摘要:
通过实验分析比较了对于相同高度不同宽度的四种矩形喷嘴，当压力在0.2 MPa到0.8 MPa 之间变动时，欠膨胀超音速自由射流的啸叫特性和对应的流场纹影结构.结果表明：不同宽高比喷嘴的超音速自由射流辐射噪声中的单频离散啸叫存在两种不同的啸叫模式，且随着射流压力的变化会出现模式间的切换.所谓模式切换是指不同模式的轮流占优和消失的现象.啸叫模式间的切换及占优区间的宽度随着喷嘴宽高比的减小而缩短.其中，宽高比为2的射流啸叫模式中的一种模式所占的射流压降区间异常小，此现象未在相关文献中提及；喷嘴宽高比为4的射流啸叫占优区间内，啸叫基频-射流压力曲线在0.49 MPa时出现了间断、跳跃现象.随着压力的降低激波纹影的轴线出现了抖动，不同宽高比下流场结构的稳定性随压力变化的规律各异.射流压力在0.70 MPa到0.45 MPa区间内，随着宽高比减小，第一波节格栅的激波致密度减弱，且出现轴向脉动，第二波节后方的流场变得紊乱；当射流压力低于0.45 MPa时，激波串结构随着宽高比的增大而趋于稳定，在此压力区间内周期性激波格栅结构较射流压力在0.45 MPa以上时有所减弱.结合啸叫频谱及纹影图分析，可初步认为，第二、三波节也会对啸叫频率的声压幅值起到反馈增强作用.
Abstract:
An experiment was carried out to analyse and compare the screech tone characteristics and schlieren structures of an under-expanded jet flowing through four rectangular nozzles, aspect ratios of which are different (having the same height but different widths), with the jet pressure ranging from 0.2 to 0.8 MPa. Results indicate that there exist two different screech tone modes in the noise generated by supersonic jet flowing through the rectangular nozzles with different aspect ratios, and a mode switching can be found by altering the jet pressure. Mode switching is a phenomenon that different mode dominates or disappears according to the change of jet pressure. The switching time of fundamental frequencies in the screech tone modes and the width of the domination interval would be shortened as the aspect ratio decreases. The jet flow pressure drop interval of one mode, whose aspect ratio is 2, is extremely small. This phenomenon has never been mentioned in the literature. When the aspect ratio of the rectangular nozzle is 4, there exists an interruption and skip on the fundamental frequency-jet pressure curve within the jet flow domination interval for jet pressure at 0.49 MPa. As the pressure reduces, the axes of the schlieren figures begin to shake, and the structure stability of the flow field with different aspect ratio varies with the jet pressure. When the jet pressure is within the range of 0.45 to 0.70 MPa, the density in the first shock-cell decreases as the aspect ratio reduces. Meanwhile, axial pulsation and disorder of the flow field behind the second shock-cell appear. When the jet pressure is under 0.45 MPa, the flow field structure of the shock wave becomes more stable as the aspect ratio increases. In this pressure region, the periodical shock-cell structure is weaker than those above 0.45 MPa. Analyzing the screech frequency spectrum and the schlieren figures, we can find that the second and third shock-cells also have feedback and enhancement for the sound pressure of the screech frequency.
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