亞聲速同軸射流波包敏感性研究(英文)
發(fā)布時間：2021-10-17 09:40
同軸射流廣泛應(yīng)用于航空航天領(lǐng)域,目前基于線性機制的理論模型對預(yù)測射流波包和噪聲普遍存在局限性,非線性效應(yīng)已成為研究熱點。本文通過在線性拋物化穩(wěn)定性方程（PSE）右端項中加入外部激勵項,研究了亞聲速同軸射流中非線性對波包演化的作用。當前工作主要聚焦于考察同軸射流中的波包特性,因為其存在兩個剪切不穩(wěn)定模態(tài)。基于同軸射流的基本流,采用求解線性PSE構(gòu)建了線性波包,進一步通過求解伴隨PSE方程得到了總體擾動能量對外部激勵的敏感性。結(jié)果表明敏感性區(qū)域和流動較大響應(yīng)區(qū)域均對應(yīng)于內(nèi)外臨界層附近,且二者對外部激勵均十分敏感,在下游區(qū)域可以看出,內(nèi)外兩個失穩(wěn)模態(tài)之間存在一定程度的相互作用。最后,采用伴隨優(yōu)化的方法獲得了關(guān)于外模態(tài)的最優(yōu)激勵,且施加最優(yōu)激勵之后波包得以更快速的增長。
【文章來源】：空氣動力學學報. 2020,38(02)北大核心CSCD
【文章頁數(shù)】：13 頁
【部分圖文】：
亞聲速同軸射流基本流流向速度場
Firstly,the local stability analysis is carried out for different Strouhal numbers St=fDs/Upat z=z0near the nozzle exit where f=ω/2πrepresents the frequency.Two meshes with 280and 300 r-grid points are adopted to show the convergence of eigenvalues.Generally,the low-frequency and low-azimuthalwavenumber components of wavepackets are recognized to be associated with the major sound sources of turbulent jet noise,thus in the following the axisymmetric mode m=0is investigated.Gloor et al.[5]have reported a representative frequency of St=0.4in the sound pressure spectrum at low polar angle in their isothermal coaxial jet.Thus,we also take the axisymmetric mode m=0and St=0.4as an example.From the eigenvalue spectrum shown in Fig.2,two distinct unstable modes can be identified.The two different modes are corresponding to the hydrodynamic Kelvin-Helmholtz(K-H)modes of the two mixing layers,where the definition of‘inner mode’refers to the K-H mode at the primary mixing region,and the‘outer mode’refers to the K-H mode at the secondary mixing region.At St=0.4,the local growth rate of the outer mode is much higher than its counterpart of inner mode,indicating that the outer mode dominates the local instability near the nozzle exit,which is consistence with the conclusion of Kwan and Ko[12],Léon and Brazier[22].The imaginary part of the eigenvalues obtained by LST representing the local growth rate with respect to the Strouhal number is presented in Fig.2(d).It is seen that the peak frequency of growth rate is reduced when the velocity ratio decreases.For St=0.4and m=0,the growth rates in the two cases are relatively high,so this frequency component is appropriate for comparison between the two cases.2.2 Non-parallel analysis
Figure 4 compares the imaginary part of local streamwise wavenumberαiand the‘N-factor’,defined as imaginary part of-χin Eq.(11),representing the spatial growth rate between the two cases for outer modes.It shows that the rapid growth of instability wave mainly occurs before 3D～4Dto nozzle exit inside the potential core in both cases.Moreover,as the velocity ratio decreases,the peak of N-factor corresponding to the neutrally stable axial location moves upstream to the nozzle exit,due to a shorter length of the outer potential core in Case 2.The change of neutrally stable axial location will affect the spatial distribution of sensitivity and flow response.3 Sensitivity analysis and optimization
本文編號：3441545
【文章來源】：空氣動力學學報. 2020,38(02)北大核心CSCD
【文章頁數(shù)】：13 頁
【部分圖文】：
亞聲速同軸射流基本流流向速度場
Firstly,the local stability analysis is carried out for different Strouhal numbers St=fDs/Upat z=z0near the nozzle exit where f=ω/2πrepresents the frequency.Two meshes with 280and 300 r-grid points are adopted to show the convergence of eigenvalues.Generally,the low-frequency and low-azimuthalwavenumber components of wavepackets are recognized to be associated with the major sound sources of turbulent jet noise,thus in the following the axisymmetric mode m=0is investigated.Gloor et al.[5]have reported a representative frequency of St=0.4in the sound pressure spectrum at low polar angle in their isothermal coaxial jet.Thus,we also take the axisymmetric mode m=0and St=0.4as an example.From the eigenvalue spectrum shown in Fig.2,two distinct unstable modes can be identified.The two different modes are corresponding to the hydrodynamic Kelvin-Helmholtz(K-H)modes of the two mixing layers,where the definition of‘inner mode’refers to the K-H mode at the primary mixing region,and the‘outer mode’refers to the K-H mode at the secondary mixing region.At St=0.4,the local growth rate of the outer mode is much higher than its counterpart of inner mode,indicating that the outer mode dominates the local instability near the nozzle exit,which is consistence with the conclusion of Kwan and Ko[12],Léon and Brazier[22].The imaginary part of the eigenvalues obtained by LST representing the local growth rate with respect to the Strouhal number is presented in Fig.2(d).It is seen that the peak frequency of growth rate is reduced when the velocity ratio decreases.For St=0.4and m=0,the growth rates in the two cases are relatively high,so this frequency component is appropriate for comparison between the two cases.2.2 Non-parallel analysis
Figure 4 compares the imaginary part of local streamwise wavenumberαiand the‘N-factor’,defined as imaginary part of-χin Eq.(11),representing the spatial growth rate between the two cases for outer modes.It shows that the rapid growth of instability wave mainly occurs before 3D～4Dto nozzle exit inside the potential core in both cases.Moreover,as the velocity ratio decreases,the peak of N-factor corresponding to the neutrally stable axial location moves upstream to the nozzle exit,due to a shorter length of the outer potential core in Case 2.The change of neutrally stable axial location will affect the spatial distribution of sensitivity and flow response.3 Sensitivity analysis and optimization
本文編號：3441545
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