The impact modulator with bounded two-dimensional configuration does not work stably under specific conditions. It is found that the impingement surface in the coaxial impingement of twodimensional jets is practically unstable and except for very short nozzle spacing, the two jets deflect and oscillate each other, and cause the unstable behaviour of the impact modulator. To clarify the whole aspect of the impingement phenomena, the impingement of jets from rectangular nozzles and that of those from circular nozzles are comparatively studied. It is revealed that while jets from circular nozzles always form a stable impingement surface, this is not always the case with those from rectangular nozzles, but the stability of the impingement surface depends on Reynolds number nozzle aspect ratio and nozzle spacing. Especially, in the case of bounded jets, the impigement surface is found to be always unstable, showing oscillatory behaviour. The frequency of the oscillation is found to be in proportion to initial velocity and inversely proportional to nozzle spacing.
The mechanism of the oscillation is qualitatively explained in terms of the static pressure field measured. The conditions for the occurrence of the jets' oscillation are found using simplified analitical model, and the calculat ion shows a good agreement with the experiments. The computation of the patterns of oscillating jets are performed in order to confirm the mechanism of the oscillation, and the results are found to agree qualitatively with the experiments.

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This paper deals with the transient phenomena of a fluid conduit in which trapped air forms an air chamber.
It has commonly been agreed that the air existing in a fluid conduit always absorbs surge pressure. However, the theory and experiments described in this paper revealed that under certain conditions the pressure surge is enhanced by such an air chamber or trapped air.
The test line is about 106 m long, 35.7 mm inside diameter. One end of it is closed and an air chamber is attached. Quick opening of a direction control valve mounted at the other end causes a fluid transient in the line.
The experiments showed that pressure surge in the conduit with the air chamber is about 2times greater than that without air chamber. Burning of oil droplets in the air chamber was observed when pressure surge was generated. This shows the temperature of the air rises high enough for the oil to catch fire. The result of numerical calculation by means of characteristics method showed a good agreement with that of experiment.
When liquid is transported through a long conduit such as pipeline, transient phenomena described in this paper can occur by pump starting or sudden valve opening. The result of this study explains why an incomprehensibly high pressure surge sometimes occurs in actual piping systems, and offers, at the same time, another important factor for the consideration of surge pressure absorption with accumulators and air chambers.

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