By K Attenborough; V I Tokarev; Oleksandr Zaporozhets
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The thickness of the separation zone is small in comparison with the typical turbulence scale. Therefore, the separation zone is considered as a geometrically random surface, distorted by the instability of the vortex sheet (see Fig. 15a). At the separation surface, there is a jump in vorticity because outside the turbulent volume V, the ﬂow is potential (the gas velocity of inﬂow into the jet over the separation is continuous). We suppose that a subsonic jet contains compact noise sources. To calculate the parameters of turbulent ﬂow, we introduce non-dimensional inner variables xi = tVj xi vi a , vi = , τ = ,a = , L Vj L a0 where L, Vj deﬁne jet turbulence length and velocity scales, respectively.
At high Strouhal numbers, the broadband noise is due the interaction of disturbances in the jet with shock waves. In experiments on supersonic jets at low to moderate Reynolds numbers, the large coherent structures in the ﬂows were found to inﬂuence the emitted sound. 17 Sound power level spectrum of jet suction. 49) where δ (f ) is the Dirac delta function, f = 0 gives the equation of the moving surface, li is the ith component of the surface force and Tij is Lighthill’s stress tensor. 49) contains the following source terms: the ﬁrst is the thickness source (monopole), the second is the loading source (dipole) and the third is the quadrupole source.
Two thresholds are speciﬁed: ‘signiﬁcant’ exposure is deﬁned by Ldn = 55 dB or higher and ‘high’ exposure is deﬁned by Ldn = 65 dB or higher. Harmonization of noise indicators (NI) is an essential component of European strategy to reduce noise. The Commission of the European Communities has suggested use of the following criteria for NI:1,5 • • • • • validity (relationship with effects); practical applicability (easy to calculate using available data, or to measure using available equipment); transparency (easy to explain, the relationship with physical units); enforceability (use of indicator in assessing changes or when set limits are exceeded); consistency.