Study on the effect of the boundary layer on the generation of aerodynamic noise and its control method.

Experimental investigations were conducted in order to understand the characteristics of the Aeoliantone generated from two-dimensional square and circular cylinders with pressure transducersembedded on the surface. the following conclusions wtainec .1) Square CylinderThe peak level ofthe Aeolian tone is maximum when the angle of attack is 0°，or，a flat surface is placed normal to the mean flow. the separated flow from the leading edge corner ofthe square cylinder reattaches to the side surface for the angle of attack between 10°to 13°. thesurface pressure fluctuation becomes minimum at this angle of attack and consequently the Aeolian2) Circular CylinderThe characteristics of the Aeolian tone radiated from acircular cylinder可以divided into three categories in the Reynolds number， Re，i.e. sub-critical (Re<3x10 - D15 - D1)，super critical (3x10 - D15 - D1<Re<10 - D16 - D1) and trans-critical (Re>10 - D16 - D1) regions. In thesub-critical region the surf…pressure fluctuation is uniform spanwise and the coherence between the Aeolian tonehigh and the Strouhal number is 0.2. In the supercritical region，Strouhal number jumps up to 0.45 and the peak level of the Aeolian tone decreases sharply. the Strouhal number jumps up to 0.45 and the peak level of the Aeolian tone decreases sharply. thesurface pressure fluctuation shows strong three-dimensionality and only a fractional part inspanwise shows high coherence with the Aeolian ire. In the trans-critical regionthe Strouhal number returns to 0.2 and the spectra of the surface pressure fluctuation and theAeolian tone show somewhat random nature with a blunt peak. The comparison between The measured andpredicted Aeolian tone level from the surface pressure fluctuation shows good agreement for the sub-and super-critical regions but the predicted level is approximately 10db lower than the measuredlevel for the trans-critical region.3) Reduction of the Aeolian ToneA half rounded square cylindershows approximately 10db lower Aeolian tone level when the rounded part is faced to the mean flowcompared to the circular cylinder with the same diameter. When a wire of diameter d is wound inspiral with a pitch p,around a circular cylinder of diameter Dthe Aeolian tone is considerably decreased when d/ d >0.2 and p/ d =3. Less

Experimental investigations were conducted in order to understand the characteristics of the Aeolian tone generated from two-dimensional square and circular cylinders with pressure transducers embedded on the surface. The following conclusions were obtainec.1) Square CylinderThe peak level of the Aeolian tone is maximum when the angle of attack is 0°, or, a flat surface is placed normal to the mean flow. The separated flow from the leading edge corner of the square cylinder reattaches to the side surface for the angle of attack between 10°to 13°. The surface pressure fluctuation becomes minimum at this angle of attack and consequently the Aeolian tone becomes minimum.2) Circular CylinderThe characteristics of the Aeolian tone radiated from a circular cylinder can be divided into three categories in the Reynolds number, Re, I.e. sub-critical (Re<3x10ィイD15ィエD1), super critical (3x10ィイD15ィエD1<Re<10ィイD16ィエD1) and trans-critical (Re>10ィイD16ィエD1) regions. In the sub-critical region, the surf … More ace pressure fluctuation is uniform spanwise and the coherence between the Aeolian tone is high and the Strouhal number is 0.2. In the super-critical region, the Strouhal number jumps up to 0.45 and the peak level of the Aeolian tone decreases sharply. The surface pressure fluctuation shows strong three-dimensionality and only a fractional part in spanwise shows high coherence with the Aeolian ire. In the trans-critical region, the Strouhal number returns to 0.2 and the spectra of the surface pressure fluctuation and the Aeolian tone show somewhat random nature with a blunt peak. The comparison between the measured and predicted Aeolian tone level from the surface pressure fluctuation shows good agreement for the sub- and super-critical regions but the predicted level is approximately 10 dB lower than the measured level for the trans-critical region.3) Reduction of the Aeolian ToneA half rounded square cylinder shows approximately 10 dB lower Aeolian tone level when the rounded part is faced to the mean flow, compared to the circular cylinder with the same diameter. When a wire of diameter d is wound in spiral with a pitch p,around a circular cylinder of diameter D, the Aeolian tone is considerably decreased when d/D>0.2 and p/D=3. Less