Nonlinear Response of a Laminar Boundary Layer to Isotropic and Spanwise Localized Free-stream Turbulence
Nonlinear Response of a Laminar Boundary Layer to Isotropic and Spanwise Localized Free-stream Turbulence
复制标题
DOI：
10.2514/6.2011-3292
复制
发表时间：
2011-06
期刊：
影响因子：
--
通讯作者：
Yongmin Zhang;T. Zaki;S. Sherwin;Xuesong Wu
中科院分区：
文献类型：
--
作者：
Yongmin Zhang;T. Zaki;S. Sherwin;Xuesong Wu
本文研究了过渡前平板边界层对各向同性和展向局域自由流湍流的非线性响应。湍流被表示为傅立叶模式的叠加，并考虑了边界层对FST的位移效应。边界层对FST的响应是低频流向条纹，其发展是通过数值求解非线性非定常边界区(NUBR)方程得到的。通过直接数值模拟对计算结果进行了验证。非线性是稳定的，因为对于小的FST雷诺数RL11，它降低了边界层内扰动速度的均方根(RMS)，而对于大的RL11，它是不稳定的。研究了上下游机制与自上而下机制的比较问题。条纹主要来自上游强迫，自上而下的强迫起次要作用。将各向同性FST的数值计算结果与Ovchinnikov等人的数值计算结果进行了比较。和Roach&Brierley的实验数据。计算的扰动没有达到域名系统和实验中的水平。然而，当考虑钝化前缘引起的FST各向异性时，得到了很好的定量一致性。结果表明，钝化前沿在解释大幅度条纹中起着关键作用，因为它导致了FST与纯各向同性的偏离。将跨向局域化FST的数值计算结果与Westin等人的实验数据进行了比较。由于实验中缺乏FST的速度谱信息，除了扰动的幅度外，其余的结果都是一致的。粘性二次失稳分析表明，在绕流过渡前的条纹状边界层中存在较强的不稳定性。在Blasius边界层中，不稳定模的最大增长率比Tollmien-Schlichting(T-S)波的增长率大一个数量级。
This paper is concerned with the nonlinear response of a pre-transitional flat-plate boundary layer to isotropic and spanwise localized free-stream turbulence (FST). The turbulence is represented as a superposition of Fourier modes and the displacement effect of the boundary layer on FST is taken into consideration. The responses of the boundary layer to FST are low-frequency streamwise streaks, and their development is obtained by numerically solving the nonlinear unsteady boundary-region (NUBR) equations. Direct numerical simulations (DNS) are carried out to validate the results. Nonlinearity is stabilizing in that it reduces the root mean square (rms) of the perturbation velocity in the boundary layer for small FST Reynolds number RL11, while it is destabilizing for large RL11. The issue of upstream-downstream versus top-down mechanisms is investigated. Streaks primarily develop from the upstream forcing; the top-down forcing plays a minor role. The numerical calculations for isotropic FST are compared with DNS results of Ovchinnikov et al. and experimental data of Roach & Brierley. The computed disturbances do not reach the levels in the DNS and experiment. However, good quantitative agreement is obtained when the anisotropy of FST induced by the blunt leading edge is accounted for. The results suggest that the blunt leading edge can play a key role in explaining the large amplitudes of streaks in that it leads to the deviation from pure isotropy of the FST. The numerical calculation for spanwise localized FST is compared with experimental data of Westin et al. Agreement is obtained except for the amplitude of the disturbances, which is due to the lack of the velocity spectral information of FST in experiment. The viscous secondary instability analysis indicates that there is strong instability in the streaky boundary layer before bypass transition. The maximum growth rate of the unstable modes is larger than that of Tollmien-Schlichting (T-S) waves in the Blasius boundary layer by one order of magnitude.