Abstract. A two-dimensional flow generated by the interaction of two opposing, symmetric curved wall jets is investigated experimentally. The overall flow field can be divided into the curved wall jet region, the interaction region, and the merged jet region; thus, the results of the measurement are discussed to characterize these three distinct regions. For the curved wall jet region, the Reynolds stress distribution, the correlation coefficient, fiu v', and the ratio of normal stresses, v2/u 2, are presented and the effects of curvature and adverse pressure gradient on these distributions are discussed. The Reynolds stress distributions in the interaction region are analyzed in detail to illuminate the negative production of the turbulent kinetic energy. The developing jet in this region is found to accelerate owing to the very high pressure arising from the collision of the two wall jets. A counter-gradient shear flow situation is also observed in this interacting region. Measured data in the merged jet region are often compared to those of plane jets and the development of the merged jet is discussed in that respect. The spreading rate of the present merged jet is found to be much larger than that of the plane jets. To account for the larger spreading rate, the intermittency distribution is also investigated.

An experimental investigation was made of a two dimensional flow formed by the interaction of two asymmetric turbulent curved wall jets past a circular cylinder. Measurements were made of velocity and turbulence intensity profiles of the two curved wall jets before the interaction, and those of the merged jet after the interaction. The location of the interaction region of the two opposing curved wall jets and the flow direction of the merged jet were found to depend primarily on the ratio of initial momentum fluxes. The velocity and turbulence intensity profiles of the merged jet were similar to those of the plane turbulent jet. However, the growth rate of the merged jet was approximately 1.5 times larger than that of the plane jet. The influence of the momentum flux ratio on the growth rate appeared to be insignificant.

The aero acoustic characteristics of a centrifugal fan for a vacuum cleaner and its noise reduction method are studied in this paper. The major noise source of a vacuum cleaner is the centrifugal fan. The impeller of the fan rotates at over 30000 rpm, and generates very high-level noise. It was revealed that the dominant noise source is the aerodynamic interaction between the rotating impeller and stationary diffuser. The directivity of acoustic pressure showed that most of the noise propagates backward direction of the fan-motor assembly. In order to reduce the high tonal sound generated from the aerodynamic interaction, unevenly pitched impeller and diffuser, and tapered impeller designs were proposed and experiments were performed. Uneven pitch design of the impeller changes the sound quality while the overall sound power level (SPL) and the performance remains similar. The effect of the tapered design of impeller was evaluated. The trailing edge of the tapered fan is inclined. This reduces the flow interaction between the rotating impeller and the stationary diffuser because of some phase shifts. The static efficiency of the new impeller design is slightly lower than the previous design. However, the overall SPL is reduced by about 4 dB (A) . The SPL of the fundamental blade passing frequency (BPF) is reduced by about 6 dB (A) and the 2 n d BPF is reduced about 20 dB (A). The vacuum cleaner with the tapered impeller design produces lower noise level than the previous one, and the strong tonal sound was dramatically reduced.

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The flow rate and the specific noise level of 18 sirocco fans were measured and these data were analyzed by the Taguchi method and the neural network. The optimal design value obtained by the neural network generally showed good agreement with that by the Taguchi method. The effects of eight important design variables on the flow rate and the specific noise level were discussed.

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