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首页> 外文期刊>Journal of Fluid Mechanics >Influence of nozzle-exit boundary-layer conditions on the flow and acoustic fields of initially laminar jets
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Influence of nozzle-exit boundary-layer conditions on the flow and acoustic fields of initially laminar jets

机译:喷嘴出口边界层条件对初始层流射流的流动和声场的影响

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摘要

Round jets originating from a pipe nozzle are computed by large-eddy simulations (LES) to investigate the effects of the nozzle-exit conditions on the flow and sound fields of initially laminar jets. The jets are at Mach number 0.9 and Reynolds number 10~5, and exhibit exit boundary layers characterized by Blasius velocity profiles, maximum root-mean-square (r.m.s.) axial velocity fluctuations between 0.2 and 1.9% of the jet velocity, and momentum thicknesses varying from 0.003 to 0.023 times the jet radius. The far-field noise is determined from the LES data on a cylindrical surface by solving the acoustic equations. Jets with a thinner boundary layer develop earlier but at a slower rate, yielding longer potential cores and lower centreline turbulent intensities. Adding random pressure disturbances of low magnitude in the nozzle also increases the potential core length and reduces peak r.m.s. radial velocity fluctuations in the shear layer. In all the jets, the shear-layer transition is dominated by vortex rolling-ups and pairings, which generate strong additional acoustic components, but also amplify the downstream-dominant low-frequency noise component when the exit boundary layer is thick. The introduction of inlet noise however results in weaker pairings, thus spectacularly reducing their contributions to the sound field. This high sensitivity to the initial conditions is in good agreement with experimental observations.
机译:通过大涡模拟(LES)计算源自管道喷嘴的圆形喷嘴,以研究喷嘴出口条件对初始层流喷嘴的流场和声场的影响。射流的马赫数为0.9,雷诺数为10〜5,并表现出出口边界层,其特征在于Blasius速度分布,最大根均方根(rms)轴向速度波动介于射流速度的0.2至1.9%之间以及动量厚度从喷射半径的0.003到0.023倍变化。通过求解声学方程,根据圆柱表面上的LES数据确定远场噪声。边界层较薄的射流发展较早,但速度较慢,产生的潜在岩心更长,中心线湍流强度更低。在喷嘴中增加低强度的随机压力干扰也会增加潜在的芯长度,并降低峰值r.m.s。剪切层中的径向速度波动。在所有射流中,剪切层的过渡都由涡旋状起伏和成对作用所主导,这些涡旋起伏和成对产生强大的附加声成分,但当出口边界层较厚时,也会放大下游占主导地位的低频噪声成分。但是,引入进气噪声会导致配对变弱,从而显着降低了它们对声场的影响。这种对初始条件的高度敏感性与实验观察非常吻合。

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