of conventional jet. The linear analysis, which was carried out by difference equation obtained by substituting perturbation into continuity and Euler equation, indicated that the amplitude of the spiral jet was much smaller than that of the conventional jet at the each, radial point. This approximate solution was qualitatively consistent with the experimental results. This high stability of the spiral jet was caused by its steeper axial velocity and its larger radial gradient change of axial velocity near the boundary as compared to the conventional jet.
